90 FUNCTIONS OF HAIRS [CH. 



jointed (Tradescantia) : such hairs may also be silky, 

 velvety, woolly, &c. as before. Silky hairs are also often 

 met with in the shape of a T with a short stem, as in 

 many species of Scabious, Aster, Artemisia, where the 

 upper cell of the series grows out in two opposite directions 

 (Fig. 55, c) : such hairs are common in Cruciferae, where 

 the hairs are also often stellate, or forked, owing to the 

 transverse growth occurring in three or more directions 

 e.g. Alyssum, Aubretia, Draba (Fig. 52,/ 1 ). In other 

 cases the outgrowths occur as peltate scales. 



Branched hairs are often divided by septa in directions 

 perpendicular to those referred to above, and we have 

 then forms such as tufted hairs (Potentilla, Cistacece), 

 scales (Hippophae and Elceagnacece), the furfuraceous hairs 

 of Bromelias, the stellate hairs of Phlomis, Verbascum, 

 Gorrea (Fig. 55, d), or the branched flocculent hairs of 

 Verbascum, and so on. 



In most of the cases referred to, the hairs, when mature, 

 have lost their living contents, and consist of merely the 

 cell-walls filled with air, and their functions are prin- 

 cipally those of protection against unduly rapid variations 

 of temperature and moisture : they cover the leaves or 

 other organs with a non-conducting layer of entangled 

 air and cellulose which acts exactly as a layer of cotton- 

 wool (the long hairs of Gossypium) would do (Fig. 53, d). 



In very many cases, however, the cell-walls of such 

 hairs are thickened and strengthened in various ways, 

 either by mere thickening of the cell-walls, as in the 

 bristles or setce of many Boraginece, &c., or with deposits 

 of silica (Urtica, Boehmeria), calcium carbonate (many 

 Boraginece). 



In another large class of cases, however, the hairs 

 preserve their living contents, and their functions often 

 depend especially on the continued activity of these. 



