The Mechanism of Leaf-Fall in Certain New Zealand Trees
[Read before the Otago Institute, May, 1935; received by the Editor, May 15, 1935; issued separately March, 1936.]
The Leaf-abscission Mechanism.
(a) Hoheria Lyallii, Hook. f.
(b) Aristotelia serrata, J. R. et G. Forst.
(c) Fuchsia excorticata, J. R. et G. Forst.
(d) Nothofagus species.
The Changes in the Nature of the Cell-Walls in the Separation-layer during Abscission.
The Deciduous Habit in New Zealand.
Although the flora of New Zealand is predominatingly evergreen, it contains a number of deciduous species, but these do not usually play a very conspicuous part in the vegetation of the country. In the following paper are described the leaf-abscission mechanisms of some of the most important deciduous trees—viz., Hoheria Lyallii, Hook, f.,Aristotelia serrata, J. R. et G. Forst., and Fuchsia excorticata, J. R. et G. Forst., and also certain species of the genus Nothofagus. The chemical nature of the changes which occur in the cellwalls during abscission are described in a separate section of the paper, and another section is devoted to a general consideration of the deciduous habit in New Zealand.
During this study it has become apparent that an exact interpretation of the term “deciduous” is necessary. In its generally accepted usage this term is applied to plants which shed their leaves at a regular season and are leafless for a certain part of the year. But there are trees, such as Aristotelia serrata, which, though they show a regular leaf-fall, are nevertheless frequently leafy throughout the year, and it appears that the most satisfactory use of the term “deciduous” is to apply it to all plants which show a more or less regular annual leaf-fall, irrespective of whether or not they are ever entirely leafless. Such deciduous trees as are never leafless may be distinguished as “semi-deciduous” or “showing a low degree of deciduousness.”
A fairly full bibliography is appended to this paper, and it is not intended to make individual detailed references to the literature on the subject. Several important papers which the writer has been unable to consult are not cited. Briefly, it may be stated that from the anatomical viewpoint the work of Lee (1911) is outstanding while Sampson (1918) has contributed the most valuable paper on the chemical aspects of leaf-shedding. The work of Lloyd (1916 b) and of Berkley (1931) is also of special interest.
The following is a general account of the leaf-abscission mechanism as it is now known:—
Abscission is effected by the disorganization of the cells of a zone of tissue, the Separation-layer, which extends across the base of the petiole. The cells of the developing Separation-layer are characterized by dense cytoplasm and large nuclei, while cell-division usually occurs in them and numerous tyloses develop in the tracheae of the petiole bundle and in some cases completely block them. The behaviour of the phloem in the Separation-layer has not been carefully described. Before the cell-walls in the plane of abscission disorganize, they swell, and separation takes place by the central region of the wall becoming mucilaginous, but there is a diversity of opinion as to the extent to which these walls are altered (Lee, 1911; Lloyd, 1916; Sampson, 1918; Dutt, 1928). Sampson has advanced the theory that the cellulose of the secondary wall layer becomes largely converted to pectose which is further transformed into pectic acid and pectin. The dissolution of the cell-wall is explained on the law of physico-chemical equilibrium. Calcium ions from the calcium pectate of the middle lamella pass out into the surrounding pectic acid till a critical concentration is reached, when there is not sufficient calcium left to retain the rigidity of the middle lamella, which becomes mucilaginous, enabling the cells to separate. Sampson further concludes that, since oxides and ferric ions are accumulated in the abscission zone, they “possibly initiate and probably accelerate” the changes in the nature of the cell-wall. Tison and Lee (1911, p. 81) have reported the presence in certain species of a layer of lignified cells in the base of the petiole immediately above the Separation-layer.
The development of the leaf-scar protective mechanism has been studied chiefly by Tison and Lee. The Protective-layer, which is initiated either before or after leaf-fall, consists of a zone of cells below the plane of abscission and parallel to it. Lee states that the Protective-layer may develop by the lignosuberization of existing cells which are otherwise unchanged; by the lignosuberization of cells which have divided irregularly; or by the lignosuberization of cells produced by cambial activity. The lignosuberization of tyloses has, in some cases, been recorded. Below the Protective-layer a periderm—the Protective-periderm—develops sooner or later and becomes continuous with that of the stem.
According to Lee (1911, p. 54) a lignosuberized wall is more or less completely lignified with a film of suberin deposited on its inner surface. He states (loc. cit., p. 105) that “in favourably stained preparations it is easy to demonstrate the localization of the suberin as an inner film on the lignified middle lamella.” Among the methods recommended for demonstrating this point is to “dissolve the film of suberin by boiling in macerating fluid and after careful washing to apply the tests for lignin.” As it is well known that suberized cell-walls are the most resistant of any to Schultz macerating fluid, it is difficult to understand Lee's method. Possibly “macerating fluid” was accidentally stated in place of “caustic potash.”
