Page:The Journal of Indian Botany.djvu/798

290 THE JOURNAL OP INDIAN BOTANY. So it can be said that intense heat and light arrest the development of the parenchymatous tissue, and that the arrest of the development of the parenchymatous tissue is followed by a proportionate development of clothing hairs. It can thus be observed that as transpiration increases, the development of parenchyma is arrested and clothing hairs are produced until the hairy covering is so dense, that it lessens transpiration towards which purpose all the adaptations of the desert plants are directed.

A dense covering of clothing hairs is of invaluable use to desert plants as it forms a non-conducting screen against heat and against the strong glare and sun-light reflected from the sand ; it also serves as a means of collecting moisture from the atmosphere at night. For instance a covering of woolly hairs in Bosaceae, or of flagellum-hairs in some Gompositae, or of candelabra hairs in some Amarantaceae forms an adequate means of collecting dew at night, which can trickle down the surface of the hairs and be absorbed by the basal cells. In cases of unicellular hair with a thin-walled bulbous basal portion in Menispermaceae, Boraginaceae and Gr amine ae, dew at night can be easily absorbed by the bulbous basal'portion. Clothing hairs, therefore, in many cases perform a double function that of forming a non- conducting screen against intense heat and light and that of absorb- ing moisture from the atmosphere.

Roughness of the walls caused either by muriculations, or warts, or papillae, as described above, forms a further evidence of the arrest of parenchyma ; and that it brings about the interlacing of clothing hairs, so as to produce a thick non-conducting screen of hairs. Clothing hairs, such as woolly, candelabra, flagellum or armed, are usually filled with air, so as to produce a silvery screen which is the best means for reflecting back the strong glare and sun-light in the desert.

For systematic investigations, much importance cannot be given to the characters of clothing hairs, as many types of hairs are found in more than one order, genus and species. It is found, for instance, that tufted hairs are characteristic of three allied orders, viz., Malva- ceae, Sterculiaceae and Tiliaccae ; so also unicellular and uniseriate hairs occur in four allied orders, viz., Scrophulariaceae, Acanthaceae, Verbenaceae and Labiatae. As regards their value in the diagnosis of genera, here again there are forms of hairs which are characteristic of more than one genus ; for instance uniseriate trichomes occur in Crotalaria, Bhynchosia, Phaseolus and Tephrosia. For diagnosis of species characters of hairs are of still less importance.

Hairiness varies with changes in the surroundings. Not only the density but also the shape and structure of the hair altogether