THE GROWTH OF SECONDARY WOOD FIBRES

Transcription

1 . (1975) 74» ,^^,^,_ THE GROWTH OF SECONDARY WOOD FIBRES BY M. W. WENHAM* AND F. CUSICK Department of Botany, University of Aberdeen, St Machar Drive, Aberdeen, ABg 2UD {Received 22 July 1974) SUMMARY Cambial derivatives differentiating as xylem fibres characteristically increase in length by intrusive growth. Radial expansion of contiguous cells is required for this intrusive growth which, in consequence, is restricted to the expansion zone of the newly formed wood. The amount of intrusion depends primarily on the position of the tip of a fibre in relation to the lines of junction of tangential and radial walls in the surrounding cells; these lines of junction are potential sites for intrusive growth. In general, the greatest intrusion occurs where there are two potential sites for intrusion (usually one on either radial face of the fibre's tip); tips with one site show less intrusive growth and those with none show least of all. These arrangements may be modified by the configuration and activity of neighbouring cells. The hypothesis formulated is that the tip of a fibre secretes an enzyme which weakens the middle lamella between the cells adjacent to it; where this occurs, these cells, during their turgor expansion, round off from each other at the corners. The thin-walled tip of the fibre fills this space as it is created. The notion that the intruding tip pushes its way into the cellular matrix is contested. INTRODUCTION Cambial derivatives which develop into xylem fibres usually increase in length during their differentiation. This longitudinal growth of particular cells necessitates intercellular adjustments within a vertically static tissue. The nature of these adjustments has been the subject of much speculation in the past, the 'sliding growth' hypothesis (Krabbe, 1886) which postulates the movement of whole cells over each other, and the 'symplastic growth' hypothesis (Priestley, 1930) which relies on the harmonious expansion of shared partition walls, being largely superseded by the concept of intrusive growth (Sinnott and Bloch, 1939). According to this concept, growth in length is localized at the tips of a fibre (see Schoch-Bodmer, i960). Percipient observers, such as Bannan (1956), have reflected on how big, in this respect, the tip of the fibre might be since any elongation behind tbe extremity would perforce result in the movement of the extremity over the surface of the surrounding cells. While the physico-chemical aspects of these intercellular adjustments may remain obscure for some time, further anatomical investigations should throw light on the situations which favour intrusive growth and should add to our scanty knowledge of the course which it follows. The present paper reports observations, mainly on Sahx viminalis L. but also on other woods with non-storied cambia, and reopens the discussion on the mechanism of intrusive growth. * Present address: Department of Education, University of Bath. 247

2 248 M. W. WENHAM AND F. CUSICK LLJ Q O m Fig. I. Viburnum opulus. Sections through the narrow, dormant, cambial zone (thin lines) and mature xylem cells (thick lines) derived from it during the preceding growing season. The sequence extends through 210 fim, the intervals between A-B, B-C etc. being 70, 60, 60 and 20 fim. The two radial rows of cells illustrated in A are progressively separated by a third (stippled) row whose cambial initial first appears in D. Derivatives of this cell numbered 3-15 project /Um beyond this level and are variously represented in higher sections whereas no. i, at the annual ring, terminates at the same level as the cambial initial and no. 2 only 15 yum beyond this point, x 460.

3 Growth of secondary wood fibres 249 PRELIMINARY DATA FROM TRANSVERSE SECTIONS Two cambial cells and their derivatives on the xylem side,fixed during the dormant season, are illustrated in Fig. i. As the series continues, the two rows are separated from each other, first at isolated locations, then completely, by the derivatives of a third cambial cell which itself appears only in the later sections. A more detailed study of the complete series enables the following comments to be made, (i) The intruding tips have elongated independently of each other. (Exceptions to this generalization will be discussed later.) (2) The amount of intrusion undergone by different fibres is not constant, even though the fibres are derived from the same cambial initial. (3) Intrusion has taken place where one (or two) tangential walls meet the radial wall which constitutes the boundary between the two 'original' rows of cells. (4) In this sample, in which the cells have been stabilized by the deposition of secondary walls, the interfaces between the intruded tips and the displaced cells are approximately plane. overlying vascular rays (parallel stippling), x 200

