Alan Buchanan Wardrop was one of Australia's most distinguished students of plant cell wall ultra-structure who made major contributions to our understanding of the structure of secondary walls and of how that structure affected the industrial uses of wood fibres. His work integrated information from observations with polarized light, X-rays and electron microscopy. Joining the Forest Products Division of Australia's Council for Scientific and Industrial Research in 1945, he became Foundation Professor of Biological Sciences at La Trobe University, Melbourne, in 1965.
Alan Buchanan Wardrop died in Melbourne on 20 May 2003. He was a distinguished student of plant cell wall ultrastructure, spending twenty years with the Forest Products Division of the Council for Scientific and Industrial Research (CSIR, later CSIRO) between 1945 and 1964 before becoming Foundation Professor of Botany at La Trobe University where he remained until retiring in 1986. Coming to the subject at the start of the electron microscopy era, his work illuminated many fundamental aspects of the heavily thickened secondary walls of wood fibres and the properties that affect their use in pulping. He also conducted many important experiments on the primary walls laid down in growing cells before secondary wall thickening occurs and on how growth modified their structure.
Alan Wardrop was born in Tasmania, in King Street, Sandy Bay, Hobart on 28 July 1921. He was the second son of James McAllan and Ethel May Wardrop (née Buchanan) and the younger brother of John Lindsay Wardrop (born 1916). His father was born in Clydebank, Scotland in about 1886 and the family migrated to Tasmania when Alan's father was about three. His mother was born in Hobart in 1889. James Wardrop worked for the Tasmanian public service (serving in several departments) while Ethel Wardrop was the homemaker. There was no tradition of science in the family but Alan noted, when interviewed many years later (Blythe 1998), that his father was always very supportive of his pursuing science whereas he viewed his mother as more protective although never opposed to his scientific ambitions. He commented that 'I don't think she wanted me to have to work too hard in life'!
He attended Albuera Street Primary School from 1928 and Hobart High School from 1934. The high school had several notable teachers and, with Hobart being so small, some teachers taught at both school and university. Alan many years later remembered Gordon Brett and Victor Crohn (Blythe 1998). Biology was not taught to male students at that time but Hobart's attractive setting and in particular walks to the summit of Mount Wellington helped foster his interest in the diversity of vegetation as well as providing a favourite recreation.
A £50 University Scholarship helped Alan's move to the University of Tasmania in 1939. Degree courses in botany had started only in 1938 and, with a background in the physical sciences, his initial goal was to study biochemistry. This he pursued through first-year studies of physics, chemistry, mathematics and botany. In 1940, he won the Florence Sprent Prize for Zoology and the University Prize for Physics. After third-year botany and chemistry, he took honours biology in a class of some fifteen students. Alan remembered some sixty years later the teaching of the professor of physics, Leicester McAulay, not only for his studies of electric fields around growing roots but also for his eccentricities in dress and manner. The influence of Dr Hugh Gordon, the university's first appointment in botany, and of Professor E. E. Kurth in chemistry were also noted.
Graduating in 1942 during the war, science students were put into reserved occupations and Alan pursued an MSc in the Chemistry Department. This involved course work and a research project and was funded by a University of Tasmania Research Scholarship and Commonwealth Research Grant. Many wartime programmes were in progress in Hobart, notably those of Professor Kurth. 'Kurth kilns' for charcoal production were widespread (one survives at Gembrook near Melbourne) and were used to generate 'producer gas' as a petrol substitute. Alan's project involved hydrolysing wood cellulose to monomer sugars that were then fermented to alcohol, which was also of interest as a fuel substitute. He harboured no illusions of changing the course of the war and confessed that it never seemed to him that they were going to make enough alcohol to affect the war effort. The project did, however, involve an interesting study of cellulose hydrolysis kinetics, a subject to which he returned later in his career. His MSc, supervised by Professor Kurth and B. J. F. Ralph, was awarded in 1944 with a thesis entitled 'An investigation of the acid hydrolysis of the wood of Eucalyptus obliqua L'Her'. Results from the study were published with Ralph in 1946.
In 1944, Alan began training as a Royal Australian Air Force navigator at Balnarring, on Western Port Bay in Victoria, after which he went to Mount Gambier, in South Australia, for air navigation training. Alan derived great satisfaction from his navigational training and found it immensely satisfying to be able to calculate where his plane should be and then look down and have the prediction verified. Shortly after the war's conclusion, he worked for a year as resident tutor in chemistry at Trinity College, University of Melbourne.
In 1946, Alan married Beulah May Brims, the daughter of plywood manufacturer Marcus Brims. Beulah worked in CSIRO as a research chemist for a short period and later as a Senior Tutor in Mathematics at the University of Melbourne. Their four children were Martin, Alison, Ann and Simon. Martin was a Rhodes Scholar (1974) and subsequently held a senior position in the Australian government service. Alison gained a PhD from the Biochemistry Department at La Trobe University and currently works in the Botany Department there. Ann graduated from the University of Melbourne with a BA and LLB and is a Senior Lecturer in the School of Law at La Trobe University. Simon won an 1851 Exhibition science research scholarship to Oxford from where he graduated in 1990 with a DPhil in mathematics.
Almost all Alan Wardrop's research concerned plant cell walls, the polysaccharide-rich structure assembled just outside the plasma membrane so forming a cage surrounding almost every plant cell. Young, expanding cells are surrounded by a thin primary wall that both controls growth and is also remodelled by it. When growth is complete, many cells – notably those forming wood – deposit a thick secondary wall. New polymers reach the wall through the plasma membrane so that continuing synthesis displaces the components of the original wall away from the plasma membrane. A mature wall is therefore layered from youngest to oldest on passing from its inner to its outer surface. Different layers can have quite different structure and composition, reflecting changes over time in the cell's activity.
Walls contain many polymers but cellulose is particularly noteworthy. Long, unbranched chains of glucose residues crystallize to form cellulose microfibrils that are only a few nm wide but many µm long. Crucially, cells control the orientation of the microfibrils they deposit. Orientation can change with time, producing the distinct layers of polylamellate walls, and cell growth can reorientate microfibrils in the primary wall after they are deposited. The structure of the primary wall is central to plant growth and the structure of the secondary wall is of great industrial interest because it determines many of the properties of timber and wood pulp. Both types of wall are dominated by cellulose microfibrils although lignin – a phenolic polymer that encrusts cellulose in the secondary walls of wood fibres – accounts for much of wood's rigidity.
