Page:The Journal of Indian Botany.djvu/29



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