4 250 M. W. WENHAM AND F. CUSICK The most obvious feature of intrusive growth the increase in length of the cells that show it tends to encourage the view that intrusion is initially an increase in length supported only subsequently by transverse expansion of the intruded tip. Transverse expansion does indeed continue in parts of the tip that have ceased to increase longitudinally, but even the initial penetration must have a transverse dimension. As penetration begins in proximity to the cambium, the initial insertion is between cells which are at the start of their phase of radial expansion. To what extent, then, is the subsequent enlargement of the intruding tip brought about symplastically with the transverse enlargement of the surrounding cells? Following the outline of a fibre back from its tip, one sees it in progressively greater contact with the surrounding cells. This does not necessarily Fig. 3. Salix viminalis. A radial group of fibres and a vessel element derived from a single fusiform cambial initial. Each pair of fibres, counting outwards from the vessel element, is presumed to have arisen by the periclinal division of a mother cell previously segmented by the cambial initial. Is it noticeable that the sister fibres of a pair often exhibit contrasting amounts of elongation, x 480. mean that the intruding tip continues to split the middle lamellae of its neighbours after the initial penetration because the more proximal the section, the longer has it been subject to symplastic expansion. Further evidence, however, demonstrates that the region of the tip recently 'left behind' as intrusion proceeds, continues to make inroads along the lamellae of contiguous cells. Thus the tips of two sister cells in a radial row of fibres may make contact with each other after a period of independent growth. This is the simplest explanation of some series of transverse sections (e.g. the contiguous intrusive tips 6, 7 and 8 in Fig. ic) and also of some groups of cells seen in macerations (Figs. 2 and 4). Similarly, the reduction in the tangential walls of the displaced rows is largely due to

5 Growth of secondary wood fibres JO 10 Fig. 4. Castanea sativa. (a) and (b) radial groupings f ^^^^^ed cells derived from two bial initials. Intrusive growth is indicated by extension beyond the cells (stippled). The lengths of fibres produced from the «^"^«siderably as do the ratios of intrusion at the opposite ends "/ has intruded equally at both ends whereas in cells 3, 6 and 9 differences in the degree of intrusive growth; cells i, 3, 7 and wards the bottom of the page, cells 4, 6, 9 and 11 more t"^^"-j ^^ cells 2 and 6 show similar, cells i, 3 and 5 disparate amounts of i and R, overlying vascular rays, x 200. two ^ js b) underlying

