Morphological and Anatomical Studies in Helobiae

MORPHOLOGICAL AND ANATOMICAL STUDIES IN HELOBIAE

I. Vegetative Anatomy of Some Members of Potamogetonaceae a.2

BY V. SrNGH (School of Plant 3/Iorphology, Meerut College, Meerut) Received May 29, 1964 (Communicated by Prof. V. Puri, F.A.SC.)

INTRODUCTION ARBER (1920) in her monumental work on water-plants has nicely reviewed the earlier literature on the vegetative anatomy of the family Potamo- getonaceae. Some of the earlier contributions are those of Rauinkiar (1903), Chrysler (1907), Graves (1908), etc. Later, Arber (1923, 1925 a) made a detailed study of minute scale-like structures--the squamulae intravaginales-- associated with the leaf-bases of the members of Potamogetonaceae and other Helobiae. Monoyer (1927) studied the stipules of Potamogeton and Campbell (1936) described the vegetative anatomy of Potamogeton pectinatus. Since the nodal anatomy and other vegetative structure of Potamogetonaceae show some interesting features, the present work has been taken up. In the present investigation seven species of Potamogeton, viz., P. indicus Roxb., P. natans Linn., P. crispus Lion., P. pectinatus Linn., P. epihydrus Raf., P. praelongus Wulfen, and P. berchtoldi Fieb., Ruppia maritima Linn. and Zannichellia palustris Linn. have been studied.

MATERIAL AND METHODS The material of P. indicus, P. crispus, P. pectinatus, and Zannichellia palustris was collected locally by the author and that of P. natans was kindly passed over to me by Professor V. Puri from his Kashmir collections. The material of the remaining four species, viz., P. praelongus, P. berchtoldi, P. epihydrus and Ruppia maritima was obtained from U.S.A., that of the former two through the courtesy of Professor Ernst C. Abbe (Minnesota) and latter two Dr. R. J. Eaton (Middlesex).

a A part of the thesis approved for the Ph.D. degree of the Agra University. z Research contribution No. 62 from the School of Plant Morphology, Meerut College, Meerut, 214 Morphological and Anatomical Studies in Helobiae--I 215

All the materials were fixed in F.A.A. and customary methods of dehydra- tion and embedding were followed. Serial transverse sections were cut 8-12 microns thick. For staining safranin-fastgreen combination was used which gave satisfactory results.

OBSERVATIONS Potamogeton Potamogeton (potamos--a river; geiton--a neighbour--the ancient Greek name of a water-plan0 is the largest genus of the family distributed chiefly in the temperate regions of the world. These freshwater aquatic herbs have a creeping root stock which gives rise to erect leafy branches. The leaves are 2-ranked except for the involucrae ones which are opposite. They are submerged (P. crispus, P. pectinatus, P. epihydrus, P. praelongus and P. berchtoldO, or floating (P. indicus and P. natans). The submerged leaves are narrowly linear or linear lanceolate but the floating ones are elliptic or elliptic-ovate. The stipules are intrapetiolar (axillary), either free from or adnate to the base of the leaf (P. pectinatus and P. crispus). Some species have special means of vegetative propagation. P. pectinatus forms tubers, while at the close of growing season P. crispus produces apically, short-leaved winter buds which detach from the plant and remain buried in the mud till the next spring when they give rise to new plants. P. natans and P. indicus grow from the rhizome of the previous season. The roots of only P. crispus and P. indicus have been studied. The internal structure of mature roots is very simple. The cortex is many layers in thickness and is composed of thin-walled cells with intercellular spaces. The endodermai cells show easparian thiekenings. While in P. crispus the xylem is represented by a centrally placed vessel (Fig. 1) in P. indicus the root shows two or four xylem vessels in the centre (Fig. 2). The walls of xylem vessels show some thickenings. The stems are always differentiated into nodes and internodes. The internode is cylindrical and almost circular in cross-section. The cells of the epidermis are squarish or radially elongated and contain numerous chloroplasts. They are covered externally by a delicate layer of cuticle, that is rather thick in P. natans and P. indicus. In P. natans cutinization extends to the radial walls of the epidermal cells (Fig. 5). The cortex is very lacunaceous with two or more circles of lacunae (Figs. 3-5). There are generally one or two layers of large thin-walled cells in between the epidermis and the outermost lacunae. Adjacent lacunae 216 V. SIN6H