Wardrop took up the challenge of describing wall structure and in particular of documenting microfibril orientation in the multilayered walls of wood fibres and the process of lignification. His work, particularly that published between 1947 and 1960, settled many fundamental issues of wood fibre structure. In this, he employed both the 'classical' tools of X-ray diffraction and optical microscopy and the techniques of electron microscopy that, beginning in the 1940s, gave more direct views of wall structure.
Alan joined the CSIR Division of Forest Products situated on the Yarra Bank in South Melbourne in 1945 as an Assistant Research Officer in the Wood Structure Section. He progressed rapidly through the various professional grades – Research Scientist, Senior Research Scientist, Principal Research Scientist and Senior Principal Research Scientist – and was appointed head of the Section of Wood and Fibre Structure, as it had been renamed, in 1961. He resigned in 1964 to return to the University of Tasmania as Professor of Botany. During his two decades at Forest Products, Wardrop made a massive contribution to plant science. His work on cell walls was largely concentrated on a few major themes: the arrangement of cellulose microfibrils, the structure of microfibrils, lignification, reaction wood, plant growth, and fibre structure in relation to utilization.
Wardrop came to these studies at a time when important conceptual and technical advances were occurring. Ideas, of which the multi-net hypothesis was the most enduring (Roelofsen and Houwink 1953), provided an explanation for how growth modified wall structure. On the technology side, electron microscopy was starting to provide refreshingly direct images of wall structure. With the methods that preceded it, 'a complex mental process intervened between the observations made and the structure delineated' (Preston 1958). It is easy to forget, however, just how much sophisticated structural information had already been deduced by that 'complex mental process' after observing how walls behaved when irradiated with polarized light or with X-rays. This information included estimates of the sizes of the crystalline regions of cellulose, their spacings, their orientation and the differences in orientation between different layers of the wall. Wardrop was skilled in the classical methods, which he applied throughout his career, but played a significant role in developing electron microscopy as a method to study walls.
Alan's studies of the arrangement of microfibrils in cell walls were by far his most extensive, and represent a major contribution to our knowledge of this subject. He employed a variety of techniques in his work but was primarily a microscopist. In his work on the organization of the cell wall, Wardrop clearly expounded the nature of the primary and secondary walls, relating them to earlier studies by Kerr and Bailey. The 'barber's pole' illustration showing the microfibrillar arrangement of the various layers, first published by Wardrop and Bland in the Proceedings of the Fourth International Congress of Biochemistry (1958), became quite famous.
Much of Wardrop's early work was in collaboration with H. E. Dadswell, head of the Wood Structure Section, who in 1960 became Chief of the Division of Forest Products on the retirement of S. A. Clarke. Their first collaboration led to a note in Nature in 1946 describing observations of cell wall deformations in wood fibres and another early study dealt with the nature of intercellular adhesion in delignified tissue.
In 1947, just six months after his marriage, Alan and his wife travelled on the converted troop ship Almeida to England where, supported by a CSIR Research Studentship, he took up PhD studies in the Botany Department at the University of Leeds. The PhD degree had not yet been established in Australia. The post-war period in the UK was a time of rationing: meat, in particular, was in short supply and the only affordable beverage was Tetley's Ale. Meetings with other Australian PhD students in Manchester, Cambridge and Oxford led to clandestine purification of industrial-grade alcohol using Raney nickel to remove impurities.
His supervisor in Leeds was R. D. Preston, who had graduated in 1929 as a physicist before moving into plant science. He became Reader in Plant Biophysics in 1948 and was later Professor and Head of the Astbury Department of Biophysics and, in 1954, Fellow of the Royal Society (Cushing 2005). In Leeds, Wardrop investigated the fine structure of the cell wall of the conifer tracheid, elucidating the dimensional relationships in the outer layer of the secondary wall, the organization of the secondary wall in relation to the growth rate of the cambium, the influence of pressure on cell wall organization and the orientation of microfibrils in the wall's different layers. His PhD was awarded in 1948 for a thesis entitled 'The submicroscopic organisation of the plant cell wall and its bearing upon the growth of the plant cell', and the results were published in a series of papers with Preston between 1947 and 1951. Wardrop came to regard Preston as his mentor and they remained in contact for many years with Wardrop contributing the introduction to a Festschrift marking Preston's 75th birthday in 1983. He looked back fondly on the Leeds interlude as most productive, and one that had set him on his life's course. His work there did much to settle important issues regarding the alignment of microfibrils in the conifer tracheid.
Wardrop described in his interview (Blythe 1998) how his work, conceived in Melbourne and brought to fruition in Leeds, settled a controversy between Preston and I. W. Bailey at Harvard University in favour of the latter. The dispute concerned the orientation of cellulose in each of the three layers of the secondary wall (S1, S2 and S3 where the S1 layer, the first deposited, is on the outside adjacent to the primary wall). Preston believed from his X-ray and optical data that microfibrils in each layer had the same mean orientation and that only the spread of microfibril angles differed from layer to layer. Bailey had proposed in 1934 that the inner and outer layers were transverse or formed shallow helices whereas the middle layer formed a steep helix. Wardrop devised an elegant method in which wood was sectioned in a series of planes starting with transverse and ending with longitudinal. The sections were examined in polarized light to find in which section each wall layer showed maximum birefringence. At this point, its microfibrils were aligned in the plane of the section. Birefringence was plotted as a function of section angle for each layer and showed unequivocally that the orientations of microfibrils differed between the layers, essentially confirming the view of Bailey. Wardrop notes that, although Preston 'pulled out every stop in arguing his case … he acknowledged that we had a clear result'. As some concession to Preston's views, the 1947 paper did provide evidence favouring his idea that there were also differences in the spread of microfibril angles between layers.
In 1949, Wardrop also published, from work in Leeds, a description of the micellar1 organization in the primary cell walls of cambium cells and oat coleoptile parenchyma. This had the interesting result that micelles in primary walls were smaller than those in secondary walls.