6 252 M. W. WENHAM AND F. CUSICK continued intrusion, though to a small extent these walls contract as the mechanical constraints on the differentiating tissue change (Priestley, 1929; Thompson, 1942). (If secondary xylem is examined from the cambium inwards, a diminution of the tangential walls is normally observed, even in the absence of intrusive growth.) Finally, it may be remarked that the appearance of the tissue while undergoing intrusive growth is also suggestive of a continuing dissolution of the middle lamella. In summary it may be said that, after the initial phase of intrusion, the subapical portion of the tip enlarges partly by continued 'splitting' of the surrounding cells (transverse intrusion) and partly by symplastic growth with the new wall contacts as these are established. In this way, the end of the fibre acquires and maintains its tapered form. FURTHER ANALYSIS OF INTRUSIVE GROWTH In approaching the problem of how fibres intrude, it is profitable to consider the circumstances associated with greater and less intrusion as this can be observed in the derivatives of a single cambial initial. An analysis of these differences includes an examination of the following factors, (i) Variation in the intrusive capacity of the individual fibres. (2) The rate of maturation of the displaced cells. (3) Variations in cellular pattern in the tissue surrounding the intrusive tip. Variation in the intrusive capacities of individual fibres It is possible that the tips of fibres mature relatively early or late, or that certain fibres elongate faster than others, according to the internal constitution of each cell. On the available evidence, largely indirect, such differences are of minor importance in the regulation of intrusive growth. Thus, in Willow, the two fibre initials which are characteristically produced by the subdivision of a fibre mother cell, and which may be assumed to be in a similar physiological situation, often exhibit contrasting degrees of intrusive activity (Fig. 3). Likewise, the upper and lower extremities of a single fibre may differ considerably in their intrusive growth (Fig. 4). Intrusive growth and the maturation of the surrounding cells The maturation of the products of cambial activity is, without doubt, an extremely complex series of events. In microscopy, the more obvious stages can be delimited as the following zones, (i) The cambial zone, consisting of the cambial initials and more or less unexpanded derivatives, all of which are very similar in appearance, (ii) The expansion zone, in which cells increase in size and become adjusted into their final shapes, (iii) The zone in which the secondary wall is deposited, (iv) The zone in which lignification of the cell wall takes place. Intrusive activity begins in the cambial zone and reaches its peak in the zone of expansion. The point at which it stops is more difficult to define, but tangential profiles of radial rows of immature fibres (Fig. 5) indicate that longitudinal growth ceases in the expansion zone and that transverse enlargement of the intruded tip proceeds more slowly, continuing for a short time after elongation has ceased. Among the possible causes of the curtailment of intrusive growth are progressive changes in the primary wall and middle lamella of the cells which so far have accommodated the expanding fibre tip; also the diminution and cessation of radial expansion in the zone. These two factors are probably linked. The second lends itself to analysis on the present data. g f.

7 Growth of secondary wood fibres 253 The enlargement of the intruding tip 'at the expense of the adjacent cells does not entail a reduction, during development, in the absolute values of either the perimeters or the cross sectional areas of these cells, because the whole zone is expanding. They simply increase less at levels where they are subject to intrusion than elsewhere. It may now be asked whether the expansion of the tissue as a whole is a prerequisite of intrusive growth. Clearly, if this expansion had stopped, any increase in the intruded tip (which could be brought about by a transfer of sap) would entail a realignment of the common walls. The fact that, at maturity, these walls are characteristically plane argues in favour of the view that, when intrusion is occurring, there is also radial growth. Further support for this view comes from the paucity or absence of intrusive growth among the last-formed I I+10 Fig. 5. Salix viminalis. Tangential profiles of a cambial initial (I) and the ten youngest cells derived from it (I + i 1 + io) obtained by measuring the tangential widths of the cells through a series of transverse sections and plotting these widths on a vertical axis (vertical scale = ^ tangential scale). Growth in length ceases before growth in width. Deposition of the secondary wall had begun in I + io, indicating that this cell had completed its surface growth. products of a season's growth, where there is little radial expansion of the cambial derivatives (Fig. 6). It is interesting to note that this zone has been moved outwards by the expansion of the preceding derivatives. It is not, therefore, radial movement with its associated changes in mechanical stress that is the causal factor in intrusive growth, but radial growth within the cells directly concerned. It is concluded that the cessation of radial growth in a zone of cells puts an end to intrusion among these cells, even if the tips of the fibres still possess the potentiality for growth. Only a limited time is therefore available for intrusion; to this extent, the maturation of the surrounding fibres (more precisely, the main bodies of these cells, since they also may have intrusive tips) regulates the extent of intrusive growth. The duration of

8 254 M. W. WENHAM AND F. CUSICK I LJ en Fig. 6. Viburnum opulus. Sections through an annual ring (position indicated by thick double lines). The sequence extends through i io ^m, the intervals between A-B, B-C etc. being 6o, 2O, 2o and io //m. At level A, the row of intrusive tips (stippled) shows a gap in the region of the annual ring which is made good at lower levels. Cell a probably over-wintered in the undifferentiated state, largely confined by the lignified fibres of the neighbouring rows; the body of the cell would be extended outwards with the new season's growth {cf. D, E) but no vertical growth has occurred. Further comment in text, x 460.

