CONTRIBUTION TO OUR KNOWLEDGE OF THE PHYSIOLOGICAL ANATOMY OF SOIVLE

INDIAN HYDROPHYTES

VI. The Stem of Hygrophila quadrivalvis Nees

BY M. V. MmAsm, M.Sc. (Department of Botany, College of Science, Nagpur)

Received May 6, 1957 (Communicated by Dr. T. S. Sabnis, F.A.SC.)

INTRODUCTION

Hygrophila R.Br. is a typical hydrophytie genus of the Acanthace~e. In India, it is represented by seven species which grow in marshy areas. H. quadrivalvis Nees is a rare species of this genus. It is an erect, stout, hairy* annual herb which grows to a height of about two and half feet. " I t helps to make the perfect transitional series from the smaller and simpler Hygrophilas to Asteracantha" (Santapau, 1951).

The plants come up by the end of August. The flowering season extends from October to December. Some plants, however, come up late in the season. These can be observed to continue even after the substratum has fairly dried up.

The stem of this species is quadrangular in outline and often red in eolour. It is profusely hairy, especially in the younger parts. It is erect or slightly decumbent near the base where it roots at the nodes.

MATERIAL AND METHOD

Hygrophila quadrivalvis Nees is restricted to a very small area near the source of the Nag River in Nagpur. It occurs in thick dense stands in the puddles where the water is almost stagnant. Collections were made at differ- ent periods during the year and the anatomical characters were studied mostly

* The species is classed as "glabrous or nearly so" by Haines, although in the detailed notes on the species, he describes it as a "rather coarse herb with strigillose or glabrescent stem and leaves which are hairy above". "'Glabrate'" is the word used for the leaves of this plant by C. B. Clarke in Hooker, f FI. Brit. India, 4,408. The word is not found in the original descriptions of the species by Nees. From these descriptions, however, it appears that there are various forms of this plant. The Nagpur plants have profusely hairy stems as well as leaves which are hairy on both surfaces. This striking hairiness is in accordance with Nees's own remarks. The author is indebted to Father H. Santapau, St. Xavier's College, Bombay, for this information about the species.

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248 M . V . MmAsrii

from free-hand sections of fresh as well as preserved material. Considerable difficulty was experienced in seclioning the material on account of the abundant occurrence of cystoliths in the epidermis of the stem.

DESCRIPTION 1. Epidermis.--Outer wails thickened, radial walls thin and undulating;

cuticle thin, even in specimens collected quite late in the season (Figs. 6 and 7). Solitary elongated cystoliths abundant in large epidermal cells (Figs. 4 and 5). Stomata situated in groups, caryophyllaceous, raised well above the general level of the epidermal cells (Fig. 1) . . Multicellular, peltate glands frequent (Fig. 2). Multicellular, uniseriate hairs with bulbous bases (Fig. 3).

Epidermal cells of the lower portion of the stem, which is under water or embedded in the saturated soil, larger in size, with the outer walls and cuticle thinner than-in the upper aerial portion (Figs. 6 and 7).

2. Cortex.--As in the stems of several species of the Acanthace~e, the outer cortex is mostly collenchymatous, being interrupted at places by radial sectors of chlorenchyma (Fig. 6). Collenchymatous thiekening less pro- nounced and the cells tangentially elongated in the lower portion of the stem (Fig. 7).

Middle cortex consists of thin-walled chlorophyllose cells separated by schizogenous air-spaces. The cortical ceils abound in raphides. In the upper portion of the stem the cells are roundish and the lacunae are smaller. The cells of the lacunar cortex in the lower portion hypertrophy, stretch in various directions and remain attached to each other by their ends only. The lacunae enlarge and become radially elongated. Thus, the cortex in the lower portion is a broad zone forming a spongy covering for the stem.

Sections of the material collected later in the season reveal a gradual narrowing of the lacunar cortex. There is a deposition of tannin in the corti- cal cells. Ultimately, along with the peripheral collenchyma, the lacunar middle cortex gets crushed so that in the older stem even the outlines of the constituent cells cannot be made out (Figs. 13 and 14).

