Page:The Journal of Indian Botany.djvu/806

298 THE JOURNAL OF INDIAN BOTANY. fied ground tissue in the wood and the narrowness of medullary raye, when present, are characteristic of desert plants.

Interfascicular wood prosenchyma is scantily developed in the herbaceous species of Violaceae, Malvaceae, Zygophyllaceae, Celastra- ceae, Ficoideae, Bubiaceae, Gompo sitae, Salvadoraceae, Gentianaceae, Boraginaceae, Convolvulaceae, Scrophulariaceae and Acanthaceae. It is altogether absent in Menispcrmaceae, some Capparidaceae, Caryophyllaceae and Bosaceae. It may be observed that interfasci- cular wood prosenchyma is abundantly developed and is composed of cells with thickened and lignified walls and with small lumina in shrubs and undershrubs ; and that in herbaceous species it is either altogether absent, or is scantily developed and is formed of cells with walls les3 thickened and with lumina much larger. The abundance of vessels is inversely proportional to that of interfascicular wood prosenchyma. The medullary rays are very narrow and are formed of thick-walled cells in shrubs and undershrubs. They are broader in herbaceous species of Menispermaceae, Tamariscineae, Bosaceae and Salvadoraceae. They are altogether absent in Cruciferae, Caryo- phyllaceae, Sapindaceae, some Compositae, some Amarantaceae and some Ghenopodiaceae.

In succulent 3pecies of Ghenopodiaceae the vascular system is composed of peripheral bundles traversing the aqueous tissue and of a central vascular cylinder. The former strengthens the aqueous tissue and brings about quick translocation of water in the aqueous tissue.

In Monocotyledons the peripheral vascular bundles are either apposed or embedded in the stereome tube or girders. Vascular bundles in Cyperaceae are enclosed in a ring of stone cells.

There are a number of anamolous structures which are perhaps brought about by the effects of environment and which are as follows : (a) Cortical vascular Bundles e.g. Crotalaria Burhia. (6) Medullary vascular bundles e.g. some species of Malvaceae and of Amarantaceae.

(c) A ring of larger vascular bundles in the medullary tissue

e.g. Nyctaginaceie.

(d) Isolated vessels in the medullary tissue e.g. Commelinaceae.

(e) Xylem bundles in the soft bast e.g. Simarubaceae and

Burseraceae. The development of a ring of large xylem bundles with large ves- sels in the pith of rhizomatous axes of Aerua tomentosa may be an adaptation to protect the axis against longitudinal pressure in the soil.