9 Growth of secondary wood fibres 255 this period may change as the season advances, but not with such irregularity as to account for the differences in length often observed in a short radial chain of fibres. Variations in cellular pattern in the tissues surrounding the intrusive tip In a non-storied cambium, the tips of the fusiform cells in a row A will commonly be interposed between the subterminal or middle portions of the fusiform cells in the rows B and C on either side. The tangential walls which have formed as a result of periclinal Fig. 7 Fi9- ^ Fig. 7. Salix viminalis. A cambial initial, I, and five derivatives seen in radial longitudinal section, x , Fig. 8. Salix viminalis. Two elongating fibres, one adjacent to a ray cell, seen in tangential longitudinal section, x divisions in rows B and C provide potential routes for intrusive elongation by the cells of row A The tangential walls in the three rows of cells coincide infrequently and only by chance, since periclinal divisions in the fusiform initials of the cambium are not synchronous, and different fusiform initials often show different rates of division. Thus it comes about that while, at each end, a fibre normally has an 'intrusive pathway or route ot intrusion' available to it, there is considerable variety in these routes and m the degree to which they can be exploited by intrusively elongating fibres.

10 256 M. W. WENHAM AND F. CUSICK Within any radial row, a fibre starts to elongate in proximity to its neighbours in the row. The size of its tip during this elongation is relatively constant; consequently, as the surrounding tissue expands radially, this part of the fibre usually becomes isolated from the corresponding parts of its sister cells, producing a characteristic comb-like appearance la) (b) (c) Fig. 9. Salix viminalis. The fibre tip during intrusion, (a) Successive levels within a single transyerse section, 14 fim in thickness (drawn from photographs), (b) and (c) Perspective drawings, based on similar series of photographs, of two other fibre tips; two views of each tip are shown, the radius of the stem being indicated by arrows which point towards the cambium. Note the strongly concave faces of the tip during intrusive growth, (a) x 2675; (b) and (c) x 5000.

11 Growth of secondary wood fibres (Figs. 2 and 4). In following an intrusive pathway, a fibre may acquire a curved or irregular form (Fig. 4a). Bannan (1956) regards the growth of conifer tracheids as following the line of least resistance between the surrounding cells. Profiles and sections of growing fibres show that the intrusively elongating tip terminates in a pyramid or a radially aligned ridge below which it has three to six concave faces, corresponding to the number of contiguous cells (Figs. 7, 8 and 9). The concavity (a) (b) Fig. 10. Salix viminalis. (a) and (b), transverse sections through the ends of mature fibres, (a) Illustrates the observation that the ultimate length of a fibre depends on the rate at which its tip can gain space by intrusive activity; as is usual, the fibre tip (B) which can intrude between two tangential walls is longest at maturity and that which has no tangential wall to intrude (A) is the shortest; C and D, each with one tangential wall abutting, are intermediate in length, (b) Shows that competition for space can restrict intrusive growth. Because the routes of intrusion x and y are very close to the common tangential wall B/C, neither B nor C was able to exploit them fully; in consequence, both B and C are shorter at maturity than A and D. x 750. of the wall diminishes during the radial expansion of the zone and in the final state, as already noted, the walls are plane. The majority of fibres, though not all, elongate rapidly at the start of their surface growth; later, some grow much farther than others. Fibres which add little to their original lengths are apparently those which are unable to elongate rapidly during the early phase of expansion of their zone. In all cases, the degree of elongation seems to