On returning to Forest Products in 1949, Wardrop continued his research into the cell wall organization of the conifer tracheid and cambium, initially in collaboration with A. J. Hodge, who was one of those operating what was then Australia's only electron microscope in the CSIRO Division of Industrial Chemistry at Fishermen's Bend in Melbourne (Rasmussen 1999). There were, of course, no well-established specimen preparation methods in those days. Electron microscopists interested in plant cell walls were fortunate in not being dependent on the emergence of methods to preserve delicate cytoplasmic structures or to cut ultra-thin sections to see fine structure. Cell walls and cellulose in particular were very durable in the face of mechanical and chemical challenges, allowing electron microscopists to use shadowing and replicas to image cellulose microfibrils at high resolution on the inner and outer surfaces of walls. (Similar methods were being used by the Melbourne group to study wool fibres and viruses; Rasmussen 1999.) Goodchild and Dowell (Goodchild and Dowell 1986) attribute Wardrop's introduction to metal shadowing to his time in Leeds where Preston, using similar methods, had published striking images of the large microfibrils in Valonia ventriculosa (Preston et al. 1948). This conclusion presumably comes directly from Wardrop whom they credit for 'reminiscences' in their acknowledgements. Rasmussen emphasises the role of visits to laboratories that A. L. G. Rees made on his way to Melbourne from England but notes that success finally came after three years 'when the group reversed standard replica procedures by first shadowing the wood, and then removing the surface layer of metal by flowing a layer of replica plastic over the shadowed surface and peeling it'. These replica and shadowing methods, with some refinement, continue in use today along with, of course, many methods involving sectioning that reveal the wall's full thickness as well as its surfaces. Some images provided by Hodge and Wardrop in 1950 would not look out of place in contemporary papers.
The papers of Hodge and Wardrop imaged the inner surface of tracheids from Pseudotsuga taxifolia where microfibrils were inclined at 80° to the cell's long axis. It is striking that Wardrop already considered that they had a clear view of tracheid wall structure from his Leeds birefringence data and that these results reinforced and confirmed that view in more direct fashion. The first paragraph of the Nature paper cites two papers of Wardrop and Preston (1947 and in press, presumably 1949) showing 'the walls of these cells to be composed of three coaxial micellar spirals such that the inner and outer spirals are relatively flat … while the central spiral is steep …'. (These are the layers now referred to as the S1, S2 and S3 layers, outside to inside.) In similar vein, the full paper's summary states that their work 'provides confirmation of the type of cell wall organization of conifer tracheids proposed in other investigations on the basis of X-ray and optical evidence and of microscopic examination'. Striking images of the walls of cambial cells showed much less strongly oriented microfibrils, again in keeping with the X-ray and optical results of the 1949 study by Preston and Wardrop. The 5 to 10 nm diameter of the fibrous structures in the cambial cell walls suggested to them that the 'microfibrils may correspond to the “micelles” or crystalline regions inferred from X-ray examination'. From 1951, the Division of Forest Products had its own electron microscope and so too did the company Australian Paper Manufacturers (Goodchild and Dowell 1986). This concentration of resources in the wood fibre area presumably testifies to the high hopes held that important insights would emerge to provide a better understanding of wood fibres and their utilization.
Wardrop's resumed collaboration with Dadswell led to insights regarding the inner and outer wall surfaces. They rejected the concept of a tertiary wall at the tracheid's inner surface as resulting from misinterpretation of cytoplasmic debris and, having traced the balloon-like swellings seen in cuprammonium hydroxide to the middle layer of the secondary wall being intermittently constricted by the outer layer, rejected the concept of a 'skin substance'. Wardrop summarized his views on the organization and properties of the outer layer of the secondary wall in conifer tracheids in 1957. Two systems of prominent striations were visible with bright-field illumination and electron microscopy showed that they consisted of two grid systems of microfibril bundles. A fine grid of bundles each about 60 nm wide lies adjacent to the primary wall, the two sets of bundles intersecting at about 80°. This is overlain by a coarse grid of microfibril bundles 200-300 nm wide, arranged in the same orientation. Taken together, the Leeds and CSIR studies settled many fundamental questions regarding the arrangement of microfibrils in secondary walls.
Wardrop also investigated the micellar structure of cellulose and related topics. Work in Leeds led to publications in 1948 (with Preston as first author) that reported crystallinity in never-dried cell walls – so disposing of the possibility that drying caused crystallization – and in 1949 that showed that micelles in primary walls are smaller than those in secondary walls. In 1951, after returning to CSIRO, Wardrop and D. H. Foster used X-ray diffraction to study the dependence of cellulose crystallinity on the degree of acid hydrolysis of wood and cotton. They suggested that regions of lower lateral order were attacked first during hydrolysis. The results were consistent with the view that the mesomorphic region surrounding the micelles was more extensive in wood than in cotton. Wardrop also studied the intermicellar spaces in cellulosic fibres by treating them with gold chloride prior to electron microscopic examination. With delignified flax, the crystal aggregates that formed between adjacent microfibrils were 7-10 nm wide, in fair agreement with conclusions from X-ray data on the spacing between micelles.
The process of lignification was another major theme in Wardrop's research. From the mid-1950s onwards, he published about a dozen papers on this subject, the last in 1981; and indeed he was still exploring new experimental systems in his final period of sabbatical leave in 1985. A note in Nature in 1956, with E. Scaife, reported the occurrence of peroxidase (a putative supplier of oxidant for lignin formation) in the tension wood of angiosperms, and a detailed paper by Wardrop in Tappi Journal in 1957 described the lignification phase in the differentiation of wood fibres in terms of the physical texture and sub-microscopic organization of the cell wall. He found that lignin occurred mainly between the cellulose microfibrils but that it can penetrate them to some extent, and that lignin is concentrated in the region of the middle lamella and primary wall. Lignification began in the primary wall at the cell corners and then extended to the middle lamella and primary wall. Wardrop and Scaife argued that at least one peroxidase-controlled phase of lignification proceeds within the cell wall and concluded that lignin precursors originate within individual cells at a particular stage of their differentiation. Wardrop and D. E. Bland showed, from ultraviolet absorption spectra of the lignin found at various stages of differentiation, that a higher proportion of conjugated units is incorporated during the initial phases of polymerization.
In addition to studying the course of lignification in wood, Wardrop and G. W. Davies studied lignification in model systems where eugenol, a lignin precursor, was used to increase lignification. In Avena coleoptile sections and internodes of Elodea densa, eugenol-induced lignification took place in the cell walls and the product was deposited between the microfibrils as with natural lignification. Finding that the treatment killed cell protoplasts, they suggested that the reaction depended on cell wall enzymes. In the woody stems of Pinus radiata, Eucalyptus regnans and Tilia americana, ultraviolet-detected artificial lignification occurred in the xylem, phloem and cambial zone although the products differed in their staining reactions.