Some of the cortical cells develop, lignified and pittedplates on their wails late in the season (Figs. 8 and 13).

The cells of the inner cortex show the presence of anthocyanin. The endodermis is distinct. While the outer and middle cortex are getting crushed, this layer is seen to become meristematic; but, it does not give rise to any secondary tissue (Fig. 14).

A subepidermal phellogen gives rise to lenticels (Fig. 15).

Physiological Anatomy of Some Indian Hydrophytes--VI

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TExT-FIGs. 1-10. Hygrophila quadrivalvis Nees.--Fig. 1. Part of the T.S. of the stem showing the raised stoma. Fig. 2. Part of the T.S. of the stem showing the multicellular peltate gland in the epidermis. Fig. 3. The multicellular, uniseriate hair. Fig. 4. Part of the T.S. of the stem showing the cystolith in the large-sized epidermal cell. Fig. 5. Part of the epidermal peel showing the elongated solitary cystolith. Fig. 6. Part of the T.S. of the upper aerial part of the stem showing the welt-developed collenchyma of the outer cortex interrupted by the radial sec- tor of thin-walled chlorenchyma. Fig. 7. Part of the T.S. of the lower submerged portion of the stem showing the tangential elongation of the epidermis and the outer cortical ceils as well as the comparatively poorer coUenchymatous thickening. Fig. 8. (A) Part of the T.S. of the stern showing the supporting cell in the cortex with the lignified pitted plate in surface view; (B) Part of the longi- tudinal section of the stem showing two supporting cells with the characteristic pitted thickening on their walls. Fig. 9. Part of the T.S, of the stem showing the thin-walled parenchymatous pith with very small intercellular spaces. Fig. 10, part of the T.S, of the stem showing the raphides in the pith cell,

250 M . V . MIRASHI

4. PericycIe.--This region is not clearly marked out from the phloem. It includes several strongly thickened fibres.

5. Vascular system.--This consists of separate vascular bundles that get joined together by the formation of the interfascicular cambium. Secon- dary growth in thickness is normal. Phloem includes a number of bast-fibres (Fig. 16). There is a well-developed secondary wood (Fig. 17).

6. Pith.--Thin-walled parenchymatous cells; intercellular spaces very small (Fig. 9). Raphides abundant (Fig. 10).

DISCUSSIOIN

1. The foregoing account of the physiological anatomy of the stem of Hygrophila quadrivalvis Nees confirms the amphibious nature of the species. It reveals the characteristic combination of hydrophytic and geophytic charac- ters met with typically in the helophytes.

The thick-walled epidermis, the profuse covering of hairs, the presence of the cuticle, the collenchymatous outer cortex and the well-developed xylem elements indicate essentially an adaptation to terrestrial conditions.

The peripheral mechanical cylinder comprising the thick-walled epidermis and the collenchymatous outer cortex, as well as the broad woody cylinder of secondary xylem, situated round the periphery of a prominent central pith, give to the stem mechanical rigidity which is further enhanced by the presence of pericyclic and bast-fibres.

2. The fragile lacunar cortex is strengthened not only by the peripheral shell of collenchyma, but also by the thick-walled, lignified and pitted cells. These are somewhat prosenchymatous and occur singly or in groups of twos or threes. They develop rather late in the season by secondary sclerosis of thin-walled cells. The distribution of these cells and the lignified thickening of their walls suggest that they serve as local mechanical supports. Such an arrangement for supporting the lacunar cortical tissue of the hydrophytic stem is not very frequently met with. It has been recorded earlier by the author (1957) in the stem of Lobelia alsinoides Lamk. in which the cells are, however, not lignified. Isolated supporting cells with peculiar thickening ridges have been recorded by Mullan (1932) in the pneumatophore of the Indian halophytes, Avicennia alba B1. and A. officinalis Linn.

