THE sea cord-grass, Spartina Townsendii Groves, grows abundantly in muddy,

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1 THE STRUCTURE AND DEVELOPMENT OF THE HYDATHODES OF SPARTINA TOWNSENDII GROVES BY A. D. SKELDING, B.SC, A.R.C.S. AND JOYCE WINTEREOTHAM, B.Sc. University College, Southampton (With 19 figures in the text) INTRODUCTORY THE sea cord-grass, Spartina Townsendii Groves, grows abundantly in muddy, coastal and estuarine soils which are regularly inundated by the tides, particularly on the south coast of England. The body of the plant consists of a horizontal fleshy rhizome invested by scale leaves and ending in a large bud, which in the spring grows out of the mud to become the annual aerial shoot of the plant. The axis of the latter is vertical, terminates in the inflorescence, and is completely enclosed in its lower half by the sheaths of the foliage leaves, which are borne alternately in two opposite rows. The mature leaf consists of a split sheath which very closely clasps the axis and the sheath of the next leaf above, a lamina attached to the top of the sheath and carried at a wide angle to the vertical, and thirdly a soft hairy ligule closely pressed to the axis at the junction of the sheath and lamina. The lamina and sheath are connected by a cartilaginous pulvinar articulation. The general anatomy of Spartina Townsendii has been described in detail by Sutherland & Eastwood (1916). LEAF STRUCTURE AND LOCATION OF HYDATHODES Hydathodes are absent from the rhizome, from that part of the stem invested by leaves, and from the inner surfaces of the leaf sheaths, but they are found with varying frequency in all other aerial parts of the grass with the exception of the flower itself. A few are usually present on the barren glumes, but we have yet to find hydathodes on the flowering glume or palea. The observations described here have been made from hydathodes occurring on the lamina of the leaf, since this material is the most convenient with which to work. The adaxial and abaxial surfaces of the leaf differ strikingly. The abaxial surface is smooth. On the adaxial surface, however, there are some forty flat-topped ridges, each about 200/u, in width at the base but tapering slightly upward and more than half the total thickness of the leaf in height. These run parallel along the full length of the lamina. In the mesophyll at the base of each ridge is a vascular bundle

70 A. D. SKELDING and JOYCE WINTERBOTHAM enclosed by two eccentric sheaths of tissue, the inner one fibrous and the outer one of large cells with colourless contents.

2 70 A. D. SKELDING and JOYCE WINTERBOTHAM enclosed by two eccentric sheaths of tissue, the inner one fibrous and the outer one of large cells with colourless contents. This outer sheath is extended on the adaxial side to form the core of the ridge immediately above. The assimilating cells with which the hydathodes are always associated occur immediately below the epidermis in zones which run the full length of the leaf. As seen in transverse section each zone extends from a point just below the flat top of a ridge, down the flank, beneath the groove and up the flank of the adjoining ridge to a similar point near the top. They extend inward as far as the core of colourless cells, but beneath the groove to within a single cell of the lower epidermis (Fig. i). The epidermal cells of the leaf are of two kinds, both roughly rectangular in shape and arranged in unbroken rows from one end of the lamina to the other. The commoner type of cell is greatly elongated in the direction of the long axis of the leaf and the average dimensions are 130/i in length, 30/11 wide and 30/^ in depth. The other type of cell is of the same width and depth but only about io/x in length; such cells usually occur singly between the ends of the long epidermal cells but are sometimes found in pairs (Figs. 6, 8). The lateral walls which come into contact with other epidermal cells are sinuous, more conspicuously so on the adaxial than on the abaxial side of the leaf. The cuticle on the adaxial epidermis is raised into large numbers of small papillae, the average number being about fifty to each long epidermal cell, but the abaxial cuticle is quite smooth. The hydathodes are epidermal structures not connected directly with the waterconducting system of the plant. They are always in direct contact with the assimilating tissue on the adaxial side of the leaf and on the abaxial side are only separated from it by a single layer of large cells with colourless contents. On the adaxial side they are to be found in a single longitudinal row on each side of the ridges, about six epidermal cells from the top and always higher than the stomata. Hydathodes in the same row are separated by between two and five long epidermal cells, i.e. by /x. On the abaxial side they occur in rows lying between the vascular bundles, sometimes a single row between each pair of bundles, sometimes two rows. The hydathodes on this surface are more widely spaced than on the adaxial surface. The relative frequency is indicated by the figures obtained when the number of hydathodes in equal areas of epidermis on the two sides of the leaf were counted. There were forty-two on the adaxial surface to twenty-seven on the abaxial surface. THE STRUCTURE OF THE MATURE HYDATHODE The complete hydathode involves four epidermal cells so arranged as to leave a roughly cylindrical opening in the epidermis, here termed the well of the hydathode, and a specialized structure of two cells constituting the hydathode proper. The latter will be described in detail first. The larger basal cell of the hydathode is attached to the four surrounding epidermal cells but is sunk in the tissue of the leaf so as to leave a roughly cylindrical depression in the epidermis above it. This depression is delimited by the end walls of the two epidermal cells in the same longitudinal row and by part of the lateral walls of epidermal cells in the adjoining rows (Figs. 2-4). The upper and smaller of

