The Journal of Indian Botany/Volume 1/September 1919/Note on the Œcology of Spinifex squarrosus L.

From Wikisource
Jump to navigation Jump to search

NOTE ON THE ŒCOLOGY OF SPlNIFEX SQUARROSUS L.*[1]

By P. F. Fyson, b.a., f.l.s., and M. Balasubrahmanyam, b.a.

The question discussed in this paper arose in the course of an investigation into the water and soil-relations of the marine strand vegetation of Madras. One of the most important species of the strand formation is Spinifex squarrosus L. It occurs on low sand-dunes as close as 20 yards only from the sea, but always raised a few feet above high-tide level. It usually grows by itself, but in open patches Cyperus arenarius Ketz. and Launasa pinnatifida Cass, sometimes occur.

The plant spreads over the sand by horizontal shoots which root at the nodes, usually beginning with the second and third node from the growing end. The adventitious roots are thick and run more or less vertically downwards for one or two feet, with only small rootlets. Root-hairs are conspicuously developed on the oldest part near the surface of the soil and seem to be persistent there. A few inches below the surface of the ground there are usually none, or only shrivelled dead hairs, and the root is sometimes thinner because of the exfoliation of the outermost layers of the cortex after the formation of an exodermis. The youngest part has no root-hairs, as a rule ; the root being quite smooth for one or two inches behind the tip, then becoming slightly lumpy on account of branch roots pushing out from below, but still without any sign of root-hairs.

If a plant be dug up without breaking the roots, and grown in a jar with some roots in water, root-hairs appear on young roots close behind the growing point, but not on older roots.

Sections of this lowest region shows the root surrounded by a highly refractive substance, which is apparently secreted by the cells of the piliferous layer. This layer and its secretion can be traced quite easily under the root-cap back to the earliest stage in the differentiation of root-cap and piliferous layer. The cells are at first isodiametric but soon become elongated radially, taking on the general appearance of secretory tissue. There are no intercellular spaces, the cells are narrow the protoplasm is dense without vacuoles and the nucleus large and situated about the middle. The outer wall is irregular in outline, and beyond it the secretion is marked by tangential and also by radial lines, which latter appear to be continuations of the middle lamellas between the cells, (fig. 3.) The tangential lines are clearly due to stratification of the substance produced by successive changes in its composition affecting its refractive index, for they are progressively further apart and fainter, and the presence of the radial lines clearly indicating the lateral limits of each cell, shows that the secretion or modification of each cell takes place independently of those of contiguous cells.

The secretion does not take up any of the ordinary protoplasmic stains, such as eosin, osmic acid, or hsematoxylon ; nor could we obtain any reaction with methyline-blue, Schultz solution or iodine.

We conclude that the secretion is due to a modification of the cell-wall, and though controlled and brought about by each cell independently, is not a direct product of its protoplasm. But that it is not a gum or mucilage in the ordinary sense is shown by the fact that it does not swell and dissolve in water at the ordinary temperature, nor take up any cellular stains nor show any cellulose reaction .

The Root-hairs.

As stated above, the root-hairs are found only on parts of the root near the surface of the sand. A few inches down they are cut off by the exodermis, which is formed as usual in monocotyledons, and shrivel up. But near the surface there is no exodermis, and the root-hairs formed while that part was still young persist apparently indefinitely.

The Regions of the root.

There are three regions in the root : —

I. From the tip to the region of the exodermis the surface is covered by the secretion. This may be two to six inches long and the surface is white, smooth and glistening.

II. In the region of the exodermis the surface tissue is dead and brown. It is in this region that the short rootlets are found.

III. From the exodermal region to the surface, for a length of two or three inches as a rule, the piliferous layer remains fresh, the root-hairs persist and adhere firmly to grains of sand.

The existence of a secretion by the piliferous layer has been noted, we believe, in only a few grasses, which grow in the Algerian Sahara. The chief of these is Aristida pungens. R. Price (2) who

has examined the secretion, pointed out that in Aristida it binds together the sand particles with a tubular sheath into which root-hairs run and are thus enabled, probably, to take up moisture from a larger volume of sand.
The Journal of Indian Botany - p31.png

1. Longitudinal section of root. 2. Longitudinal section of the tip magnified showing the layer of secretion. 3. Small part of some magnified showing to the left two loose cells II of the rootcap. The secretion shows striation tangentially and readily.

With Spinifex squarrosus no tubular sheath is formed, for the secretion does not cause the sand particles to adhere. It appears to act rather as a lubricant or perhaps as a resisting layer to protect the piliferous layer from damage by sharp grains of sand. Spinifex is not related closely to any of the grasses noted by Price as possessing the mucilage layer. The genus consists of only four species, occurring on sandy shores of India, Malaya and N. Australia.

The Water-supply.

Every one knows that sand a very few inches below the surface is damp, as also is any ordinary soil. With ordinary clay or loamy soil this is usually taken to be due to an upward movement from low levels of water drawn up by capillarity through the fine cracks developed by the drying of the soil. This water being continually dried at the surface of the ground. There may be a slow upward movement of water also through sand ; but it is very slow. This was determined by us both in the laboratory and in the sand of the beach. In the laboratory two wide glass tubes were filled with dry sand, the lower end closed with muslin and the tubes supported with their lower ends, one in fresh water, the other in salt water. At first the rise of water as shown by the darkening in colour of the sand was rapid, but after the first day the rate fell off and after seven days the level was practically stationary at 24 cms; a slow rise went on for several days till it reached 30 cms. Then no further rise was noted. There was no appreciable difference between the two tubes. The experiment was continued for 4½ months without any change in the level of the dampness being seen. It came to an end with the rotting of the muslin holding in the sand.