Wardrop and J. Cronshaw (a recent graduate from Preston's laboratory) described the formation of phenolic substances in ray parenchyma of Eucalyptus elaeophora. They were enclosed in vesicles within a structure resembling a chloroplast.
Wood is not, of course, a material with constant structure, and the morphology and chemistry of reaction wood received Wardrop's attention, initially in co-operation with Dadswell and later with G. W. Davies and G. Scurfield. These are the types of wood formed under the mechanical stresses acting on tree branches that are angled away from the vertical. They described the variation in cell wall organization between compression wood formed on the lower side of softwood branches and tension wood formed on the upper side of hardwood branches. Staining techniques and ultraviolet microscopy revealed that compression wood tracheids were highly lignified, while tension wood tracheids were virtually unlignified. They identified three types of tension wood in which the number of secondary wall layers varied along with the degree of lignification. They suggested that lack of lignification may be the first stage in tension wood formation. Microscopic compression failures are common in tension wood and can be related to the phenomenon of 'brittle heart'. Tension wood is associated with the extreme 'wooliness' observed when hardwoods are sawn in the direction of the grain. Other manifestations include high longitudinal shrinkage and a remarkable tendency to collapse when dried from the green condition.
From early in his career, Wardrop took a keen interest in the process of plant growth as it contributed to both the elongation of stems and roots (primary growth) and the increase in girth of woody stems (secondary growth), the process generating the cells that go on to form thick, lignified secondary walls. His PhD supervisor, Preston, had investigated the relationship between cell dimensions and cell wall organization since 1934 and in their 1950 paper, Wardrop and Preston derived a relationship between micellar angle and the rate of growth of cambial initials. In a 1953 paper with Dadswell, Wardrop described the development of the conifer tracheid, including the nature of cell division in the cambium and the dimensional changes involved in tracheid differentiation. Differentiating tracheids increase in length at their tips where only the primary wall is present. Secondary wall formation commences before the dimensional changes of differentiation are complete. This is consistent with the observed increase in the number of turns of the micellar helix with increasing cell length. They suggested that the cytoplasmic surface governed the helical orientation of micelles in the secondary wall. In a subsequent paper they pointed to the correlation between the size of fibrils seen by electron microscopy and the size of micelles as determined by X-ray methods. They also noted the relationship between the degree of cell wall lignification and the grinding quality of eucalypt woods used for groundwood pulp production.
In the mid-1950s, Wardrop investigated the mechanism of surface growth in the parenchyma of Avena coleoptiles incubated in vitro with 14C-glucose. From electron microscope observations, A. F. Frey-Wyssling and K. Muhlethaler in Zürich favoured tip growth occurring in these cells. With autoradiography, Wardrop found a uniform distribution of 14C-labelled cellulose over the cell surface. Moreover, by using pit-fields as markers to estimate extension in different parts of the cell directly, he was able to show that growth was also uniformly distributed. Electron micrographs indicated a multi-net type of growth in the coleoptile parenchyma as proposed by Roelofsen and Houwink (Roelofsen and Houwink 1953) in which microfibrils deposited transverse to the major axis of growth were passively reorientated (strain realignment) towards the axis of growth. As a result, the wall's inside surface showed transverse microfibrils whereas its external surface showed microfibrils that had been reorientated towards the longitudinal. In an experiment where longitudinal growth was inhibited by colchicine, an alkaloid from the autumn crocus Colchicum autumnale, Wardrop showed that the microfibrils on the outer surface did not develop the longitudinal orientation they showed after normal growth, so confirming an important prediction of the multi-net theory.
Wardrop in 1959 also studied root hairs that, with clear evidence that growth was highly localized at the tip, he saw as a useful model for interpreting experiments on cells such as tracheids where growth localization was less clearly established. Roelofsen had suggested that microfibrils seen on the wall's inner surface (and which in this case were parallel to the cell's long axis) represented a secondary wall. Using various pulse-chase experiments with labelled glucose and providing simple diagrams predicting the distribution of label with and without secondary wall deposition, Wardrop's autoradiographs elegantly showed that secondary wall deposition probably began immediately growth stopped at a site just behind the tip and continued along the cell's whole length. He contrasted the results with what happened in tracheids that showed tip growth but with a more diffuse growth zone.
Although most of Wardrop's research was of a fundamental character, he was also interested in the influence of fibre and tracheid structure on the properties and utilization of wood and paper. About three-quarters of the publications devoted to these practical ends are in collaboration with Dadswell, to whom considerable credit must be given for leading Wardrop in this direction. Several joint papers were with G. W. Davies and other collaborators were A. J. Watson, C. F. James, W. E. Cohen and F. Addo-Ashong.
As early as 1951, Wardrop and Dadswell discussed the relationship between cell wall structure and fibre properties and stressed the importance in pulp and paper research of changes to the fibre surface before and during their bonding to form the paper sheet. The pulping processes – which convert wood into free fibres suitable for papermaking – received considerable attention, particularly the so-called semi-chemical processes using a combination of chemical treatments and mechanical energy. The structural changes occurring during preparation of cold soda pulps from both eucalypts and pines showed that the mechanical action breaks the cell wall external to and including the outer layer of the secondary wall. This exposed the lightly lignified middle layer of the secondary wall as a potential bonding surface between the fibres. Hardwoods provide superior cold soda pulps to softwoods, and they reached the important conclusion that this was related to hardwoods having less lignification of the intercellular layer. These studies were extended to several pulp types (Asplund, Masonite, neutral sulphite and groundwood). In processes that required high temperatures, fibres separated in the middle lamella, whereas they separated within the cell wall itself during low temperature processes. Others later related these observations to the glass-transition temperature of lignin.
Beating is an essential prerequisite to papermaking and is notoriously inefficient in energy terms, prompting much research to identify the essential changes that occur with a view to improving efficiency. With C. F. James, Wardrop studied the structural changes occurring during beating of wood pulp fibres. Using shadow casting in conjunction with the optical microscope, they distinguished the primary wall and the outer and middle layers of the secondary wall of fibres and tracheids. At various stages in the beating of eucalypt kraft pulp, they observed changes in the amount of primary wall debris and – in sheets formed from the pulp – in the number of primary wall inter-fibre connections, in the damage to the vessel members and in the occurrence of fibre splitting. They found that in pine kraft pulp, the outer layer of the secondary wall, as well as the primary wall, was removed, facilitating the formation of inter-fibre connections during papermaking.