3 Structure and development of hydathodes of Spartina Townsendii 71 the trwo specialized cells is the cap cell; this may be regarded as an outgrowth from the outer wall of the basal cell, since, apart from its attachment to the basal cell, it is quite free in the well of the hydathode which it more or less fills (Figs. 2, 4). B Fig. I. Transverse section of part of the mature leaf to show the general distribution of tissues and the location of the hydathodes. A, adaxial hydathode; B, well of ahaxial hydathode; C, phloem detail omitted; D, mesophyll; E, stoma. Fig. 2. Longitudinal section through hydathode of adaxial epidermis to show sections of pits leading from the basal cell into the epidermal and mesophyll cells. The basal cell is spindle-shaped with its long axis directed along the length of the leaf. Its dimensions are approximately ioo/t in length, 30/A in width and 30 ;u. in

4 72 A. D. SKELDING and JOYCE WINTERBOTHAM depth. Because it is immersed in the tissue of the leaf its upper half abuts on parts of the inner walls of the four epidermal cells which define the well of the hydathode. The lower half of the basal cell comes into contact with many assimilating cells of the mesophyll in the case of hydathodes on the adaxial surface of the leaf and with large cells of the water-storage type which sheath the vascular bundle in the case of those on the abaxial surface. Communication with both epidermal and mesophyll cells is made by large variously shaped pits (Figs. 5, 7, 9, 14). The large mesophyll cells which surround the basal cell on the abaxial side of the leaf have walls as thick as those of the basal cell itself and the pits can be seen in both (Fig. 14). The assimilating cells which surround the basal cell on tbe adaxial side have extremely thin walls and the pitting is much less evident (Figs. 2, 7). The wall of the basal cell is composed of cellulose with the exception of an annular segment of the outer portion between the line of its attachment to the four epidermal cells and its attachment to the cap cell, which is also cuticularized. This cuticularized area is exposed to the cavity of the well (Figs. 2, 3, 5, 10, 14). The cap cell is dome-shaped with the flat side applied to the outer wall of the basal cell. Its height is about ioja and the diameter of its base about ;ti. The cellulose wall is strongly cuticularized on the exposed outer surface and is devoid of pits. That part of the wall which separates the cap cell from the basal cell is also quite devoid of pits (Figs. 2, 14). Both the cap cell and the basal cell are typically glandular in that each is full of protoplasm and has a large strongly staining nucleus (Fig. 3). The epidermal cells framing the well of the hydathode have external walls which are very thick and lateral walls which are very thick at the exterior but become progressively thinner toward the interior. Thus the walls of the actual hydathode well are thick at the mouth of the opening and become thinner toward the line of articulation with the basal cell where they are sufficiently thin for the joint to be somewhat flexible. The outer surface of the well has a thick cuticular covering which, apart from certain perforations described below, forms a complete coating over the well Legends of Figs. 3 to 9 Fig. 3. Transverse section of mature hydathode. A, cap cell; B, basal cell. Fig. 4. Surface view of ahaxial epidermis showing the well and pits in the wall of the basal cell. A, wall of hydathode well; S, wall of cap cell; C, contents of cap cell (shaded); D, pits connecting basal and epidermal cells. Fig. 5. Composite diagram from three optical sections of the adaxial hydathode viewed from above. A, wall of well; B, surface view of pit connecting epidermal and basal cell; C, wall of basal cell in section, with pits communicating with mesophyll cells; D, line of articulation of basal cell with epidermal cells; E, lateral wall of epidermal cells. Fig. 6. Surface view of the abaxial epidermis to sbow the location of the hydathode. A, long epidermal cell; B, short epidermal cell. Fig. 7. Longitudinal section of the basal cell cut parallel to the leaf surface showing the pits in section. Fig. 8. Hydathode in surface view seen through the abaxial epidermis. A, cap cell in well; B, basal cell of the hydathode showing pits communicating with the epidermal cells; C, mesophyll cell. Fig. 9. Hydathode basal cell viewed through the underlying mesophyll cells, showing surface view of pits connecting them. A, basal cell; B, mesophyll cells.

