TRICHOMES 



125 



Fig. 34. 



T-sbapcd unicellular hair [Malpighian hair] from the 

 abaxial leaf-surface of Cheiranthus Cheiri. After Ue Bary 

 (Com j}. Amit.). 



C. APPENDAGES OF THE EPIDERMIS. 71 



Ill most plants a certain proportion of the young epidermal cells 

 grow out either singly, or less often in groups, to form uni- or multi- 

 cellular appendages of. diverse shapes, which are included under the 

 general categories of hairs or trichomes. While these epidermal 

 outgrowths most frequently take the form of ordinary hairs, they may 

 also develop into scales, prickles, warts or vesicles (bladders). 



One can obtain a rough idea of the extraordinary variety of shapes 

 which trichomes may assume by reflecting, first, that there are only a 

 very few groups of plants (e.g. the Potamogetonaceae and Lemnaceae 

 among Angiosperms) in which these structures are entirely or almost 

 entirely absent ; secondly, 

 that there is hardly any 

 physiological function in 

 connection with which hairs 

 cannot be utilised ; and, 

 finally, that one and the 

 same ors-an often bears 

 several different kinds of 

 epidermal appendages. 



In its simplest form a 

 trichome merely represents a tubular outward prolongation of an 

 epidermal cell. This condition is illustrated by the Campanulaceae 

 and Cruciferae; in the latter family the hairs often fork or 

 branch repeatedly, but without undergoing septation (Fig. 34). In 

 most cases, however, the formation of a hair is accompanied by cell- 

 division. The first wall laid down usually separates a basal portion or 

 foot, which remains embedded in the epidermis, from the body of the 

 hair or hair proper. Sometimes no further divisions take place ; more 

 often, however, the hairs ultimately come to consist of several, or even 

 of numerous cells, and assume the form of branched or unbranched 

 cell-filaments, of cell-plates (scales), or even of bulky cell-masses (villi 

 or shaggy hairs, warts and prickles). The foot of a trichome may lie 

 at the same level as the rest of the epidermis, or it may be sunk in a 

 small depression, or, on the contrary, raised up on an emergence 

 composed of subepidermal tissues. Hairs which are directed obliquely 

 to the surface are often provided with a little pad or cushion, situated 

 on the convex or on the concave side of the base ; this pad is usually 

 made up of some of the adjoining epidermal elements, but may in 

 certain cases also include a few subepidermal cells. Eenner asserts 

 that such basal cushions serve to effect the erection of hairs which at 

 first lie closely appressed to the leaf, stem [or other organ]. It is, 