Wardrop and Davies also studied the morphological factors related to the penetration of liquids into wood – an important issue in chemical pulping and areas such as wood preservation. Penetration in Pinus radiata proceeded from one tracheid to the next via the bordered pits, with lateral spread through the rays and resin canals facilitating longitudinal penetration. In Eucalyptus regnans, penetration was through the vessels and then to adjacent fibres and rays. Tyloses blocked some vessels in the heartwood. After entering the cell lumen, reagents diffused centrifugally through the cell wall, so that the middle lamella and especially the cell corners were the last regions to react.
Wardrop and F. Addo-Ashong also discussed the anatomy and fine structure of wood in relation to its mechanical failure, but concluded that knowledge of the mechanical properties of the cell wall and middle lamella did not permit accurate assessment of their contribution to the strength of wood. However they could deduce that the structure of the complex comprising the middle lamella, the primary wall and the S1 (first deposited) layer of the secondary wall exerted considerable influence on such properties. Wardrop also studied the variation of breaking load in tension of the xylem of conifer stems. Tangential longitudinal sections taken from successive growth rings showed an increase in breaking load with distance from the stem centre, accompanied by an increase in tracheid length, basic density (a measure of the concentration of dry matter in the wood) and cellulose content, and a decrease in micellar spiral angle. They concluded that cell wall organization and basic density governed the breaking load of the wood sections. They attributed the increased breaking load on drying to changes in the intercellular layer.
Given these and other findings, Dadswell and Wardrop also helped guide foresters and others involved in tree breeding to recognising those wood properties required for specific end uses, such as the production of pulpwood for the paper industry. As early as 1959 they recognised the importance for papermaking of a high proportion of thin-walled cells, with a low extractives content and a certain proportion of latewood. As these are subject to genetic control, they pointed out that it should be possible to develop suitable trees by selective breeding. As discussed later, these remain important goals over forty years later, and data Wardrop painstakingly collected and analysed in the 1940s and 1950s is now collected in seconds using SilviScan technology.
In 1964, Alan Wardrop left CSIRO to take up the Chair of Botany at the University of Tasmania on 1 September. He succeeded Professor Newton Barber FRS, an expert in plant cytogenetics and ecological genetics of eucalypts. Asked about the move when interviewed some thirty years later, Wardrop found it hard to give a specific reason but mentioned a wish to teach and changes going on in CSIRO. He mentioned that he had little sympathy with the organization's discussions about what constituted pure and what applied research. Things moved quickly in Hobart, where he secured a large grant to purchase a Siemens electron microscope and enjoyed a full introduction to teaching by lecturing to students in each of the three years of the BSc course.
Unfortunately, his younger daughter's illness meant she could not be moved to Hobart away from Melbourne's medical facilities. In late October 1965, Wardrop wrote to the Registrar at the newly created La Trobe University enquiring about that university's Foundation Chairs in Biological Sciences and was told (in a letter of 8 November) that the selection committee might consider a late application, should he wish to apply. His application was forwarded to the committee and Wardrop was offered the post on 29 November. In announcing his appointment in a press release and in a personal letter to Professor Keith Isles (Vice-Chancellor of the University of Tasmania), Dr David Myers (La Trobe's Vice-Chancellor) was at pains to stress that Wardrop's departure from Tasmania was for personal reasons and did not reflect adversely on the University of Tasmania. The latter agreed to release him and he joined La Trobe on 15 January 1966.
Wardrop's appointment to the Foundation Chair in Biological Sciences came very early in La Trobe's history. The first meeting of the Interim Council had been held only a year before (on 19 December 1964) and Wardrop, together with three other newly appointed professors, was elected by staff to join the Interim Council in 1966. The following year, the first year that undergraduates were admitted, he was appointed first Dean of the School of Biological Sciences, one of four schools then in existence. Wardrop's vision as Dean was that organisms – be they plant, animal, fungal or microbial – had many unifying principles and that these should emerge in the School's teaching and research. He was very disappointed when separate Departments were formed within the School.
The new university provided remarkable opportunities to shape a substantial department both in personnel and in facilities. He appointed staff in the areas of plant anatomy (Ian Staff), algal physiology (Dilwyn Griffiths), mycology (Alan Griffiths) and ecology (Bob Parsons). Later he appointed a biochemist (John Anderson), a biophysicist (Charles Pallaghy) and a chemo-taxonomist (Trevor Whiffin). In the mid-1970s, the two Griffiths took chairs in Townsville and Hong Kong respectively, allowing appointment of a cell biologist (Richard Williamson) and a plant pathologist (Philip Keane); a little later, he was also able to appoint a phycologist (Bill Woelkerling). While providing wide coverage of plant science in teaching, these appointments also provided a strong core of researchers interested in cellular processes that was further strengthened by the establishment of a Biochemistry Department unusually strong in plant research and, in particular, in the chemistry of cell wall polysaccharides. While supporting this direction for the new department, Wardrop took care to ensure that first-year teaching, including its biochemistry components, remained under the Botany Department's control – along with, of course, the associated resources! This began when candidates being interviewed for the Biochemistry Chair were presented with course summaries and questioned on their attitudes to first-year teaching.
Publications reflecting work done at La Trobe began emerging in 1968 using a newly acquired electron microscope (Siemens Elmiskop 1A), an ultramicrotome, a well-equipped Zeiss Photomicroscope with microphotometer, and good Melbourne connections that provided access to a scanning electron microscope at the Defence Research Laboratories at Maribyrnong. A venerable X-ray machine was acquired from Wardrop's old CSIRO Division and a convenient site for glasshouses and plant growth cabinets developed. A Balzer's freeze-etch apparatus was also acquired with contributions from three Australian paper companies.
In teaching, Wardrop always began the first-year biology course with cell biology lectures emphasizing the unity of organisms at that organizational level. He was also determined to present something of the history behind current ideas, rather than just presenting these in isolation. His quiet, rather shy demeanour perhaps counted against him as lecturer to large, first-year audiences, although his audience's attention was reputedly held as they watched him being progressively 'reeled in' as his wanderings around the stage wrapped the microphone cable ever more times around the lectern! On one occasion his meanderings led him to fall backwards off the platform, fortunately without physical harm. Teaching styles were under discussion. While perhaps somewhat sceptical, he supported Ian Staff's efforts to develop audiovisual teaching methods for plant anatomy, using slides and audio tapes in the pre-computer 1970s.