From the phylogenetic viewpoint Bews (1927) has considered the origin of the deciduous habit. He suggests that this habit with its specialized leaf-abscission mechanism evolved from the evergreen habit as a physiological response to seasonal periodicity.
Lloyd (1916 b) and Sampson (1918) have discussed the external factors influencing leaf-fall.
Little has been written on the leaf-shedding of New Zealand trees. Rutland (1888) published a paper based on limited field observations, while Cockayne (1904 and 1906) touched briefly on the subject. More recently Cockayne (1928, pp. 143–6) supplied a list of deciduous species which, though not complete, contains the most outstanding New Zealand examples.
Aristotelia serrata and Fuchsia excorticata grow abundantly in the Dunedin “Town Belt,” where the material used in this research was procured. Material of Hoheria Lyallii, which is not found in the native state near Dunedin, was obtained from the Dunedin Botanical Gardens. In each case the trees were kept under observation for twelve months, and material was collected at regular intervals. The investigation of leaf-fall in Nothofagus has been less extensive.
Formalin-acetic-alcohol was the most frequently used killing and fixing reagent. Microtomed sections, prepared by the usual paraffin method, were employed throughout. Considerable difficulty in obtaining good paraffin infiltration was remedied by a method of gradual infiltration in which dehydrated material was kept in xylol for a week at room temperature under a copper gauze table supporting a block of slowly dissolving paraffin. In Fuchsia complications arose due to air held in the dead cortical cells, but were largely overcome by frequent evacuation. The “Cambridge Rocker” microtome was used extensively, but the more rigid “sliding” microtome was preferable for large woody objects. The majority of preparations were stained with safranin and light-green, several other well-known stains being also used. Numerous microchemical tests were employed, especially in the examination of the Protective-layer and the disorganization of the Separation-layer.
All the drawings illustrating this paper were made with the aid of either an Abbé camera lucida or a Leitz reflex drawing apparatus, while the photomicrographs were made using Imperial “Fine-grain” plates and appropriate “Wratten” filters.
The research on which this paper is based was carried out in the Department of Botany, University of Otago. To Dr. J. E. Holloway, who suggested this investigation and has given helpful encouragement and advice throughout, the writer owes a deep debt of gratitude. For the use of photomicrographic apparatus he desires to record his thanks to Professor Bevan Dodds, Dean of the Otago Dental School.
The Leaf-Abscission Mechanism
(a) Hoheria Lyallii, Hook. f.
The natural habitat of Hoheria Lyallii is among the mountains of the South Island of New Zealand. Frequently found at the margins of the forest, it occurs on the banks of streams and near
the timber-line, and outside the forest it may grow on sub-alpine river beds among the shrubbery or upon ancient moraines. It will be seen that the habitat is essentially an exposed one. During the winter, and occasionally in the summer as well, heavy snowfalls occur, and though no meteorological data are available from situations such as those in which Hoheria grows, it is safe to say that there exists a markedly greater climatic alternation between winter and summer than in most lowland situations.
Hoheria Lyallii is usually leafless in winter in its natural habitat and also in Dunedin. However, this condition is by no means invariable, and warm periods in late autumn may cause renewed apical growth when practically all leaves have been shed. This occurred in Dunedin during the autumn of 1934, and the recently formed leaves persisted throughout the winter so that the tree was never entirely leafless.
Anatomical. The leaf-trace in Hoheria Lyallii consists of three vascular bundles which leave the stem stele accompanied by a strand of sclerenchyma, and, sloping steeply upwards through the cortex, enter the petiole. The sclerenchyma usually disappears before the leaf-base is reached. The position of the Separation-layer is shown in Fig. 1. In young leaves, the cortical cells of the leaf-base are somewhat shorter than those of the petiole or stem, but their contents are the same throughout and consist of well-vacuolated cytoplasm with occasional starch grains and calcium oxalate crystals. In slightly older leaf-bases the cells are distinguished by the increasing denseness of their cytoplasm, their enlarged nuclei, and very numerous starch grains. This is the first sign of the development, of the Separation-layer which is a dozen or more cells thick. The activity of this region is soon shown by irregular cell division. Leaves collected towards the end of October, or in early November, show a few cell divisions (x in Fig. 2). Throughout the summer the Separation-layer develops steadily, cell division continues, the cytoplasm becomes denser, and the calcium oxalate crystals become more abundant, especially in the inner cortex, and the starch grains disappear entirely. The volume of the cells increases slightly so that the tissue is more compact with fewer air spaces, and the contents of the petiolar-cells gradually disappear. By the time the leaves have turned yellow (March-June) the cell-division in the Separation-layer is almost at its maximum (Fig. 3), but it is still possible to trace the outline of the original cells, and it is seen that frequently three or four divisions have taken place in each cell.