are separated by a single stratum of cells. In those species where cortical bundles are present the walls in between the lacunae are thicker particularly in the region of the bundles. There are only three (P. pectinatus) or many (P. natans, P. praelongus and P. epihydrus) cortical bundles distributed in the peripheral zone of the cortex. The cortical bundles are generally poorly developed and in some cases they are represented by a group of thin-walled cells. The cells of the endodermis are usually thin-walled and show some casparian thickenings. In P. berchtoldi walls are slightly thickened and in P. pectinatus the inner tangential and radial walls are very much thickened (Fig. 8). The central vascular cylinder in P. natans, P. indicus, P. praelongus and P. epihydrus consists of a ring of generally 8-12 distinct vascular bundles (Fig 6). The number varies even in the same species. In P. crispus the bundles of axial cylinder are condensed into three groups (Fig. 7) which are separated by two or three intervening layers of thin-walled cells. In the centre of each group is present a xylem cavity. Arber (1920) mentions that in this species the xylem cavities of two groups of bundles may fuse and in such cases the distinctness of bundles is maintained by phloem alone. In P. pectinatus and P. berchtoldi all the bundles of the axial cylinder fuse to form a complete vascular cylinder with a central xylem cavity surrounded by a ring of phloem (Fig. 8). Within the central cylinder of the internode of P. epihydrus there are two groups of three leaf-trace bundles each, similar to those observed by Chrysler (1907) in P. pulcher. Following the terminology of Chrysler (1907) the bundles t 1, T1, tl are traces of next higher leaf and t2, T~, t2 of the leaf next to that (Fig. 6). The remaining bundles are purely cauline. In P. n atans, P. indicus and P. praelongus the three traces belonging to next higher leaf are not free as in P. epihydrus but they unite into one bundle. In P. crispus where the bundles are fused in three groups the central group represents four leaf-traces T~, h, Tz, tz (Fig. 26). Inthe vascular bundles of Potamogeton the phloem is normally developed. The sieve tubes are very distinct and stand out dearly because of their large size and empty cell cavities. However, the xylem is very much reduced and is represented by a lysigenous cavity formed by the disorgan- isation of xylem elements. The xylem cavity is bounded by a distinct layer of thin-walled cells. The xylem vessels have been observed in the internodal regions of very young stems of P. pectinatus. Morphological and Anatomical Studies in Helobiae--I 217

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Fins. 1-14. Figs. 1--2. Transverse sections of the axial cylinder of roots of P. erispus and P. indicus Iespeetively. Figs. 3-5. Portions of cortex showing lacunae from transverse sections of stems of P. pectinatus, P. rrispus and P. naCans respectively. Figs. 6-8. Transverse sections of central vascular cylinder of stem of P. praelongus, P. erispus and P. pectinatus respectively. Fig. 9. Transverse section of central vascular cylinder of stolon of P. pectinatis. Fig. 10. A portion of vertical section of leaf of P. pectinatus. Figs. 11-12. Portions of vertical sections through the margins of leaves of P. berchtoldi and 1). crispus respectively. Figs. 13-14. A -squamulae intravaginales (sq.) associated with leaf axil of P. pectinatus cut in transverse and longi- tudinal sections respectively. 218 V. SINGrt

The nodal anatomy of all the species other than P. natans has been investigated. Since some differences have been observed in the departure of the leaf-traces the species are described in three groups.

In P. pectinatus and P. berchtoldi 'there is an axial cylinder in the internode. In addition to this in P. pectinatus three small cortical bundles are also present (Fig. 15). In the sub-nodal region the central stele expands and the xylem cavity is replaced by vessels and tracheids showing spiral thickenings on their walls. From this one prominent leaf-trace passes out closely followedby a weak trace on either side (Figs. 16,17). Before entering the leaf-base each of these weak traces takes an outward turn within the cortex. Thus, each leaf receives three traces. In P. pectinatus the cortical bundles are independent and they do not join the central stele even in the nodal region. Before the leaf-base separates from the axis each of the cortical bundles gives off one or two branches which move outward to enter the leaf-base (Figs. 18, 19).

