ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES. 



229 



tion of water is not veiy considerable. But it often happens that the uppermost 

 cells of a septate hair are developed into absorption-cells. The terminal cell is then 

 usually spherical or ellipsoidal and larger than the rest, or else this cell is divided 

 into two, four, or a greater number of cells, which together form a little head, whilst 

 the lower cells constitute a stalk supporting it (see fig. 53 ^ and 53 *). In botanical 

 terminology structures of this kind are named capitate or glandular hairs. The 

 protoplasm in the cells of the head is, for the most part, of a dark colour, and the 



Fig. 53.— iHairs from stem of SfeZtona media; xllO. 2 Lowest cells of the same hairs ; x200. s Capitate hairs ot 

 Centaurea Balsamita; xl50. * Capitate hairs of Prfarfiromurn Zmdmn. ; x 150. 



cell-membranes are readily permeable by water, which is attracted with great 

 energy by the cell-contents. The cell-membrane is often very thick, it is true, but 

 as soon as water comes into contact with it the outer layer is discarded, the inner 

 layers swell up and the water passes through these swollen layers into the interior 

 of the cell. This happens, for instance, in many pelargoniums and geraniums, 

 wherein the capitate cells go through a process of excoriation on every occasion of 

 the imbibition of water (see fig. 53"^). In other plants the walls of the capitate 

 cells are everywhere thin, and not only do the cell-contents consist of a viscid gum- 

 like mass, but the external surface of the wall is also covered by a layer of viscid 

 excretion. In many cases the viscid matter excreted by the glands spreads over the 

 entire surface of the leaf, so that the latter feels sticky and looks as if it were 