The atmosphere in the Department under Wardrop's leadership was non-confrontational. A bottle of wine often assisted staff meetings and invitations to staff for lunch on or off campus kept busy colleagues in easy contact with each other. He owned an impressive-looking brief case from which, rather than agenda papers, bottles of wine would emerge, having been cosseted by the case's carefully moulded inserts. Meetings did not, however, shirk thorny issues and 'academic standards' were frequently discussed, sometimes with quite heated exchanges. He readily took responsibility for the Department's standards and explained to new staff members that, although La Trobe's first-year student intake might not be very well credentialled because students valued proven institutional reputations, graduates (BSc or PhD) should stand both national and international comparisons. His study leave reports always remarked on teaching in the universities he visited, including entry standards, degree of specialization permitted, and so on.
Wardrop was unsympathetic to reforms suggesting the uncoupling of professorial rank from departmental chairmanship and so remained head, although absences on sabbatical leave and through ill-health left John Anderson acting ably in the position for extended periods. Wardrop had strong views on the responsibility of academics to undertake research and, in particular, favoured pressing Demonstrators (mainly responsible for organizing practical classes) to do research if they were to have continuing appointments. An issue that surfaced intermittently was whether to rename the department 'Plant Science' or some other term more fashionable than Botany. It never was.
Academic life as Chairman of the Department of Botany, and on occasion Dean, brought with it a high administrative load and in this Wardrop was something of a procrastinator. As the administrative load grew, teaching became more of a chore because, as he put it, 'You dared not take your eye off what money was coming in, and that sort of thing'.
Wardrop and the other professors of biological sciences faced unwanted publicity in the pages of Nature and elsewhere. Charles Pallaghy, senior lecturer in the Botany Department, espoused creationist views. The School prohibited Pallaghy from using the curriculum, or departmental facilities or materials, to promote his ideas. Coverage of Pallaghy's views made their way from Melbourne's Age newspaper to Nature's correspondence section (Pallaghy 1985, 1986). In the first letter, Pallaghy asserted that one reason for the scarcity of scientifically well-credentialled creationists was that 'creationists are threatened by their own colleagues and by legal action by their own school boards and institutions if they do not keep a strictly low profile either publicly or in their own class rooms'. A letter in Nature in 1986, signed by all professors in the School of Biological Sciences, was the final public offering in the controversy. It affirmed the 'right of a tenured academic to espouse whatever views he believes worthy of promulgation, through whatever extracurricular forum he is able to capture'. It went on, however, to dissociate the professors and other staff from creationist views and to affirm that they remained 'totally committed to rational, scientific explanation of biological and all other natural phenomena'.
Wardrop served several times as Acting Vice-Chancellor, including in 1973 when the jailing of La Trobe students for trespass or damaging the US Consulate in Melbourne led to violent demonstrations on campus. Against the advice of administrators, Wardrop initiated a dialogue with the students. At the meeting, he was interrupted by a staff member who had experienced totalitarianism in Nazi Germany and spoke about the evils of totalitarianism of any kind. Wardrop turned the situation around by directing the staff member to sit down and shut up. He went on to tell the meeting that he had been to see the jailed students and proceeded to explain what the University stood for. He won applause from students, staff and the administration and the violence receded.
Wardrop also served on the Council of the Royal Society of Victoria from 1972 to 1982 and the Council of the Victorian Institute of Marine Science from 1977 to 1982. Membership of the School Biology Committee of the Australian Academy of Science (1976-1986) brought involvement with production of the Web of Life textbook. He was also a member (1979-1981) of the Academy's Plant Sciences Section.
Clearly, there were considerable calls on Wardrop's time and energy in establishing a school and department, plus a requirement to assemble research equipment. Nonetheless, beginning with papers in 1968, Wardrop continued his interests in cell wall ultrastructure, publishing in his own right and with some of the department's first visitors and PhD students. These included S. M. Jutte (Nijmegen), B. W. Thair (Commonwealth Exchange student), S. C. Chafe (Forest Product Laboratory, Ottawa), L. R. Jarvis and E. F. Schneider (Ottawa). They used the recently developed freeze etching technique to study, among other topics, the mechanism governing the formation and orientation of microfibrils, nuclear pore structure and the microfibrils in the test of an animal making cellulose – the ascidian, Pyura stolonifera.
An essential contributor to the success of this research and that at CSIRO was Wardrop's senior technician, Fred Daniels. He followed Wardrop to La Trobe and, as well as scrupulously maintaining the electron microscopy facilities, carried out much research and taught electron microscope techniques to students (undergraduate and graduate), staff and visitors. It could be a chilling experience to walk to Fred's office to break the news that your carelessness had led to a malfunction that Fred would have to fix.
In the final decade of Wardrop's career, his research increasingly depended on sabbatical leave. A stay in 1978 with Professor M. M. A. Sassen in the Department of Botany, Nijmegen University, allowed him to renew his long-standing interest in microfibril orientation in elongating plant cells. Sabbatical leave in 1986, with R. Malcolm Brown Jr in the Department of Botany, University of Texas at Austin, furthered his interest in the plasma membrane structures synthesising cellulose microfibrils.
Microfibril alignment in the cell wall was a recurrent theme in Wardrop's work at CSIRO. At that time, few techniques were available to look for cytoplasmic structures that might align the microfibrils. That had changed by the late 1960s and important conceptual advances regarding the orientation mechanisms had occurred. Wardrop's work at La Trobe concerned two major issues: first, determining the orientation of newly synthesized microfibrils and identifying structures that aligned them, and secondly, determining what changes in alignment the microfibrils undergo as cells grow and new synthesis displaces them towards the wall's outer face. Both topics remain live issues (Baskin 2005).