The separation of leaf-base starts flora the middle region and extends towards the edges. The sheathing leaf-base is broad and membranous and completely surrounds the axis. Subsequent to the separation of the leaf- base from the stem a branch trace separates from the central stele. It occu- pies an axillary position and constitutes the vascular cylinder of the axillary branch (Figs. 19, 20).

At a higher level the sheathing leaf-base become differentiated into a median swollen portion and a membranous wing on either side (Fig. 21). The two wings joined together by a strip of tissue separate from the median portion of the sheathing leaf-base as an intrapetiolar structure still surround- ing the node (Figs. 22-24). The wings are 3-4 cell layers in thickness and the cells are without chloroplasts. Before separation, the wings receive branches from the marginal bundles of the leaf-base (Fig. 22). In addition to these in P. peetinatus branches received from the cortical bundles also adds to its vascular supply.

P. crispus has three traces for each leaf. The median trace is prominent and is followed closely by two lateral traces which arise in opposite directions at right angle to the median trace (Figs. 26-29). The wings which are attached to the sheathing base receive one branch from each of the two marginal bundles of the leaf. They separate in similar fashion as described for P. pectinatus (Figs. 30-32). Morphological and Anatomical Studies in tIelobiae--I 219

FIGS. 15-32. Figs. 15-20. Serial taansections through nodal region of P. pectinatu~ showing divergence of leaf and its axillary branch. Figs. 21-24. C-'ross-seetions through leaf-base of P. pectinatus. In Fig. 24 ,stipular sheath has completely separated from leaf-base. Fig. 25. A portion of diaphragm from a cross-section of node of 1). pectinatus. Figs. 25-32. Serial cross- sections passing through a node of 1). crispus. Fig. 32 is at a slightly higher level where sfipular sheath has separated from leaf-base. (sti, stipular sheath; sq, squamulae intravaginales.} B5 220 V. SINGIJ

In the sub-nodal region of P. indicus, P. praelongus and P. epihydrus the bundles show some anastomosing and their xylem cavities are replaced by lignified elements (Figs. 33, 34). At this level a large number of small groups of thick-walled cells also make their appearance in the cortex. Three traces pass out almost simultaneously for a leaf along with some thick- walled cells (Fig. 35). Subsequent to the departure of the leaf-traces, there depart in all directions many traces from the central mass of vascular tissue (Figs. 36, 37). A number of traces are also given out by cortical bundles in P. praelongus and P. epihydrus. All these traces supply the membranous sheath of the leaf (Figs. 38, 39). The membranous sheath also derives two main bundles from the marginals of the leaf (Fig. 38). They lie in the two angular projections of the sheath which clamp the petiole (Figs. 39--41). As in other species described earlier, the sheath is intrapetiolar in position but is free from the leaf-base and is prominent with rich vascular supply. After the divergence of the traces for the leaf-base and sheath, branch traces begin to set off from the bundles of the central vascular cylinder(Fig. 40). These traces constitute the vascular cylinder of the axillary branch (Fig. 41). The air cavities in the nodal region are transversely perforated by one cell-layered plates of somewhat hexagonal cells with small and rounded intercellular spaces at their angles. These are the diaphragms characteristic of many of the aquatic plants (Fig. 25). The leaves of all the seven species have been studied. While in P. peetinatus, P. crispus and P. berehtoldi they are not differentiated into petiole and lamina, in others they have distinct petioles. A cross-section of the petiole appears dorsiventral,ly flattened. The anatomy is very similar to that of stem. The petiole receives three bundles, one median and two laterals. The latter while traversing the petiole divide into five to seven bundles arranged in a single file. The bundles are collateral and their xylem is represented by a narrow canal. The cells of the upper and lower epidermis of the lamina have slightly thickened outer tangential walls. The lower epidermis is not distinct in P. crispus. In the floating leaves (P. natans and P. indieus) the upper epidermis is covered by a distinct cuticle and has stomata. Though the stomata are present in P. indieus, they are covered with cuticle and are permanently closed. Submerged leaves lack stomata. The mesophyll is composed of loosely arranged roundish or irregular cells enclosing a number of air cavities. There are only two large air cavities in P. peetinatus, one on either side of Mo~Thological and Anatomical Studies in Helobiae--I 221 the midrib bundle. The air cavities show perforated diaphragms similar to those observed at nodes of the stems.

