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 develop- 

 ment 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 