In another series of experiments 8-inch drain pipes were sunk in sand in an enclosed space but open to the air. The pipes were 36 inches long. They were filled with dry sand, and some left open, others closed at the top. No appreciable rise of water could be detected after periods of three to six weeks except in one case where the sand had a musty smell and was slightly more damp below than above, pointing to a rise of water from below. Heavy rain had fallen and the sand outside had been saturated, so that the water might have come up as much by hydrostatic pressure as by capillarity.[2]

Other workers have observed the same thing, that water does not rise by capillarity in sand; the explanation probably being that there are no capillary tubes formed as in a fine grained clay. The dampness of the sand just below the surface is then not due to a rise from below.

The only other source of water is from above, as rain or dew; and it seems clear that the water in sand even by the sea is not salt water drawn up from below nor even brackish but quite fresh rain water, preserved by the inability of the sand to draw it up to the surface when it would quickly be dried by the sun.

To test this point, we dug pits in the sand till free water was obtained, the depth varied from 2 to 6 feet. The salt in the water was estimated by standard silver nitrate solution. Samples were taken from different parts of the beach with the following results:

Plant growing Salt.
Cyperus arenareus 0·2—0·5 per cent.
Do. with Launæa pinnatifida 0·25 per cent.
Hydrophylax maritima 0·35 per cent.
Spinifex squarrosus 0·85 per cent.
(No plants) near sea 6·3  per cent.

It will be seen that except on the narrow stripe which is periodically inundated by the sea, the salt-content is very low. It should be noted that the water obtained at, say, 3 feet is not all water which has sunk down from above but some naturally free at that depth. The sinking is clearly seen when the pit is dug. It seems therefore that Spinifex squarrosus and other strand-formation species are not halophytes at all as suggested by Schimper (3) Warming at one time, (4) and others, but rather xerophytic psammophytes, depending for their water-supply on the rain-water and dew retained by the sand. The former, it may be noted, though it sinks through the surface layers almost or quickly as it falls, would not pass through the lower layers quickly, for the sand on these low beaches must be saturated at no great depth by the sea-water which has filtered through. As regards dew we have noticed, during the hot weather, in the early morning before sunrise, distinct deposits of dew on the seaward face of every little lump of sand, e.g., the sides of a foot-print and round plants, as if deposited when a slow moving moisture-laden breeze passing over the cooling sand was delayed by the small obstruction. The accumulated affect of this dew, slight as it is, would keep the sand below the surface damp and supply fresh water to plants whose root-hairs are near the surface.

Nor are strand-plants however ordinary xerophytic psammophytes. Psammophytes are as a rule deep rooted, and draw on deep-lying water for their supply. These strand -plants do not. Their roots do not grow deep, and in Spinifex at least the root-hairs are, as we have seen, close to the surface and depend apparently on water close to the surface of the ground. This water would, we have seen, he fresh. A certain amount of salt must be blown in from the sea as spray and this would be leached though the surface layers by the next shower of rain down to a certain depth. Perhaps this accounts for the formation of an exodermis and the absence of root-hairs on the lower roots of Spinifex squarrosus.

Finally there is another condition different from that of either a desert or sand-field. The air blown across from the sea is damp. It is never dry by the sea, except when the land breeze blows strongly it is damp close to the water. The strand-plants therefore are xerophytic in the sense of having to depend on very little fresh water, but in regard to the water lost by evaporation from the leaves have much less to fear than even mesophytic inland plants. They are surrounded by air as damp as that round a lake. They are not halophytes. They are not xerophytes in the ordinary sense, but subject to the peculiar condition of a shortage of water available to the roots, yet without liability to extensive loss from the leaves. Perhaps their chief physiological characteristic is therefore the ability to carry on metabolism with a minimum of water passing through the system.

As regards the xerophytism of strand-plants, Kearney (5) came to similar conclusion after analysis of the salt-content of the soil. He found that seashore sand contains less salt than some cultivated soils. Kearney insists on the xerophytic character of plants as being due to dry winds and dry sand. But a distinctly moist air seems in Madras to be the rule near the sea.

Literature Cited.

(1) Olssoh-Seffer. New Pbytologist VIII (1909), p. 38.

(2) Price, B. New Phytologist X (1911), p. 328 et seq.

(3) Schimper, A. F. W. S.B.K. Preuss, Akad, Wiss., 1890, pp. 1045-1062 extr. in J. R.M.S. 1891, p. 214. Plant Geography, p. 184.

(4) Warming E. (Ecology— Eng. Ed. (1909), p. 227.

(5) Kearney. T. H. Bot. Gazette, XXXVII (1904), p. 424.

  1. * Paper read at the Indian Science Congress, 1919.
  2. The use of pipes in this way has been objected to by Olsson-Seffer (1) on the ground that no lateral movement of the air enclosed in the pipe is possible as would, he considers, occur in nature, and that the free movement of the water would be prejudiced thereby. But a little consideration would show that in a homogeneous medium of practically limitless extent (as the sands of the seashore) there can be no balance of lateral air-movement inwards or outwards from any imagined volume, and that it is therefore per- fectly justifiable to draw conclusions from experiments with definite volumes enclosed in non-porous pipes.