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FIGS. 33-41. Serial cross-sections passing through a node of P. ind/cus. Note very prominent stipular sheath (sti) and squamulae (sq) in Figs. 40 and 41. 222 V. SINGH The vascular bundl6s are of usual collateral type, the middle being very prominent. In very young leaves the xylem is represented by a few tracheids, but in a mature leaf the tracheids disorganise and their place is taken by a small cavity bounded by a distinct layer of thin-walled cells. The bundles are surrounded by a distinct sheath, the ceils of which have slightly thickened walls (Fig. 10). Some of the marginal bundles are very poorly developed and are represented only by a group of thin-walled cells. Bast bundles have been observed along the margins running throughout the length of the leaf in P. crispus and P. berchtoldi. They are represented by a few thick-walled cells in cross-sections (Figs. 11, 12).

Within the axils of the leaves of all the species studied small scale-like structures--the squamulae intraveginales---have been observed (Figs. 13, 14) while in P. pectinatus there are only two squamulae in an axil, one on either side of the median line (Figs. 19, 20), in other species their number is variable (Figs. 31, 32, 40, 41). The squamulae have densely staining cells with prominent nuclei and are devoid of any vascular supply. Besides the leafy shoots some species also produce creeping branches. According to Irmish (•858) they represent certain lateral members of the extensively branched system. The stolons of three species, viz., P. pectinatus, P. crispus and P. indicus are investigated. They bear scale leaves at their nodes and have vascular cylinder similar to that of the leafy shoots. The stolons of P. pectinatus, however, differ from the leafy shoots in the absence of endodermal thickenings and cortical bundles (Figs. 9, 43). Chrysler (1907) has also noticed the absence of cortical bundles in the creeping stems of P. natans. The cortical cells of the stolons contain a large number of starch grains. P. pectinatus forms small tubers towards the end of autumn to tide over winter. They are formed at the tips of the stolons which would have other- wise given rise to erect leafy shoots. Each tuber has two internodes and is covered by two scale leaves, one of which arises from the last node of the stolon and covers half of the tuber while the other arises from the node of the tuber ~nd surrounds the remaining tuber and a part of the apical bud. Each tuber ends in a bud which germinates to form a new plant. A tuber may bear a bud opposite the scale leaf at the node, which may develop into a daughter tuber (Fig. 42). In a cross-section the tuber is bounded by a distinct layer of epidermis. The cortex is broad and consists of large, irregular and compactly arranged cells containing a large number of regularly distributed starch grains within MoJThological and Anatomical Stud?es in Helobiae--I 223 them. The small axial cylinder surrounded by a faint layer of endodermis is centrally disposed and contains a central xylem cavity surrounded by phloem on outer side.

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42 44 45

50 53 FIGS. 42-53. P. pectinatus. Fig. 42. External diagram of a tuber with its daughter tuber. Figs. 43-53. Serial cross-sections of tuber. In Fig. 45 note a root trace (ft.) which diverge out with leaf-trace. Fig. 53 is through apical bud of tuber. Growing axis is surrounded by numerous young leaves. (ax, growing axis; dtu, daughter tuber; 1, leaves; sel~ and sell, first and second scale leaves of tuber; stow stolon.) B6 224 V. StNGI~

Serial cross-sections of a tuber reveal that at the last node of the stolon one trace is given off for the first scale leaf while the other trace is given off at the node of the tuber for the second scale leaf (Figs. 43-47). The remaining vascular cylinder splits up into two, one-half remains in the portion that continues in the tuber, while the other half, opposite the second scale leaf, moves outwards to enter the bud present at the node of the tuber (Figs. 48-51). The vascular cylinder of the tuber extends into the apical bud and there it gives off one trace for each of the many young leaves (Figs. 52, 53). Ruppia maritima It is a completely submerged aquatic herb, common in salt or brackish water of temperate and tropical zones of the world. The stem is simple or branched giving tufts of fibrous roots from the lower nodes. The leaves ore alternate or opposite, linear and filiform with a membranous sheathing base. In the axil of a leaf ~e present two small, ovate and scaly squamulae, one on either side of the median line. The internal structure of the internode is very similar to that of Potamo- geton pectinatus. There is only a single ring of lacunae in the cortex. Some of the cells of epidermis and cortex are filled with brown granular mass. Graves (1908) following Sauvageau (1891) referred to them as secretion cells. The vascular supply of the internode consists of a central vascular cylinder and two small cortical bundles (Fig. 54). The vascular cylinder consists of a central xylem region surrounded by phloem. In very young stems the xylem is represented by a few tracheids and vessels but in older ones they disorganise to form a xylem cavity. The cortical bundles consists of a few closely packed thin-walled cells, later some of these disorganise to form a cavity. In sub-nodal region the xylem cavity of the central cylinder is replaced by lignified elements and the number of secretary cells also increases. From the axial cylinder a trace passes out for a leaf, soon followed by a branch trace (Figs. 55, 56). Before the separation of the leaf from the node the two cortical bundles give off one trace each which move outward and form the lateral bundles of the leaf (Figs. 57, 58). The basal sheath of the leaf which clasps the stem (Figs. 58, 59) is made up of two layers of cells devoid of chloroplasts. The squamulae are two cells in thickness. Zanniehellia palustris It is cosmopolitan in distribution, occurring in fresh and brackish waters. The plants are submerged, perennial herbs with a cylinder creeping rhizome. Morphological and Anatomical Studies in Helobiae--I 225