While these changes are proceeding in the cortex, the parenchymatous cells in the vascular cylinder of the petiole are behaving similarly, but do not commence to divide as early as the cortical cells. The cells of the phloem parenchyma increase considerably in volume and distort the sieve tubes, whose behaviour is entirely passive. Callus formation is suggested by some sections, but its presence has not been definitely proved.
In all the xylem vessels numerous tyloses develop from neighbouring parenchymatous cells. Several may develop from a single parenchyma cell (Fig. 4). Frequently they are nucleate, and the
presence of two nucleate tyloses attached to a single parent cell proves that mitosis must accompany tylosis development. Fig. 5 shows that vessels may be completely blocked by tyloses, and that the large tyloses are so flattened against the walls of the vessels that they simulate the appearance of cross septa. Tyloses are as common for some distance below the Separation-layer as in it.
At the time that the first cell-divisions in the Separation-layer take place the stem periderm makes its appearance by the periclinal division of epidermal cells (Fig. 2). During the first season three or four layers of phellem are cut off. Occasionally periderm continues for a short distance up the petiole, but usually it stops near the level of the plane of abscission.
Shortly after the leaf becomes yellow a change is seen in the appearance of the Separation-layer cell-walls, which become swollen and indefinite in appearance. Numerous microchemical tests have been applied to determine the nature of the changes in the walls of the Separation-layer and these will be discussed later. At present it is sufficient to say that the middle lamella and much of the secondary wall layer become mucilaginous so that the tissue is disorganised. Each cell-cavity remains surrounded by a thin tertiary cellulose membrane, and the individual cells separate from one another. Fig. 11 (X) shows the two cellulose membranes separating after the dissolution of the central portion of the wall. These changes are complete at only one level—the plane, of abscission—which is situated towards the petiolar side of the Separation-layer (Figs. 6 and 10).
Shortly before leaf-fall a Lignified-layer develops by the lignification of a zone of the Separation-layer between the plane of abscission and the petiole (Figs. 6 and 10), the lignification being usually greatest at the angles of the cells, but there is considerable variation in the amount of lignification which occurs. Little or no cytoplasm remains in the cells of the fully-developed Lignified-layer.
As it is seldom that material showing the dissolution of the cell-walls at an advanced stage has been observed, it is concluded that the final stages are rapidly followed by leaf-fall. Separation commences at the abaxial side of the leaf-base, and is seldom complete right across the Separation-layer when the leaf is severed from the tree by a mechanical force, such as the wind. In all cases, the vascular elements are ruptured mechanically.
The above is the main leaf-abscission mechanism of Hoheria Lyallii. There is, however, a subsidiary mechanism which has been observed fairly frequently and may be termed “dipping periderm.” In many cases, before the alteration of the cell-walls of the Separation-layer has commenced, longitudinal sections of the leaf-base reveal a peculiar “nick” (Fig. 9) which is caused by the stem periderm dipping down into the cortical tissue. The “nick” may continue as a groove for a consderable distance round the leaf-base and is developed by the periclinal division of cells progressively deeper into the cortex. This feature is clearly not the result of injury, as at an early stage the outermost layer of the periderm is not ruptured.
It is obvious that whatever may be the cause of “dipping periderm,” it will assist in the separation of the leaf by diminishing the crosssectional area of the plane of abscission.
The Scar Protective Mechanism. No protective mechanism has developed at the time of leaf-fall, but very soon after changes may be detected in the cell-walls of the uninjured cells immediately beneath the scar. The zone of cells whose walls are transformed is usually 3–5 cells deep and is known as the Protective-layer (Fig. 7). A microchemical examination has proved the walls to be lignosuberized. Tests for lignin and suberin show the entire cell-wall (including the middle lamella) to be lignified with a film of suberin deposited as an inner film on the wall. In some cases the middle lamella has been seen to be stained more deeply by phloroglucin than the lamellae on either side. The suberin forms a distinct superficial layer. When lignosuberization is complete no cell-contents remain, but a few granules which take the suberin stains adhere to the wall. It was possible to remove the lignin and cellulose by mounting in concentrated sulphuric acid a section which had been treated previously with Eau de Javelle. After this treatment the suberin lamellae alone remained. At least some of the tylosis walls in the region of the Protective-layer also become lignosuberized.
After the Protective-layer has developed, cells immediately beneath it become meristematic and divide by walls parallel to the surface of the scar to produce the scar Protective-periderm which is continuous with that of the stem (Fig. 12). Across the xylem of the leaf-trace the periderm is continued by a division of tyloses (Fig. 8). In the figure it will be seen that many segments have been cut off from the tyloses, and the xylem vessels are consequently ruptured. By this time the phloem elements are so compressed that they can scarcely be recognised in the neighbourhood of the scar-periderm. Within three weeks of leaf-fall the periderm initials have appeared, and scar protective mechanism is complete. Several abscissed axillary buds have been observed, the sequence of changes being similar to that of leaf-falls. The leaf and bud scars frequently appear quite continuous.
