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. 