54 55

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63 ~5~ 67 FIGS. 54-67. Figs. 54-59. Serial cross-sections passing through a node of Ruppia maritima. Note in Fig. 57 cortical bundles giving rise traces for leaf. Figs. 60-67. Zannichellia palustris. Fig. 60. External diagram of a node. Figs. 61-67. Serial transections through nodal region. Note in Fig. 64 that vascular cylinder splits into two after giving rise to trace for first leaf. (l1 and I=, first and second leaf of main axis; 13, first leaf of its axillary branch; sh, stipular sheath; sq, squamulae; 1, axillary branch of first leaf; 2, pistillate flower; 3, staminate flower; 4, axillary branch of second leaf.) B7 226 V. SINGI-/

The leafy shoots are filiform and much branched bearing one-nerved linear opposite or sub-opposite leaves having membranous sheathing bases which envelope the node. The internal structure of the internode is very similar to that of Ruppia maritima except for the absence of cortical bundles. At each node there are two sub-opposite leaves. The main axis terminates in a pistillate flower while the axillary branch of the upper leaf of a node acts as the main axis. The axillary branch of the lower leaf terminates in a staminate flower after giving rise to a leaf at its very base. The lateral branch arising in the axil of this leaf acts as the axillary branch. Thus, at each node there are two sub-opposite leaves, two vegetative branches and one staminate and one pistillate flower (Fig. 60). At the nodal region the axial cylinder gives off a trace which passes outwards to form the vascular supply of the first leaf (11) at this node (Figs. 61-63). Just before the level where the leaf separates from the node the remaining vascular tissue splits up into two parts which give off one trace each in almost opposite directions at close intervals (Fig. 64). The lower trace supplies the second leaf (/2) of the node while the upper one forms the vascular supply of the leaf of the axillary branch (13) (Fig. 65). The vascular tissue left after the departure of the trace for the second leaf of the main axis splits up into two, one part supplies the axillary branch (i) and the other extends into the female flower (2) (Fig. 66). The remaining vascular tissue gives off one more trace at right angle to the first which forms the vascular supply of the staminate flower (3) (Figs. 66, 67), and then organises to form the vascular cylinder of the axillary branch (4). A vertical section of the lamina above the region of the sheath is somewhat elliptical in outline. The epidermis is devoid of cuticle and stomata and there are present numerous chloroplasts in each cell. The single vascular bundle which takes a median position has a central xylem cavity surrounded by an undifferentiated mass of phloem and parenchyma. There are present two large air cavities, one on either side of the vascular bundle. The cells of the mesophyll are large, thin-walled, loosely arranged and contain numerous chloroplasts. The membranous sheath of the leaf which envelops the node is made up of two layers of colourless cells. No vascular supply has been observed in the sheath. DISCU~ION AND CONCLUSIONS A study of the vegetative structures of different species has brought to light some points of interest to which a brief attention is drawn here. Morphological and Anatomical Studies in Helobiae--I

A reduction series can be drawn in the vascular cylinder of stems in various members studied. Not only the vascular elements are reduced quanti- tatively but the reduction in the number of vascular bundles is also seen in different species and ultimately the vascular system is concentrated into a single axial cylinder.

