Page:The Journal of Indian Botany.djvu/538

106 THE JOURNAL OF INDIAN BOTANY. very large and with vessels large and many and the other two small with vessels small and few. Soft bast forms groups on the outer side of xylem bundles.

The central wood cylinder occupies nearly the whole of the pith. It is, I should say, secondarily developed when the axis took to a rhizomatous habit of growth ; it affords mechanical support against longitudinal pressure. In a rhizomatous axis the mecha- nical tissue, when necessary as a protection against longitudinal pressure cannofe develop in any other form than in the form of a central cylinder. This is analogous to what is seen in the case of roots. Abundance of very large vessels is necessary on account of greater abundance of moisture in the soil than in the dry air outside. This affords an example of the great plasticity of plant structures which can admirably adapt themselves to changes in their sur- roundings.

Another anamolous structure is presented in the form of groups of water-storing tracheids in the middle of the cork-tissue, with clustered crystals in their neighbourhood. The occurrence of water- storing tracheids in the cork tissue can be accounted for by the abundance of lenticels.

The fact that the conjunctive tissue of medullary rays comes into direct contact with the cork tissue at certain spots where the medul- lary rays are very broad may be accounted for as means of bringing the lenticel tissue into direct contact with the water conducting tissue and thus increasing transpiration, as there may be abundance of water in the rhizomatous axis with numerous large vessels and with numerous water-storing tracheids.

The occurrence of vascular bundles in the pith of A. aspera forms an anamolous structure. They may be necessary for supplying the extensive thin-walled pith tissue with water and also for strengthening the same against the winds of the desert, as they are developed in a plane which corresponds with the direction of the prevailing wind.

General Review. — Epidermis consists of tabular cells with outer and inner walls thickened and convexly arched outwards and inwards respectively. Stomata are accompanied by ordinary epidermal cells. Hairy covering consists of candelabra hairs or of uniseriate trichomes, with walls smooth or muriculate. Collenchyma is developed at the angles. The pericycle consists of groups of stone-cells. Wood is composite. Medullary rays are absent except in the rhizomatous axis of A. tomentosa. The pith consists of thick-walled or of thin- walled cells. Some of the members present many anamolous struc- tures which prove the remarkable plasticity of their tissues.