142 ELEMENTS OF STRUCTURAL BOTANY 



and carpels the general term phyllome is applicable. 

 The characteristic of the phyllome is that it is a lateral 

 outgrowth of the stem or its branches. 



226. The term trichome, on the other hand, is applic- 

 able to any hair-like appendage on the surface of the 

 plant generally, whether of root, stem, or leaf. The 

 commonest form of trichome is the hair. The root- 

 hairs which generally clothe the surface of young roots 

 are of great importance as absorbing agents. Each 

 root-hair consists of a single, delicate, tube-like cell with 

 extremely thin walls. Other hairs may consist of several 

 such cells placed end to end. Others, again, may branch 

 extensively. It sometimes happens that the terminal 

 cell of a hair produces a gummy substance which comes 

 away with the slightest touch. The sticky surfaces of 

 many common plants are due to the presence of such 

 hairs, which are then described as glandular. Gummy 

 matters are also secreted by glands close to the surface 

 of the plant. Peltate hairs are occasionally met with, 

 as in the leaves of Shepherdia. They give a scurfy 

 appearance to the surface upon which they grow. Then 

 there are hairs which secrete odorous fluids, as, for 

 example, those upon the surface of the Sweet Brier- 

 These probably serve to attract insects. Stinging hairs 

 are also common. They contain an irritating fluid. 

 When the point of the hair pierces the skin it is broken 

 off, and the fluid then escapes into the wound. 



227. Besides the trichome forms just mentioned, there 

 are also bristles, formed from hairs by the gradual thick- 

 ening and hardening of their walls, and prickles, such as 

 those of Sweet Brier (Fig. 199), which consist of many 