Schenick (1886) has nicely discussed the origin of the axial cylinder in aquatic plants. He concluded that the axial position of the cylinder in the stem is attained phylogenetically by a gradual shifting of the peripheral bundles towards the centre of the axis and their ultimate union into a single concentric bundle. He further adds that this is the case with both monocotyledonous and dicotyledonous aquatic plants. Graves (1908) while working out the morphology of Ruppia supports Schenick's interpretation and believes that the axial cylinder of Ruppia descends from four distinct vascular bundles whose phloem regions are apparent at a node even at present.

The present author also agrees with the views expressed by Schenick (1886). A reduction series can be drawn in various members of the family investigated. The vascular system of some species of Potamogeton having floating leaves (P. epihydrus, P. praelongus and P. natans) is close to a normal terres- trial monocotyledonous stem. Besides the central axial cylinder which consists of 8-12 vascular bundles, there are many, small, scattered bundles in the cortex. Further, the cortical bundles are absent in P. indicus. In P. crispus the bundles of central cylinder have further fused and are collected in more or less three clearly separated groups each with a xylem cavity in the centre. The vascular system is in the form of a single axial cylinder in certain species of Potamogeton (P. pectinatus and P. berchtoldi), Ruppia maritima and Zannichellia palustris.

The nodes of 8 species of Potamogetonaceae are investigated in the present study. While in Potamogeton each leaf receives three traces, it receives only one in Ruppia and Zannichellia. The present author is inclined to believe that the one-trace condition has been derived from three-trace condition by fusion of the three traces. The two lateral traces appear to arise from the base of the median trace in P. pectinatus and P. berchtoldi. In a generl sense this may lend support to the view expressed by Sinnott (1914) for dicotyledonous nodes. Sinnott and Bailey (1914) compare the three trace condition of the node of Potamogetonaceae -to that of trilacunar condition in dicotyledons. They further remark that in its nodal anatomy Potamo- getonaceae approach the dicotyledons than do any other monocotyledons. 228 V. SINGH

Three different conditions of the stipular sheath have been observed in the various species investigated. The basal sheath remains completely fused in Ruppia. In some species of Potamogeton like P. pectinatus and P. crispus the membranous sheath is adnate with the leaf-base for a short distance but it becomes free above. The membranous sheath is completely free from the leaf-base in broad-leaved species of Potamogeton such as P. natans, P. praelongus and P. epihydrus and also in Zannichellia. Hov~ever, in Zannichellia the sheath surrounds the node completely. In these cases the sheath takes an intrapetiolar position. The sheath does not receive any vascular supply in Ruppia where it is completely fused with the leaf-base. However, in the species of Potamogeton where the sheath is adnate with leaf base or completely free from it, one branch each from two marginal bundles of leaf contribute to its vascular supply. In addition to this it may also receive traces directly from the vascular cylinder of the node. Though the sheath is completely free in Zannichellia, it receives no vascular supply. It is sometimes stated that monocotyledons lack stipules (see, Arber, 1952 b; Eames, 1961). However, this statement needs some qualification. As pointed out b,y Eames (1961) that, " there is little agreement in the interpretation of the sheathing base of the monocotyledon leaf. The thin margins are commonly interpreted as representing adnate stipules that have lost identity as such in adaptation of the general monocotyledonous habit of ' tele= scop.ed shoot' ". He further states that there is evidence of reduction of stipules in many families. They have completely disappeared from most of the highly evolved dicotyledonous families, but as adaptation to the domi- nant monocotyledonous habit they may survive in monocotyledons as part of the sheathing leaf. Thus, it is evident that the membranous sheath of the leaves of the members of Potamogetonaceae are nothing but the stipules. Like that of the typical stipules of dicotyledons they also derive their vascular supply from the marginal bundles of the leaf. Sinnott and Bailey (1914) while discussing nodal anatomy and morphology of stipules also point out that the vascular anatomy indicates that the stipular appendages of Potamo- getonaceae are homologous with the stipules of dicotyledons. However, the two stipules have fused in the species of Potamogeton. Their double nature is evident from the fact that they receive their vascular supply from both the marginal bundles of the leaf-base. In still other cases they have completely fused with the leaf-bases and lost their independent identity and also their vascular supply. Morphological and Anatomical Studies in Helobiae--I 229 The stipules of Zannichellia can be compared with the oehreate stipules of the Polygonaceae, though they have lost their vascular supply. The stipules of the broad-leaved species of Potamogeton show some resemblafice with those of the various species of Ficus and Artocarpus (cf Goebel, 1905; Sharma, 1962). Goebel (1905) describes the stipules of Ficus as axillary and derives them from lateral ones by fusion. The same appears to be true for Potamogeton.