(b) Aristotelia serrata, J. R. et G. Forst.
Trees of Aristotelia serrata growing near Dunedin are not entirely without leaves at any season of the year. However, there is a genuine though most indefinite period of leaf-fall which may be extended over four months or even longer. Aristotelia serrata therefore has a low degree of deciduousness. Field-observations show that under local conditions the leaf-fall is more marked and occurs earlier in exposed situations than in shaded ones, but in all cases apical growth is more or less suspended from June to September.
Early in June leaf-fall commences in exposed situations when only a few leaves are affected, but the leaf-fall continues steadily, so that there are always a few yellow leaves on the tree. In the middle of September, when many trees show but few old leaves remaining, the resting buds resume vigorous growth. In shaded positions the leaf-fall is more gradual, and commences later, while the previous season's leaves linger on until December or even longer,
by which time the new season's leaves have fully developed. Cockayne (1928, p. 146) states that this tree varies from evergreen to deciduous, and its deciduous habit is stronger in more frosty localities.
Anatomical. Since the leaf abscission mechanism of Aristotelia serrata is closely similar to that of Hoheria Lyallii described above, it will be necessary merely to mention the points of difference between the two species. The triple leaf-trace of Aristotelia is in arrangement similar to that of Hoheria, but it leaves the cauline stele more sharply. Slightly more sclerenchyma is present, and occasionally it continues up into the petiole, while calcium oxalate crystals are not as abundant as in Hoheria. In its origin and development the Separation-layer (Figs. 13–16) is exactly similar to that of Hoheria, but its extent is considerably less, being only 5–8 cells deep, while in Hoheria the depth is 12–20 cells. Tylose development (Figs. 17–19) is not so extensive, but the Lignified-layer is more strongly developed. The dissolution of the cell-walls (Fig. 16) is a slower process, though the changes in the two species are identical in nature. Following the disorganization of the parenchymatous cells leaf-separation is effected by the rupture of the leaf-trace.
After leaf-fall the Protective-layer and periderm develop similarly to those in Hoheria, but in Aristotelia the cauline periderm has not developed at the time of the leaf-fall. The Protective-periderm is thus at first unconnected with any other development in the stem. Eventually the cauline periderm develops and becomes continuous with the Protective-periderm. This process is not complete till five or more years after leaf-fall.
(c) Fuchsia excorticata, J. R. et G. Forst.
Fuchsia excorticata is undoubtedly the best-known New Zealand deciduous tree. Accurate field-observations over a large area are wanting, but it is probably more consistently leafless in winter than any other tree. In Dunedin, during the winter of 1934, it was the only native tree entirely leafless. Leaf-fall commenced in trees in all situations during the early part of May, and was complete within three or four weeks. The trees were without leaves during the greater part of June, throughout July, and in early August, the new leaves making their appearance before the middle of that month. Exotic deciduous trees cultivated in Dunedin show a much longer leafless period. Cockayne (1928, p. 146) states that F. excorticata “may fluctuate, according to circumstances, from truly deciduous to evergreen,” but no case of this tree being leafy in winter has been met with during the present study.
Forced Leaf-fall. A series of experiments, similar to those of Sampson (1918) have been performed and have yielded corresponding results. Two series of experiments were carried out. Series I was carried out about three months before leaf-shedding would normally commence. The lamina of every second leaf was removed from a number of healthy twigs, and the time of shedding of normal and mutilated leaves was noted. After fifteen days 97% of the
mutilated and 2% of the normal leaves had fallen. Series II was conducted nearer the time of leaf-shedding and yielded similar results.
Anatomical. As in the description of Aristotelia serrata, only points of difference from the mechanism already described will be discussed. In Fuchsia the cauline periderm is deep-seated and develops before leaf-shedding (Fig. 20). This is responsible for several modifications in the abscission-mechanism. The leaf-trace consists of a single vascular strand unaccompanied by sclerenchyma. The Separation-layer is similar in extent to that of Aristotelia (Fig. 21). Cell-division is somewhat more regular, the majority of new walls being perpendicular to the axis of the petiole, while the Lignified-layer is developed only within that portion of the leaf-base which is inside the cauline periderm. Rows of mucilage cells develop in the parenchyma of the vascular strand, and tylose development is greater than that observed in any of the other species investigated. Several tyloses were seen which appeared to be cut off from their parent cells by cross septa, a clear example of this being illustrated in Fig. 22. There seems no doubt that this tylosis is cut off from its parent cell by the septum, as two facts preclude the explanation that there are two distinct tyloses in contact. In the first place, the use of the 1–12 inch oil-immersion objective failed to show the septum to be double, and its thickness appeared to be the same as that of the remainder of the wall surrounding the tylosis. In the second place, a slight triangular thickening is seen at either side of the septum where it joins the outer wall of the tylosis. Such thickenings occur where three walls join, but would not be present if the internal wall were merely the result of two tyloses being in contact.