12 258 M. W. WENHAM AND F. CUSICK depend on the type of intrusive pathway available to the tip of the fibre and on the extent to which a potential route can be exploited; this, in turn, depends on the number of tangential walls in contact with the tip and their positions relative to one another. The critical factor is probably the rate at which the tip can make space for itself by intrusive activity. This principle is illustrated in Fig. 10. Fibre tips which can intrude between two tangential walls gain space most rapidly; those which can intrude only one tangential wall are less successful and consequently show less growth in length. A tip which has no tangential wall adjacent to it cannot intrude unless vigorous intrusive growth by an adjacent fibre introduces a new line of junction which can serve as an intrusive pathway; the initial delay, however, prohibits a substantial increment (Fig. 6, cell 7). The ability to use a potential route of intrusion may be reduced by competition between sister cells. Fig. iob illustrates an example in which the ends of two fibres B and C were bounded by their neighbours A and D in the radial row. The routes of intrusion x and y were so close to the common tangential wall B/C that neither cell was able to gain space rapidly during the early stages of expansion of the zone; as a result, fibres B and C were much shorter than A and D when fully grown. In view of the evidence presented above, it can be proposed as a working hypothesis that the course and extent of intrusive growth in the developing wood fibre is determined principally by the spatial relationships of the fibre with the cells that surround it. DISCUSSION The radial expansion of developing wood is resisted by the tissues to the outside, which become stretched and eventually ruptured as the girth of the stem increases. This resistance is overcome by the high turgor of the thin-walled tissues of the cambial zone and the expansion zone. When a cell is turgid, the cell wall is under tension and, in an expanding tissue, the primary wall responds to this stress by elastic yield or 'creep', causing symplastic growth to take place (Preston, 1964). The pattern of walls and the relative sizes of cells that are seen in transverse sections of the cambial zone are maintained, apart from adjustments due to intrusive growth, throughout the expansion phase. The fully expanded fibres of any radial row are normally remarkably uniform in diameter and, since symplastic expansion is a response to turgor stress, this indicates that fibres undergo very similar patterns of turgor stress as they develop. The major response to turgor is radial and at right angles to the long axis of the fibre. As a part of the secondary body of the plant, wood fibres cannot expand longitudinally by symplastic growth; their behaviour is probably related to the mechanical anisotropy of cells referred to by Preston (1964). Ultrastructural studies of developing fibres have not yet produced a definitive picture of the structure of expanding fibre walls (see Wardrop, 1961, 1964) but it is clear that the major orientation of the microfibrils follows the long axis of the cell, this arrangement being modified by 'multinet growth' (Roelofsen and Houwink, 1951, 1953) as expansion proceeds. Priestley (1929), in his account of the development of parenchyma, points out that, in the vacuolation phase, the internal, hydrostatic pressure directed against the elastic cellulose wall brings about a rounding off of the originally angular corners of the cells, the pectin 'cement' of the middle lamella yielding to the tension thus generated to produce intercellular spaces. Similar forces might be expected in the expansion zone of secondary xylem, but the situation differs from that described by Priestley in the following