5 Structure and development of hydathodes of Spartina Townsendii 73 Figs. 3-9

6 74 A. D. SKELDING and JOYCE WINTERBOTHAM walls, the cap cell and the exposed annular part of the basal cell. Like the cellulose part of the walls the cuticle becomes progressively thinner from the mouth of the well inwards and is quite thin at the line of articulation (Figs. 2, 3, 10, 14). Treatment of transverse sections of the epidermis with Schultze's solution shows a thin layer of cutinized material immediately within the primary layer of the outer wall. This cutinized layer is somewhat thicker at the corners of the cells and extends for some distance inward along the lateral primary wall separating adjoining epidermal cells (Figs. 10, 14). It does not seem to be present in the walls which form the well of the hydathode, nor in the wall of the cap cell or that of the basal cell. The cuticle of the adaxial epidermis is thinner than that of the abaxial epidermis and unlike it is provided with papillae which closely border the opening of the well. The walls of the epidermal cells are deeply pitted. In the thick outer walls there are numerous pits which are relatively narrow and penetrate no further than the primary wall separating the cellulose layers from the cuticle (Figs , 14). In the walls bounding the well of the hydathode the pits are wider and pass through both the cellulose wall and the cuticle and are closed only by a very thin membrane in the region of the primary wall (Figs ). It has not been possible to determine the chemical nature of the pit-closing membrane with certainty. It seems likely, however, that this membrane is of a pectic nature like the primary wall, since, when seen in surface view in sections stained with Ruthenium Red, the pits have a pale pink colour. The surface of the cuticle within the wells of hydathodes in the abaxial epidermis of the leaf, forms a few, fairly extensive but very shallow depressions into each of which two or three of the larger pits open (Figs ). These depressions have not been seen in the adaxial hydathode where the well is much shallower and the cuticle thinner. The part of the pit which passes through the cuticle often becomes wider toward the exterior giving a trumpet-shaped appearance to the pit as a whole (Figs. 12, 13). In all there are twenty to thirty pits of various diameters opening into the well of the hydathode. The inner ones are oval and large, greater than 2/A in diameter, whereas the outer ones tend to be small and circular in section, e.g. f fi in diameter. The length of the pits naturally depends upon the thickness of the wall and so it is greater in the case of those opening into the outer part of the well. A pit opening into the outer part of the well of an abaxial hydathode measured 7 /^ in length, whilst one of the innermost in the same well was only 3IX long. As the cap cell and the cuticularized area of the basal cell are both devoid of pits, it is presumed that the secreted salt solution passes from the epidermal cells into the well cavity of the pore through the pits in its sides. Legends of Figs. 10 to 14 Fig. 10. The well of the hydathode and part of the basal cell in transverse section, showing pits opening into the well (diagrammatic). Tbe cap cell is shown by a broken line since it was below the plane of the optical section in which the pits vi'ere visible. Figs. II, 12, 13. Various views of the well of the hydathode showing pits and pit openings in various positions (cuticle shaded). Fig. 14. Transverse section of mature abaxial hydathode showing the pits opening into the well and pits in the wall of the basal cell and mesophyll cells beneath it (diagrammatic, cuticle shaded).

7 Structure and development of hydathodes of Spartina Townsendii 75 Figs

8 76 A. D. SKELDING and JOYCE WINTERBOTHAM THE DEVELOPMENT OE THE HYDATHODE The two-celled glandular part of the hydathode is derived from a single epidermal initial cell which becomes distinguishable from the other epidermal cells whilst the young leaf is still in the bud. The hydathode initial grows more rapidly than the other epidermal cells and assumes a characteristic shape by tbe extension of the basal part into the mesophyll and the projection of the outer part beyond the surface of the epidermis. The nucleus is unusually large (Fig. 15). The nucleus of the initial cell undergoes normal mitotic division (Figs ), one daughter nucleus remaining in the basal region of the young hydathode and the other passing to the outermost part of the cell. In cell division, which follows immediately, a cell wall formed in the plane of the leaf surface cuts off the knoblike projection of the initial cell to form the cap cell (Figs. 18, 19). The basal cell grows rapidly and soon reaches its final shape and size but the cap cell grows very little after cell division. Shortly after cell division, the well of the hydathode appears as a result of the overgrowth of the epidermal cells and the consequent sinking of the glandular cells into the tissue of the leaf until the cap cell no longer projects from the surface (Figs. 3, 19). Cutinization of the leaf begins early. Before the majority of the hydathode initial cells have undergone cell division, small discontinuous areas of cuticle can be seen on the surface of the cap cell and the epidermal cells. The early stages in the development of the cuticle were best seen in sections which had been stained in a cold solution of Sudan III in a mixture of equal parts of 25 % glycerine and 25 % alcohol for 24 hr. and then mounted in glycerine jelly. The discontinuous areas of cuticle rapidly become larger until by the time the hydathode initials have divided there is a thin continuous cuticle covering the whole surface of the leaf and the cuticular papillae have appeared on the adaxial epidermis. The authors are not aware that the first appearance of cuticle in the form of discontinuous areas has been described previously. Development to the mature condition consists of the rapid deposition of cellulose on the inside walls of the epidermal cells, the cap cell and to a much less degree of the basal cell and also of a strong thickening of the cuticle particularly on the abaxial side of the leaf. The pits in the wall of the basal cell of the hydathode and those opening into the well become visible for the first time during this thickening process. MODE OF ACTION OE THE HYDATHODE The hydathode secretes salt solution of a fairly high concentration. Measurements made upon drops of solution from the leaves of pot-grown plants watered with a culture solution of approximately the same composition as sea water, using Barger's method gave a value of nearly \ M strength or about 25 g. per litre calculated as NaCl. The course which the liquid takes is not yet certain but the structure of the hydathode is suggestive of what happens. The fact that pits opening to the exterior occur only in the walls of the epidermal cells delimiting the well makes