3. As is characteristic of the stems of marsh plants, the conducting system of this stem is well developed in the form of a broad zone of wood and the correspondingly elaborate phloem,

Physiological Anatomy of Some Indian Hydrophytes--VI

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TEXT FIGS. 11-17. Hygrophila quadrivalvis Nees.--Fig. 11. Part of the T.S. of the upper aerial portion of the stem showing the thin-walled spherical cells and the rather small intercellular spaces of the middle cortex. Fig. 12. Part cf the T.S. of the lower submerged portion of the stem showing the hypertrophied cells of the middle cortex and the radially elongated intercellular spaces. Fig. 13. Part of the T.S. of the stem, collected rather late in the season, showing the narrowing of the middle cortex and the development of supporting ceils. Fig. 14. Part of the T.S. of the stem, collected still later, showing the lacunar middle cortex getting crushed so that the outlines of the constituent ceils can hardly be made out. Note the secondary meristem. Fig. 15. Part of the T.S. of the stem showing the lenticel. Fig. 16. Part of the T.S. of the stem showing the endodermis, the pericycle, that includes some sclerenchymatous fibres, and the phloem tissues, Fi$. 17, Part of the T,S. of the stem showing the well-developed wood elements.

252 M . V . Mmasm

4. The primary lacunar cortex constitutes the aerating tissue of the stem. As in the stems of the hydrophytic members of the Composite, Scro- phulariace~e and Lobeliace~e, described earlier by the author, the upper aerial portion of this stem and its lower part, which may be submerged or situated in poorly aerated soil, show significant differences in their anatomy. In the latter, the walls of the epidermal cells are thinner and the collenchyma of the outer cortex shows poorer development. The aquatic environment also seems to act as a stimulus for the tangential expansion of these peripheral tissues so that they could keep pace with the elaborated aerating system of the middle cortex in the lower portion of the stem.

The expansion of the cortex, consequent on the enlargement of the air chambers and the elongation of the thin-walled parenchymatous cells of the middle cortex, brings about a more central disposition of the mechanical ele- ments of the wood in the lower portion of the stem which has got to be inextensible rather than inflexible, unlike the aerial counterpart.

A similar development of a profusely lacunar cortex has been described by Solereder (1908) in the allied species, Astercantha longifolia Nees (Hygro- phila spinosa T. Anders.), and by Mullan (1932) in the stem of the Indian halophyte, Acanthus ilicifolius Linn.

5. With the improved aeration of the soil, consequent on its gradual drying up with march of the season, the secondary wood shows a rapid and pronounced development, the need for aeration is less keenly felt and the lacunar cortex goes on narrowing till it is ultimately crushed. These changes are significant in view of the fact, pointed out earlier by Saxton (1924), while explaining his theory o f " mixed formations in time ", that in India the same species has to live often under two different sets of environmental conditions during its life-time and that the responses shown by the plants to these are striking.

ACKNOWLEDGEMENT

The author wishes to express his gratitude to Dr. T. S. Sabnis, D.SC., F.A.SC., for kindly going through the manuscript of this paper and making valuable suggestions.

LITERATURE CITED

Mirashi, M.V. .. "Contribution to our knowledge of the physiological anatomy of some Indian Hydrophtes, VI. The stem of Lobelia alsinoides Lam.," Proc. Ind. Acad. Sci., 1957p 45 (4), 181-185.

Physiological Anatomy of Some Indian Hydrophytes--VI Mullan, D.P. ..

Santapau, H.

Saxton, W. T.

Solereder, H.

253

"Observations on the biology and physiological anatomy of some Indian Halophytes," J. Indian bet. Soc., 1932, 11,285-302.

. . The Acanthace¢e o f Bombay, University of Bombay Botani- cal Memoirs, No. 2, Bombay, 1951.

.. "Phases of vegetation under monsoon conditions," J. Ecol., 1924, 12, 1-38.

. . Systematic Anatomy o f the Dicotyledons (English Transla- tion), Oxford, 1908.