By the time the leaf falls the cortical cells outside the cauline periderm are in most cases dead. Death of the remaining cortical cells soon follows, and is preceded by the lignosuberization of the cell-walls. The Protective-layer and the Protective-periderm develop only in the tissue within the cauline periderm, and, in nature, are identical to the corresponding mechanisms of the other species.
(d) Nothofagus Species.
The genus Nothofagus is widely distributed in the Southern Hemisphere, five species being in New Zealand, three in southeast Australia, and eight in southern South America. An evergreen section is recognised with five New Zealand, two Australian, and three South American species. The remainder of the genus is deciduous.
Field observations have shown an interesting series of conditions, intermediate between the evergreen and deciduous habits, to occur in the New Zealand Nothofagus species. Periodic leaf-shedding is most pronounced in N. fusca, which on occasions show a much more marked leaf-shedding than does Aristotelia serrata (see above) in the same localities. In all species the leaves are always abscissed before the end of the summer following that in which they develop.
In its anatomy the leaf abscission mechanism of N. fusca is very similar to that of Aristotelia. However, the stem-periderm is present before abscission as in Hoheria, while the development of tyloses is less marked than in any of the other trees examined. This approach to the deciduous habit shown by members of the evergreen section of Nothofagus suggests that the distinction between the evergreen and deciduous species is not fundamental.
An interesting matter for speculation is whether the ancestral stock of this genus was deciduous or evergreen. This question will receive mention in a later part of this paper, but the behaviour of the New Zealand Nothofagus species suggests their descent from a deciduous ancestry.
The Changes in the Nature of the Cell-walls in the Separation-layer during Abscission.
The changes which occur in the nature of the cell-walls during the develoment of the separation-layer have been investigated in equal detail in Hoheria Lyallii and Aristotelia serrata. No differences have been found between these species either in the original nature of the cell-walls or in the changes which take place in them. In Fuchsia excorticata a less detailed examination has been made, but so far as it has been studied the sequence of changes is similar to that in the other two species. An account of these changes will now be given:—
In material stained with safranin and light green, changes in the appearance of the cell-walls are first observed near the time when the leaf turns yellow. The regular outline of the walls is lost, their staining becomes indefinite, and finally at one level the tissue is seen to disorganise. Closer examination reveals that each cell-cavity in the disorganized region is surrounded by a thin more or less distorted cellulose membrane which takes the green stain. The membranes of individual cells are independent of each other, and by their separation and rupture the severance of the petiole from the twig is effected. Special staining methods shed light on this process.
If sections showing swollen cell-walls are stained with ruthenium red and counter-stained for a very short period with light green, it is found that a central region of the walls has stained strongly with ruthenium red, indicating the presence of pectic substances, while a thin layer on either side takes the green stain, suggesting the presence of cellulose. Occasionally a darker red is taken by a band in the centre of the cell-wall—the middle lamella—than by the remainder of the red-staining region. As leaf-fall is approached the red-staining zone is found to increase, but no alteration is observed in the green-staining membranes. Normal cell-walls stained by the same method show a central region deeply stained with ruthenium red on either side of which is a region stained less strongly, and outside this again is a green-staining layer of the same thickness and appearance as that found in the swollen cells of the Separation-layer. In some cases it was impossible to distinguish between the two red-staining regions. If sections are treated with cuprammonia
before staining with ruthenium red and light green it is found that the membrane which would take the green stain has been removed, showing it to be composed of cellulose.
Treatment with chlorzinc iodine gives a sharp differentiation between the Separation-layer which stains yellow and the normal tissue which stains violet. Closer examination shows that all but the narrow middle lamella of the normal cell-wall is stained violet, indicating the presence of cellulose, while the middle lamella appears yellow. In the mature Separation-layer the entire cell-wall usually stains yellow, but occasionally the violet colour is seen in a thin layer bounding the cell-cavity. During the development of the Separation-layer all inter-grades between these two conditions are found—the violet-staining region shrinking progressively as the central yellow-staining region expands. The application of the hydrocellulose reaction produces results in agreement with those obtained by the use of chlorzinc iodine.
The above reactions suggest that the cell-walls of normal parenchymatous cells consist of three regions:—
(i) The middle lamella which is pectic in nature and gives pectic reactions with all reagents.
