HAIRS 103 



are the' source of cotton. 1 They are unicellular and may reach 

 a length of 2-5 centimetres and, in the plant's economy, serve 

 for the dispersal of the seeds by the wind. Similar hairs, with 

 the same function, occur on the seeds of the Willow, Willow- 

 herb, and many other plants. 2 



The epidermal cells of many petals are drawn out into very 

 short processes, or papilla, which are the cause of the velvety 

 surface and prevent wetting. A similar production of papillae 

 is not uncommon on the stomatal surface of the leaves of tropical 

 plants growing in damp situations (e.g. tropical rain-forest). 



Branched hairs assume very diverse forms, only a few of 

 which can be mentioned. In the Mullein and Plane they are 

 tree-like, and consist of an erect multicellular axis from which 

 numerous tapering branches radiate at 

 intervals. More frequent are so-called 

 stellate hairs, in which there is but one 

 set of radiating branches terminating a 

 short stalk which, however, is some- 

 times practically absent (e.g. Deutzia). 

 Multicellular hairs of this kind are 

 typical of the Lime-family (Tiliaceae) 



and Mallow-family (Malvaceae), whilst FIG. 49 . Peltate hair of 

 .... ' , . Sea Buckthorn (Hit- 



unicellular ones are exemplified in p pha rhamnoides}. 



Deutzia (Fig. 48, A). Stellate hairs are 



also found in the Cruciferae, but here they are accompanied by 

 simpler forms with only two or three branches and by un- 

 branched hairs. 



A very efficient transpiration-check is afforded by the peltate 

 hairs, which are well seen in the Sea Buckthorn (Hippopha, 

 Fig. 49), and in Elczagnm, where, owing to the large number of 

 these scale-like structures, the under-surface of the leaf has a 

 characteristic silvery appearance. The short stalk of these hairs 

 is surmounted by a horizontal expansion, consisting of a large 



1 The walls of these hairs are practically pure cellulose. Many hairs, 

 which from their length might be used for the manufacture of textiles, 

 are slightly lignified, and consequently too brittle to spin. Kapok of 

 commerce is obtained from the slightly cuticularised hairs lining the fruit 

 of the Silk-Cotton tree (Eriodendron anfraotuosum), winch is common in 

 the Tropics. 



2 See F. & S., p. 286. 