9 Structure and development of hydathodes of Spartina Townsendii 77 Fig. 15. Transverse section of young leaf and hydathode, showing nuclear resting stage in the initial cell. Fig. 16. Transverse section of hydathode initial cell showing prophase of mitosis. Fig. 17. Transverse section of hydathode initial cell showing metaphase of mitosis. Fig. 18. Transverse section of initial cell showing cell division and the early stages in cuticularization. A, small discontinuous areas of cuticle. Fig. 19. Transverse section of young hydathode in the two-celled stage, showing the wall separating the cap cell and the hasal cell and a continuous though thin covering of cuticle.

10 78 A. D. SKELDING and JOYCE WINTERBOTHAM it almost certain that the liquid emerges from these cells. If a leaf of a shoot standing in salt solution is enclosed in an air-tight chamber and observed under a microscope by means of an ultrapak lens, drops of liquid may be seen appearing very quickly on either side of the grooves of the adaxial surface. The glandular nature, size and freely pitted walls of the basal cell suggest that it acts as a centre into which the secreted fluid is drawn from the numerous mesophyll cells in contact with it. Presumably the salt solution passes from the basal cell to the epidermal cells and so out into the well of the hydathode. The hydathode possibly acts as a valvular mechanism. The articulation of the basal cell to the four epidermal cells at the base of the well is thin enough to be flexible and so may act as a hinge. When the leaf is in a turgid condition we may suppose that the basal cell is distended and the well kept open so as to permit secretion from the pits. When the turgor of the leaf is reduced the rigidity of the epidermis would cause the well to close by bringing the walls of the epidermal cells into contact with the cap cell, which is the condition generally found in sections. We have not been able to confirm this mechanism by direct observation. The fact that secretion occurs freely when a detached leaf is placed with its cut end in water, indicates that the process is essentially an active one. Further work is necessary before the exact mode of action of the hydathode can be clearly understood. SUMMARY 1. The structure, distribution and function of the hydathodes of Spartina Townsendii Groves are described. 2. The hydathode is an epidermal structure consisting of a well-like opening (the well) in the epidermis, bounded by four epidermal cells and two specially modified glandular cells (the basal cell and cap cell). The basal cell is much larger than the cap cell and is sunk in the assimilating tissue of the leaf with which and also with the adjacent epidermis it is connected by pits. The cap cell is attached to the outer surface of the basal cell and almost fills the well. Its wall is not pitted. 3. The walls of the epidermal cells which define the well of the hydathode are perforated by numerous pits which pass through both the cuticle and the cellulose part of the wall and are closed only by a thin membrane in the region of the primary wall. 4. The glandular part of the hydathode arises in the young leaf from a single epidermal cell which grows inward into the tissue of the leaf and then divides to form the basal and cap cells. Both cells have large nuclei and abundant cytoplasm. 5. The cuticle was observed to make its appearance in the young leaf in the form of small discontinuous areas which rapidly extended until a thin uniform covering was produced. The surface of the well and the cap cell are covered with cuticle in the mature hydathode. 6. The hydathode secretes a salt solution consisting mainly of sodium chloride with a concentration of about \ g.mol. per litre.

11 Structure and development of hydathodes of Spartina Townsendii 79 The work done by one of us (J. W.) was carried out whilst in receipt of a research grant from University College, Southampton. In conclusion the authors wish to express their gratitude for valuable suggestions to members of the staff of the Botany Department of University College, Southampton, and especially to Prof. S. Mangham whose advice and criticism has made this publication possible. REFERENCE SUTHERLAND, GEO. H.& EASTWOOD, A. (1916). The Physiological Anatomy of 5/>arftna Townsendii. Ann. Bot., Lond., 30,

Non Permanent Tissues - Meristematic Tissue

PLANT TISSUES Non Permanent Tissues - Meristematic Tissue Undifferentiated plant cells that are continually dividing by mitosis Large thin walled cells No vacuole Dense cytoplasm Large nucleus Found at