In 1970, Chafe and Wardrop used the freeze-etch apparatus to fracture the plasma membrane of celery collenchyma and examine its particles (membrane proteins embedded in the lipid bilayer). They were looking for structural features that might orientate microfibrils and so chose a polylamellate wall in which microfibril alignment changed with the deposition of successive layers. The dominant view developed during the 1960s was that microtubules on the cytoplasmic face of the plasma membrane controlled the alignment of microfibrils growing on the membrane's outer face (Newcomb 1969). Preston (Preston 1964), however, argued that particles in the plasma membrane were decisive (see also Wardrop's comments in the Preston Festschrift of 1983). Chafe and Wardrop's statistical analyses did not reveal ordering of the plasma membrane particles that would support a role in aligning microfibrils. In contrast, they found that many microtubules did coalign with microfibrils and, given the regular changes in microfibril orientation seen in collenchyma, they could rationalize non-coalignment as the microtubule orientation changing first to set up the next shift in microfibril orientation.
There is an interesting sidelight to the microtubule-microfibril story. Part of the supporting evidence was that colchicine depolymerized the microtubules and often changed microfibril arrangement on the wall's inner surface. Wardrop had used colchicine in 1956 to inhibit oat coleoptile and onion root elongation and show that microfibrils on the wall's outer surface remained more nearly transverse than in untreated cells. Tantalizingly, he did not mention microfibrils on the wall's inner face where, over thirty years later, Iwata and Hogetsu (1989) found some changes in microfibril orientation after depolymerizing microtubules in oat coleoptile cells. Any changes in microfibril orientation on the inner face seen in 1956 would, of course, have been very hard to rationalize since the discovery of the relevant microtubules in plant cells and a demonstration that they were colchicine's target was still several years in the future.
Between May and November 1975, Wardrop made his first extended overseas trip since joining La Trobe. At the 12th International Botanical Congress in Leningrad, he discussed the possibility of sabbatical leave with Professor M. M. A. Sassen of the Botanisch Laboratorium at Nijmegen, whom he had met in 1974 at the 8th International Congress on Electron Microscopy held in Canberra. The leave went ahead in 1977.
Sassen and Wardrop found common interest in the 'multi-net growth' theory of Roelofsen (then at Delft) that had formed the backdrop to much work on primary walls (including Wardrop's) since the 1950s. In Nijmegen, Wardrop did much research on his own but was ably assisted by Mieke Wolthers-Arts whose speed and precision, he confessed to Sassen, at times challenged his ability to keep her occupied! They focused on the detailed arrangement of microfibrils in the primary cell wall and the way cell elongation reorientated them from transverse to longitudinal as envisaged in the multi-net model. Exceptions to the simple transverse-to-longitudinal gradient through the wall were known in the 1950s but subsequent work, particularly by Jean-Claude Roland in Paris, emphasized just how many primary walls were polylamellate with, for example, layers having transverse microfibrils alternated with layers having longitudinal microfibrils. These walls seemed to lie outside the multi-net paradigm of growth-induced realignment of microfibrils. Wardrop, Wolters-Arts and Sassen, using collenchyma cells, provided evidence suggesting that those microfibrils deposited in the layers of polylamellate walls with transverse microfibrils showed signs of strain realignment towards the longitudinal after elongation. This important finding pointed to multi-net realignment subtly affecting the structure of polylamellate walls and so extended the theory's reach to encompass complex walls that had seemed subject to separate rules.
Wardrop and colleagues recognised that their case was not fully watertight and Wardrop recorded a somewhat different view in his 1998 interview (Blythe 1998). There he commented that he was greatly impressed by Roland's advocacy of a helicoidal wall structure in which newly synthesized microfibrils self-organized into a structure in which microfibril alignment gradually shifts over time, a change recorded by continuous variations in alignment at different depths in the wall (Roland et al. 1987). Although his own observations on collenchyma with Sassen and Wolters-Arts still seemed to support the multi-net hypothesis, Wardrop admitted that Roland's sophisticated staining methods left him with serious doubts. By 1998, he thought the accumulating evidence suggested that the helicoid concept was pretty generally applicable to extending cells. It is probably fair to say that many remain unconvinced of the helicoid model's generality and doubt whether a single model or structural principle can describe all primary walls.
Apart from pursuing his research, Wardrop lectured on cell wall investigation for the advanced students and others working in Nijmegen. His study leave report noted that the course finished with a visit to the pulp mill near Deventer and to the Institute of Forestry and Landscape at Wageningen where, he noted, the breeding programme for hybrid poplars did not select for particular end uses in the way this was done in Australia and the USA. Wardrop also noted that his limited attempts to learn Dutch were inhibited by the excellent English in which the local shopkeepers insisted on replying to his faltering efforts in Dutch!
As if in a final flourish of his skill in deciphering complex wall structures, Wardrop described in 1983 the mechanism leading to the opening of Banksia cones after bushfires. Using all his analytical techniques – and a great deal of that 'complex mental process' to which Preston had referred in 1958 – he elegantly documented the variations in the intricate, layered walls of the sclereids found in different parts of the cone. He argued that fire melted the resin holding the different parts together, allowing the different alignments of microfibrils in the sclereids to lead to differential shrinkage and hence opening.
Wardrop's Nijmegen sabbatical led to a meeting that became known as the 'The First International Cell Wall Meeting'. Sassen and Wardrop, following discussions with interested botanists at the Leningrad Botanical Congress, resolved to organize a cell wall meeting and plans were finalized during the sabbatical. Wardrop's study leave report records its growth from an intention to invite seven or eight people for informal discussions to a meeting with thirteen invited speakers with Wardrop giving the introductory address. International harmony was not all it might have been, however, with tensions surfacing between two of the field's major figures, Preston of Leeds and Frey-Wyssling of Zürich. When both were invited, Frey-Wyssling immediately accepted, while Preston (according to Sassen) told Wardrop that he would not be coming if Frey-Wyssling was! The meeting in Nijmegen on 25 and 26 May 1978 drew sixty or so participants mainly from Germany and Holland and established an enduring series of meetings of which the eleventh is planned for Copenhagen in 2007.