A variable number of small scale-like structures have been observed associated with the leaf-bases of all the species investigated. These structures have densely staining cells with prominent nuclei and are devoid of any vascular supply. They have been named as ' squamulae intravaginales ' or 'squamulae intra-axillaries' by Irmish (1858).

Most of the older workers regarded that the squamulae belong to the leaf below them and they occupy a more or less axillary position (see Arber, 1923). But Arber (1923, 1925 a) who has made an extensive study of these structures in all the families of Helobiae regarded that these structures do not belong to the leaf in whose axil they sometimes appear to arise and instead they originate from the surface of the internode separating this leaf from the next leaf above. Therefore, they cannot be regarded as axillary structures. The present study also confirms the view expressed by Arber. It has been seen in serial cross-sections through young apical buds that the squamulae appear in the majority of cases after the leaf-base has separated from the node.

The tubers of Potamogeton pectinatus are formed towards the end of growing season by the swelling of the tips of stolons. Arber (1925 b) commenting upon the tuberisation of Arrhenantherum aranaceum var. bulbose points out that it takes place due to the hypertrophy of the cells of pith and the ground tissue. Shiam (1962) while studying tuberisation in Cyperus esculentus holds a similar view. The present investigation also shows that the tubers in P. pectinatus are formed by the enlargement of the cells of the ground tissue of the stolon tips. They store a large amount of food material.

Another interesting poifit which deserve some attention here is the pre- sence of daughter tubers at the node of parent tuber opposite the saele leaf. Serial transverse sections of the tuber show that the trace for the leaf and daughter tuber are given off in opposite directions. Thus, it indicates towards the sympodial type of branching, i.e., the main axis has continued to form the daughter tuber while its axillary branch ends in the terminal bud of the parent tuber, 230 V. SINGH SUMMARY In the present work vegetative anatomy of nine species of Potamo- getonaceae, viz., Potamogeton indicus, P. natans, P. crispus, P. pectinatus, P. epihydrus, 1". praelongus, P. berchtoldi, Ruppia maritima and Zannichellia palustris have been described. The vascular cylinder of the stem shows a reduction series in the various species investigated. The axial position of the cylinder is considered to have been attained phylogenetically by a gradual shifting of the peripheral bundles towards the centre of the axis where they unite to form a single concentric ' bundle' While each leaf receives three trace in Potamogeton, it receives one trace in Ruppia and Zannichellia. It is believed that one-trace condition has been derived from three-trace condition by fusion of three traces. The basal sheath of the leaf is either completely fused, adnate at the base or completely free from the leaf-base. The vascular supply shows that these sheaths are equivalent to the stipules of dicotyledons. A variable number of non-vascularized small scale-like structures--the squamulae intra;caginales--are associated with the leaf-bases. The view that they do not belong to the leaf in whose axil they appear to arise but originate from the surface of the internode separating this leaf from next leaf above is supported. The tubers of Potamogeton pectinatus develop at the tips of the stolons and are formed by the enlargement of the cells of the ground tissue of the stolons. The vascular anatomy indicates that the stolons show a sympodial type of branching. ACKNOWLEDGEMENTS The author expresses his deep sense of gratitude to his esteemed teachers, Dr. Y. S. Murty for his valuable guidance and encouragement throughout the course of this investigation and Professor V. Puri for his unfailing interest and numerous valuable suggestions. He is grateful to Prol~ssor Ernst C. Abbe (Minnesota, U.S.A.) and Dr. R. J. Eaton (Middlesex, U.S.A.) for their generous help in the collection of material. He is also thankful to Dr. Naresh Chandra, Dr. G. Gopal Krishna and Mr. N. P. Saxena for their valuable help and to the Ministry of Education, Government of India, for, the award of a Senior Research Training Scholarship during the tenure of which a part of this work was done, Morphological and Anatomical Studies in Helobiae--I 231

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