(ii) A secondary layer which is partially pectic and partially cellulose. With ruthenium red this region has been shown to take a red colour, indicating the presence of pectic substances, while with chlorzinc iodine and in a hydrocellulose reaction the presence of cellulose has been shown.
(iii) A thin outer layer surrounding each cell-cavity. This contains no pectic substances and is composed chiefly, if not entirely, of cellulose. The solubility of this layer in cuprammonia has already been mentioned, and its failure in many cases to give the reaction of cellulose when treated with chlorzinc iodine and in the hydrocellulose reaction may be due to its extreme thinness. This layer may be described as the “tertiary membrane.”
During the development of the Separation-layer there appears to be a progressive conversion of the cellulose of the secondary layer into pectic substances. Judging by the hydrocellulose and chlorzinc iodine tests this change, which is accompanied by a certain amount of swelling, has occurred, in the greater part of the Separation-layer, before leaf-fall.
The next and final stage in the development of the Separation-layer is a conversion of the pectic regions of the cell-walls into mucilage. The rigidity of the walls is thus lost and the tissues disorganized. The tertiary cellulose membranes either separate (Fig. 11) or collapse together, becoming much folded and finally rupturing. Disorganization does not take place in the entire Separation-layer, but only in a narrow zone—the plane of abscission—which is situated towards the petiolar side of the Separation-layer. In all cases it has been noticed that the thin recently-formed walls are practically unchanged during these abscission processes.
In an attempt to determine the nature of the pectic substances present in the walls of the cells of the Separation-layer the following experiments were carried out on material of Aristotelia serrata. Sections from a leaf-base shortly before leaf-fall were treated with 2.5 per cent. ammonium oxalate solution at 50° C. for 24 hours, and at 100° C. for a short period. The sections thus treated showed the same staining reactions to ruthenium red as they had previously. Similar sections were treated with one of the following reagents:—5 per cent. hydrochloric acid, 5 per cent. sodium bicarbonate, and 3 per cent. caustic potash. In each case the sections were heated in the reagent for an hour and a-half on top of a hot-water bath. The material treated with acid was stained with ruthenium red after thorough washing, and showed no alteration in the amount of pectic substance present. When the potash-treated material was similarly stained it was found that little or no ruthenium red was held by the Separation-layer, though outside the Separation-layer the staining was normal. In cases where a certain amount of ruthenium red was held by the Separation-layer the part stained was invariably a thin band in the centre of the wall—the middle lamella. The material treated with sodium bicarbonate behaved similarly.
It was not expected that the material treated with ammonium, oxalate would stain deeply with ruthenium red, showing the pectic substances present to be insoluble in that reagent. It was found that the same result was obtained if the material was treated with acid after the oxalate. Failure to obtain the removal of the middle lamella under these conditions has also been experienced by Davison and Willaman (1927). Treatment with caustic alkalis, carbonates and certain other reagents is a standard method for removing pectic acids. It appears probable from the above evidence that, shortly before leaf-fall, the pectic substance in the cell wall is pectic acid and that the middle lamella, which is at first not composed of an alkali-soluble substance, is finally converted into pectic acid also.
The facts established here are in agreement, so far as they go, with the findings of Sampson (1918) quoted previously. However, it must be remembered that Sampson assumed the middle lamella to be composed of calcium pectate—a view which has now been largely abandoned (Nangi, 1925; Norris, 1925). No light has been shed by the present investigation on either this question or the mode of conversion of cellulose into pectic acid, but it has been shown that, during the development of the Separation-layer in the species here investigated, the secondary membranes of the cell-walls are converted directly or indirectly into pectic acid, and that finally the middle lamella is also converted into pectic acid. Separation of the cells is effected by the pectic acid becomng mucilaginous and thus freeing the individual cells surrounded by their tertiary membranes.
The Deciduous Habit in New Zealand.
The climate of New Zealand is of an equable rather than a truly alternating type, and these islands are thus able to support an evergreen flora. For this reason Rutland (1888) and Cockayne (1928, p. 143) have assumed that the leaf-shedding of the indigenous
deciduous trees is largely unconnected with present climatic conditions, but is “the result of long ‘habit.’” It is the opinion of the writer that to dismiss the present climatic conditions in this way is unjustified, but at the same time, it is abundantly clear that present conditions do not provide a complete explanation of the existing situation. A tentative attempt is now made to determine what conclusions, if any, are justified by the available facts. According to Cockayne (loc. cit.), leaf-fall in some trees is connected with flowering, but these are not within the scope of the present discussion.
