MATERIAL FOR STUDY OF TRICHOMES 53 



sticky exudation found on certain foliar structures is secreted 

 from colleters (cf. Foster, 1929. pp. 457-458). 



4. Water vesicles or bladders. Triehonies of this type con- 

 sist of greatly distended epidermal cells which presumably are of 

 physiological importance as reservoirs for water (cf. Ilaber- 

 landt, p. 116). In the so-called "Ice Plant" {Mesemhrijanthe- 

 mum crystallhium) , the water vesicles are so large and so numer- 

 ous that the leaves and young stems appear to be covered with 

 minute translucent beads of "ice." 



VI. Material for the Study of Trichomes.— 



1. Hairs. Obtain thin trans-sections of the petiole of the ger- 

 anium (Pelargonium) and after mounting them in water/ study 

 carefully the structure of the unicellular and the multicellular 

 unhranched hairs. Note the relatively thick outer walls of the 

 body of these hairs. The foot, especially of the multicellular hair, 

 consists of an enlarged bulbous cell, separated by a transverse 

 wall from the body, and surrounded by a circular group of more 

 or less elevated subsidiary cells. The true relationship of th& 

 subsidiary cells to the foot of the hair is clearly seen in thick 

 trans-sections of the petiole as well as in strips of epidermis 

 removed from the lower surface of the lamina. Note that cer- 

 tain of the hairs are (jlandular, terminating in a single large 

 secretory cell filled with dense, yellowish-brown ergastic mate- 

 rial. Excellent material for a study of multicellular branched 

 hairs is atforded by the leaves of various members of the Ma\- 

 vaceae, where typical stellate hairs occur. Each hair of this type 

 consists of a number of radiating unicellular "branches" which 

 have arisen from the subdivision of a single epidermal cell. Fur- 

 ther illustrations of multicellular branched hairs are provided 

 by the leaves of mullein (Verbascum Thapsu.'^). Scrape a small 

 amount of hairs from the leaf into water, carefully tease them 

 apart with dissecting needles and examine under low magnifica- 

 tion. Note that these complex hairs are "dendroid" in form; 

 i.e., each hair consists of a main "axis" (made of a vertical series 

 of cells) and whorls of radiating unicellular or bicellular 

 "branches." Trans-sections of very young mullein leaves are 



1 Cf. Appendix, pp. 130140. 