In his final period of sabbatical leave, in the first half of 1985, Wardrop turned to the electron microscopy of the plasma membrane enzymes (terminal complexes) that synthesized microfibrils. He did this in Austin, Texas, with their discoverer, R. Malcolm Brown Jr. Like others using freeze-etching around 1970, Chafe and Wardrop failed to see the labile terminal complexes that Brown and colleagues described a few years later in algae (Brown and Montezinos 1976) and higher plants (Mueller et al. 1976). (Chafe and Wardrop had, like many others, probably spoilt their chances by infiltrating with glycerol to minimize freezing damage.) Their discovery by Brown's group delighted Wardrop who applauded the group's technical excellence. His introduction to Preston's Festschrift in 1983 noted an earlier occasion when arrays of membrane particles in yeast and algae had produced transient excitement at a conference before doubts emerged as early as dinner time! Wardrop's study leave report recorded how he chose the topic: 'I had some doubts as to the interpretation of the role of these structures in relation to the microfibril formation in the cell wall and felt that it was a weakness in the published evidence that they had not been demonstrated by other techniques of electron microscopy'. Working with Krystyna Kudlicka and Takao Itoh (Kyoto), he therefore took up the challenge of demonstrating them in fixed and sectioned cells of the green alga Boergesenia forbesii. They observed the linear terminal complexes in glancing sections of the plasma membrane as deeply staining structures of comparable size to the structures seen by freeze-etching. He noted that work was continuing at La Trobe and at Austin; it was published in 1987 as his final publication.
While in Austin, Wardrop enthusiastically seized the opportunity to learn from Candace Haigler how to make leaf parenchyma cells of Zinnia elegans differentiate into lignified tracheary elements. His study leave report noted that 'Over an extended period I have studied lignification in intact plants and … this system seemed ideal for a more experimental study of the problem'. In Austin, he worked on the effects of calcium and boron on lignification; the technique was later established at La Trobe but no work was published.
Noting the high cost of living in Texas, Wardrop returned a month early to La Trobe. In a final comment to the university hierarchy after his experiences in Texas, he lamented that La Trobe's students were being trained on 'obsolescent/obsolete equipment' and the trend 'to expend funds for equipment on facilities to analyse experimental results, rather than to acquire equipment of contemporary sophistication designed to acquire data to be analysed'. It was a remark entirely in keeping with an approach to research that he had always emphasized – the adoption of novel technologies to see cell walls in new ways and so further to test established theories.
After retiring in December 1986 as an Emeritus Professor of the University, Wardrop organized lunches for current and retired professors that provided convivial gatherings with speakers. He also established a connection with the Botany School at the University of Melbourne but did not strongly pursue the connection, or indeed research, after leaving La Trobe. He served on the Council of the Australian Academy of Science from 1988 to 1991 and was Chair of the Academy's Plant Sciences Section between 1987 and 1989.
One retirement activity is worth recording for the light it sheds on his own contribution in the 1950s and how this was perceived almost fifty years later. From 1992 to 2000, Wardrop served on the Scientific Advisory Board of the Co-operative Research Centre for Hardwood Fibre and Paper Science, in which CSIRO Forestry and Forest Products was a major partner. Through this connection, he was an adviser to the Centre's research programme that developed CSIRO's SilviScan-1 into SilviScan-2. Robert Evans gave a very readable account of the development of this remarkable instrument in a 2002 lecture (Evans 2002). He notes that CSIRO researchers of the 1950s such as Wardrop, Watson and Dadswell asked and in part answered many of the questions that still concerned the Division in the 1990s, questions such as 'What are the key wood fibre properties influencing the properties of pulp, paper and wood products'. SilviScan-2 uses X-ray diffraction to collect in seconds information about microfibril (micelle) angle, crystallite size, degree of crystallinity and so on. Data is collected at 50 µm intervals along a thin wood core that can be extracted from a tree being assessed as part of a breeding programme. The properties at each site are separately analyzed and displayed in striking 3-dimensional plots. These were precisely the issues that Wardrop's painstaking work in the 1940s and 1950s had dealt with. It was a technical advance that Wardrop would undoubtedly have delighted in and, some fifteen years after his final complaint that La Trobe was emphasizing data analysis at the expense of data acquisition, he would probably have been only too willing to agree with Evans that here was a development in data acquisition that left the capacity for data analysis and storage struggling to keep pace!
Alan Wardrop was undoubtedly one of the most distinguished and influential forest products scientists of his generation in both the Australian and international contexts. As a colleague he was invariably courteous and co-operative and he gave freely of his knowledge and ideas. As a Section Leader at CSIRO he provided inspiration to those working under his direction and strove to develop a spirit of co-operation. The Section of Wood and Fibre Structure that he came to lead was a most effective research unit. One measure of his influence is the number of prominent international scientists (for example, W. Liese, H. Harada, V. Cheadle, F. Addo-Ashong and J. Cronshaw) who came to CSIRO's Yarra Bank laboratories to work with him in the 1950s and 1960s. A second is Evans' recognition that the work of Wardrop and colleagues in the 1950s identified many of the features of wood fibres that are assessed fifty years later with SilviScan-2, as a central part of the Division's goals to select trees with desirable properties for particular end uses.
At La Trobe, Wardrop was the dominant influence on the Botany Department in its first twenty years and, as first Dean of the School of Biological Sciences, on the establishment of other departments within the School. He never built a large group of collaborators at La Trobe, although his enthusiasm for cellulose research perhaps contributed to one of us (R. E. W.) moving into studying mechanisms of cellulose synthesis and microfibril alignment after leaving La Trobe, a conversion that Wardrop regarded as better late than never!
Wardrop had a mastery of the indirect methods of deducing wall structure from X-rays and polarized light, and was one of the important pioneers of electron microscopy in Australia. Practically all of his contributions were profusely illustrated by elegant light and electron micrographs. These in themselves provide great enlightenment, but they are particularly powerful when supplemented by data derived by other methods and by Wardrop's sophisticated interpretations. His work on cell walls drew on several of the basic sciences in pursuit of his botanical questions. He was not interested in just describing things – he was interested in understanding their function and the detailed processes that brought them about. He used whatever techniques were available to help him arrive at an answer – and those answers are recognised as major contributions to the study of plant cell walls.
This memoir was originally published in Historical Records of Australian Science, vol.18, no.1, 2007. It was written by:
We thank Carolynn Larsen, Librarian, CSIRO, Clayton and Rosanne Walker, Basser Library, Australian Academy of Science for their expert assistance, and Professor Sassen of Nijmegen for his comments on Wardrop's 1977 sabbatical leave and the origins of the International Cell Wall Conferences. The portrait was taken at La Trobe Universityc. 1976.
Thair, B. W., Wardrop, A. B., 1976. Chloroplast development in Selaginella kraussiana (Kunze) A. Braun. (Unpublished manuscript in Basser Library, Australian Academy of Science, Canberra.)
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