The New Zealand deciduous trees are as a whole characterised by very irregular leaf-shedding, and great variation is found in the degree of deciduousness shown by individuals of the same species growing in different parts of the country. A short annual resting period occurs in the growth of the majority of New Zealand trees, and this may be taken to indicate that there is a certain degree of alternation in the climate. This alternation, however, is not of similar character throughout the region, but is somewhat more marked in the southern parts, and especially in the montane districts of the South Island. Certain of the deciduous species belong to such districts, e.g., Hoheria Lyallii, Carmichaelia grandiflora, Olearia odorata, and others. These species occur in places where in winter considerable snowfalls may occur. The mechanical injury of New Zealand evergreen forest by snowstorms has been mentioned by Rutland (1888), and in unusually severe winters may be readily observed. This suggests that the deciduous habit of the above-mentioned species is, to some degree at any rate, in harmony with the environment and of definite survival value. To this extent it is suggested that present climatic conditions are conducive to the deciduous habit.
On the other hand, the systematic relations of the deciduous species suggests that the deciduous habit is not of recent origin in New Zealand. Of the more markedly deciduous species, six belong to the genus Olearia, four to Carmichaelia, three to Fuchsia, two at least to Hoheria, and two to Muehlenbeckia (Cockayne, 1928, p. 143). This suggests that these genera probably possessed the deciduous habit before the present-day species differentiated from the ancestral stock. In almost every case the closest relatives of any of the more strongly deciduous native plants show the deciduous habit also.
The geological history of the New Zealand region is imperfectly known, so it is with little hope of definite result that the geological record can be examined for evidence as to the origin of the deciduous habit in New Zealand. At first sight the early Tertiary uplift, when the region was virtually a continent, suggests itself for two reasons:—
(i) A continental climate is, in the main, conducive to the deciduous habit.
(ii) In the Tertiary or Cretaceous it is believed that New Zealand and South America were connected to a greater or less extent by land. [The details of the various theories on this subject are of no concern here (Skottsberg, 1915).]
Tertiary fossils from Graham Land, Seymour Land and other Antarctic localities lend colour to this theory. It will be noticed
that several of the New Zealand genera which show the deciduous habit belong to the South American “element” or the old Antarctic element in the flora, e.g., Fuchsia, Discaria and Muehlenbeckia, and also Nothofagus with its slight degree of deciduousness. These genera may be considered to have occurred in Tertiary times on the Antarctic continent and adjacent lands. If the deciduous habit had been possessed by the flora of that time we might reasonably expect it to be surviving here and there in the floras of southern South America and New Zealand. Little data on the South American flora are available to the writer, but it is known that of the seventeen species of Nothofagus six are truly deciduous, and of these five are in South America and one in Australia, while the New Zealand species (see above) show a mild degree of deciduousness. If the origin of the deciduous habit in this genus were of later date than the Tertiary it must have evolved independently in two distinct regions, and a marked tendency to it must have developed in a third—developments which would be unlikely in largely evergreen floras. The same is the case in Fuchsia, as some of the South American Fuchsias show leaf-fall. It must be remembered that, if the climates of the three regions were of strongly alternating type, this parallel development of the deciduous habit would be expected.
The fact that the South American genera in New Zealand and vice versa are not all deciduous could be explained by the assumption that the deciduous habit has been gradually abandoned in a more or less equable climate. That this is possible has been shown experimentally by Flammarion (quoted by Hicks, 1928). On account of the greater climatic alternation in the situations in New Zealand in which Hoheria Lyallii and certain other deciduous species grow, other things being equal, they would be likely to retain the deciduous habit longer than lowland forest-plants.
This discussion suggests the following tentative deductions, which, it must be remembered, are highly speculative:—
(i) That the present climate of New Zealand is in certain habitats favourable to the deciduous habit.
(ii) That the deciduous habit is probably not of recent origin in New Zealand.
(iii) That, although there is at present little information to suggest the past history of the deciduous element, it might conceivably have originated in Tertiary times on the Great Antarctic Continent, and since then been gradually suppressed (so far as New Zealand is concerned) by the more equable climate.
When the leaf-fall mechanisms of the New Zealand trees which are here described are compared with those found in Northern Hemisphere deciduous trees it will be seen that in all cases the mechanism is fundamentally of the same type. No useful purpose would be served by making a detailed comparison with individual types described by Lee (1911) and other workers, as the details of abscission mechanism are largely determined by the anatomical
individuality of the plants. There are, however, a number of general points which merit discussion; but no further reference will be made to the changes in the cell-walls of the Separation-layer, as these have already been discussed.
The Lignified-layer. Tison and Lee (1911) described in many cases a Lignified-layer similar to that of Aristotelia and Hoheria. Lee states (loc. cit., p. 61) that the Lignified-layer “gives rigidity to the whole layer, making the adjacent and ever-weakening Separation-layer much weaker by comparison, and thus aiding the final separation which soon follows.” When it is remembered that separation usually commences from the abaxial side of the leaf-base and that there are unlikely to be any oblique tensions or movements in the Separation-layer, it will be seen that “the effect of lignification” is not as obvious as Lee suggests.