13 Growth of secondary wood fibres 259 respects, (i) The growth of wood is essentially radial, whereas primary parenchyma usually expands in three directions simultaneously, (ii) Developing wood expands against a considerable resistance from the outer tissues of the stem, whereas the early maturing primary parenchyma expands under much less mechanical constraint, (iii) During development, secondary xylem fibres undergo proportionately less expansion than primary parenchyma. In view of these differences, it is not surprising that in developing wood schizogenous intercellular spaces are normally absent or very small. Such spaces are, however, a characteristic feature of the compression wood of conifers and have also been recorded for the normal wood of a few species (Jane, 1956). Although they are not a marked feature of hardwoods, it seems likely that schizogenous spaces between fibres could be produced by a chemically induced weakening of the middle lamella. Most accounts of intrusive elongation by wood fibres incorporate, implicitly or explicitly, the idea that the advancing tip is pushed forward, but it seems unlikely that this is so. There is no convincing evidence of differences of hydrostatic pressure among intruding fibres; indeed, the fact that cells which at one level are displaced by intrusive growth are themselves intruding at another level, indicates that such pressures are uniform among expanding fibres. It might be thought that the terminal extension of fibres is an automatic adjustment to a local reduction in wall pressure, comparable to the emergence of a root hair. The root hair, however, only protrudes from the surface of the root because its turgor pressure is greater than the resistance offered by the soil. The turgor pressure of the fibre tip, on the other hand, does not, apparently, exceed that of the surrounding cells, a point substantiated by the markedly concave surfaces of the tip during intrusion (Fig. 9a). Furthermore, the diminution of intrusion in the summer wood bordering the annual ring need not occur if local modifications of wall pressure initiated the process of intrusion. A more acceptable hypothesis, based on the evidence presented and discussed above, is that intrusive growth is initiated by a weakening of the middle lamella between the cells adjacent to the intruding fibre tip. It is possible that the modification consists of a partial and reversible solation of the pectic gel which makes up the middle lamella, effected by enzymes secreted by the tip of a differentiating fibre. Changes in the middle lamella in front of advancing cell tips have been reported (Esau, 1965) and the swollen appearance of the cell walls at junctions with the fibre tips shown in Fig. 9a may be interpreted in this way. A weakening of the middle lamella at the junction of three or more fibres, bringing about a situation comparable to Priestley's parenchyma, would cause their angular corners to round off as the cells expanded radially. Simultaneously, it is suggested, the tip of the fibre which has brought about this change and which is turgid and notably thin-walled, extends to occupy the space made available by this moving apart of the fibres around it. A continuation of this process would account for the continued lateral intrusion of the now subterminal portion of the fibre, immediately behind the advancing tip. The mechanism of intrusive growth postulated above accounts for all the observed features of the process. It clearly requires that there should be a continuous and relatively rapid synthesis and extension of the cell wall at the intruding fibre tip. Preston (1964) has proposed a model of cell wall synthesis which has been further investigated and discussed by Preston and Coodman (1968). If this model is essentially correct, the extension of the cell wall at the sites of intrusive growth involves the continuous synthesis of microfibrils, coupled with a multiplication of the microfibril-synthesizing units as the cell membrane is extended.

14 26o M. W. WENHAM AND F. CUSICK Intrusive growth of wood fibres is also dependent upon the radial expansion of the developing xylem as a whole. If the tissue were not growing, the cells adjacent to the tips of the fibres could not move apart if the middle lamella were weakened because to do so in such circumstances would entail a decrease in their volumes. The space occupied by the intruding fibre tip is available only because the partially displaced fibres around it expand (symplastically) to a lesser degree that they would otherwise do. Variation in fibre length has been the subject of many studies (Dinwoodie, 1961), but the present paper appears to contain the first analysis of the factors leading to variation in the intrusive growth of fibres produced by the same cambial initial. In Picea, successive tracheids derived from the same cambial initial were found by Vasiljevic (1955) to increase in length through the annual ring and decrease towards its end, but the mechanism underlying these changes was not resolved. Most studies of softwoods and hardwoods employ maceration techniques which destroy the cellular interrelationships on which a clarification of the process of intrusion depends. In non-storied hardwoods, the variation in the lengths of fibres refiects, to a large extent, the wide variation in the lengths of the fusiform cambial cells. A coefficient of intrusion the ratio, within a sample, of mean fibre length to mean vessel length has been used by Hejnowicz and Hejnowicz (1958) to demonstrate seasonal changes in the amount of intrusive growth but is not designed to reveal how intrusive growth takes place. In storied woods, all the fusiform cells in the cambium are of similar length; consequently, variation in fibre length is almost wholly due to variations in intrusive growth. Hejnowicz and Hejnowicz (1959) have demonstrated seasonal fiuctuations in intrusive growth in the storied wood of Robinia; intrusion increases from a low level in the spring wood to a maximum which is maintained through most of the summer wood and declines only in the outermost layers of the season's increment. In the present paper, it has been suggested that the absence of radial expansion at the close of the growing season precludes intrusive growth; conversely, the greater the amount of radial expansion occurring in any particular zone within the seasonal increment, the more intrusive growth is to be expected in this zone. A further, localized factor which reduces the intrusive activity of developing fibres in angiospermous woods is the expansion of vessel elements. Spring wood, therefore, if it contains a higher frequency of relatively large vessels, provides a less favourable environment for intrusive growth than does summer wood. This interaction of differentiating vessels and fibres will be examined in a later paper. ACKNOWLEDGMENTS M. W. Wenham expresses his gratitude to the Science Research Council for the award of a Research Studentship which made possible a detailed study of wood development in Salix viminalis. Both authors thank Mr Edward Middleton and Mr Ian Moir for help in photography. REFERENCES (1956). Some aspects of the elongation of fusiform cambial cells in Thuja occidentalis L Can. y. Bot., 34, 175. DINWOODIE, J. M (1961). Tracheid and fibre length in timber. A review of literature. Forestry, s ESAU, K. (1965). Plant Anatomy, 2nd edition. John Wiley, New York HEJNOWICZ, A. & HEJNOWICZ, Z. (1958). Variations of length of vessel members and fibres in the trunk of Populus tremula L. Acta Soc. Bot. Polon., 27, 131. HEJNOWICZ, A. & HEJNOWICZ, Z. (1959). Variations of length of vessel members and fibres in the trunk of Robtma pseudoacacia. Acta Soc. Bot. Polon., 28, 453. BANNAN, M. W.