The origin of such a layer in a tissue about to be shed is unexpected. Still more striking is the formation of a poorly developed periderm after leaf-fall in the petioles of Juglans and Pyrus, which Tison reported according to Priestley and Woffenden (1922).
“Gummy Lignin.” Lee (loc. cit., p. 53) reports that, along with tyloses in xylem vessels, there occurs “a gummy substance, which from its property of taking the lignin stain has been called ‘gummy lignin.’” No such substance has been found during the present study.
Forced Leaf-fall. Though not extensive, the experiments on forced leaf-fall described in this paper indicate that when the whole leaf-blade is amputated, abscission occurs very rapidly. When only half the leaf-blade has been amputated no acceleration of abscission has been observed, but the number of experiments on this point was too small, and the length of investigation too short, for the results to be conclusive. Sampson (1918) has conducted similar experiments with Coleus blumei, and has decided that the mutilations accelerate rather than initiate the abscission mechanism. The acceleration obtained in Coleus by amputating the lamina was considerably greater than that in Fuchsia.
Tyloses. Leaf-bases shortly before leaf-fall are particularly favourable for the investigation of tyloses, and certain features which have been observed during this study are of interest.
It has been found that tyloses are frequently provided with nuclei by the division of the nucleus of the parent cell; that in some cases lignosuberization takes place; and that at certain stages the division of tyloses—accompanied by mitoses—occurs. In the development of the protective-periderm, tylosis division is invariably found, and the occurrence of tyloses cut off from their parent cells by septa has also been shown. It will be seen that at their fullest development tyloses are independent cells and not merely “intrusions” from parenchymatous cells as they are commonly described. Tylosis formation may therefore be regarded as in irregular mode of cell division.
Fig. 1.—Diagrammatic longitudinal section of leaf base showing position of Separation-layer (S.L.), Lignified-layer (Lig.), Periderm of stem (S.Pd.) and Leaf Trace. X 15.
Fig. 2.—Early stage in development of Separation-layer in region indicated in Fig. 1. One of the earliest cell-divisions in developing Separation-layer Is indicated (X). X 100
Fig. 6.—Portion of Separation-layer shortly before leaf-fall, showing Lignified-layer (Lig.), Separation-layer (S.L.) and disorganization of cell-walls in plane of abscission (A) in cortex. X 200.
Fig. 7.—Longitudinal section of leaf-scar shortly after leaf-fall, showing lignosuberized Protective-layer (PrL.). X 150.
Fig. 9.—Longitudinal section on abaxial side of leaf-base showing early stage in Separation-layer (SL) and deeply-stained periderm of stem, also “dipping periderm.” X 75.
Fig 10.—Separation layer at late stage Note disorganization of cells in plane of abscission (A.A.), Lignified-layer (Lig.) and dense contents of Separation-layer. X 130.
Fig. 11.—Same section as Fig. 10 showing (X) separation of the two cellulose membranes of a disorganized cell-wall in plane of abscission. X 650.
Fig. 13—Diagrammatic longitudinal section of leaf-base showing position of Separation-layer (S.L) and Lignified-layer (Lig.). (A B. = axillary bud.) X 15.
Figs 15 and 16—Early and late stages in development of Lignified-layer (Lig.) and Separation-layer (SL.). In Fig 16 disorganization of cell-walls has commenced. X 200.
Fig. 20.—Diagrammatic longitudinal section of leaf-base showing Separation-layer (S.L.) and periderm of stem (S. Pd.). x 10.
Fig. 21.—Cellular details of region indicated in Fig. 20, showing mucilage cells (M.C.), xylem parenchyma (x. par), xylem vessels (xyl.), phloem sieve tubes (sv.), and cortex (cx.). x 100.
The significance of tyloses in the abscission mechanism may be interpreted in various ways. Lee apparently regards them as part of the Separation-layer, while Lloyd (1916, p. 227) suggests that tyloses are insufficient to block the vessels, and are a wound response. To the latter view is the objection that when tyloses develop there has been no wounding, for, as Lloyd himself points out, the disorganization of the Separation-layer is not a degeneration, since the cytoplasm and nuclei of the cells remain healthy. Tyloses are known to occur as wound responses in other cases, and if in the present case they are a wound response, it would be suspected that the Protective-layer is a wound response also rather than a development of the abscission mechanism—an hypothesis difficult to reconcile with the fact that in some trees (Lee, 1911) the Protective-layer develops before leaf-fall. The lignosuberization in Fuchsia of cortical cells which have been cut off by the stem-periderm has been described, and, from this, it might be argued that lignosuberization is a wound response. Experimental investigation of wounding would possibly shed light on this, but at present there are no facts to justify such a view. We can only conclude that the origin of the abscission mechanism is traceable to internal physiological stimuli, and that these stimuli are largely controlled by external factors.
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