15 Growth of secondary wood fibres 261 TANE, F. W. (1956). The Structure of Wood. A. & C. Black, London. KRABBE, G. (1886). Das Gleichende Wachstum bei der Gewebebildung der Gefdsspflanzen. Gebriider Bomtraeger, Berlin. PRESTON R. D. (1964). Structural and mechanical aspects of plant cell walls with particular reference to synthesis and growth. In: The Formation of Wood in Forest Trees (Ed. by M. H. Zimmermann), p Academic Press, New York. PRESTON, R. D. & GOODMAN, R. N. (1968). Structural aspects of cellulose microfibril biosynthesis. J. Roy. PRIESTLEY, J. H. (1929)- Cell growth and cell division in the shoot of the flowering plant. New Phytol., 28, PRIESTLEY, J. H. (1930). Studies in the physiology of cambial activity. II. The concept of sliding growth. New Phytol., 29, RoELOFSEN, P. A. & HouwiNK, A. L. (1951). Cell wall structure of staminal hairs of Tradescantta vtrgtmca and its relation with growth. Profo/>/asOTa, 40, I. ^,. 1 RoELOFSEN, P. A. & HouwiNK, A. L. (i953). Architecture and growth of the primary cell wall m some plant hairs and in Phycomyces sporangiophore. Acta bot. Neerl., 2, 218.,., ^ ^ _ ScHOCH-BoDMER, H. (i960). Spitzenwachstum und Tiipfelverteilung bei sekondaren Casern von bparmannia. Beih. z. Zeitschr. des Schweiz. Forstv., 30, 107.u A Air *o SiNNOTT E. W. & BLOCH, R. (1939)- Changes in intercellular relations during the growth and ditterentiation'of living plant tissues, ^mer. X 5of., 26, 625. ^ ^^ ^,., ^^.. _, THOMPSON D'ARCY W. (1942). On Growth and Form (2nd ed.). Cambridge University Press. VASTLTEVIC' S (19"?0. Duzina traheida u. granicama prstena prirasta. Glasn. sum Fak. Beograd JO, ibi. WARDROP A B (1961) The structure and organization of thickened cells walls. In: Recent Advances tn Tofa^y, International Botanical Congress. 9th. Montreal 1959, P- 74O. University of Toronto Press WARDROP A B (1964). The structure and formation of the cell wall in xylem. In: The Formation of Wood in Forest Trees (Ed. by M. H. Zimmermann), p. 87. Academic Press, New York.

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