52 THE EPIDERMIS 



The inurphological distim-tioii between trielioiiie>s and emergences 

 is of further interest in those eases where hairs or scales are 

 borne upon an emergence (Cooper, 1932). 



Trichomes furnish a rich fiehl for morphogeiietic investiga- 

 tions because of their great diversity and because their super- 

 ficial position and relatively simple structure facilitate onto- 

 genetic studies with living material. As an introduction to the 

 problems in this field, a brief characterization of the four com- 

 monest "types" of trichomes is now given, viz. : 



1. Hairs. In form, hairs are thread-like in appearance and 

 are either uniceUular (e.g., root hairs) or muliiceUulay. The 

 latter type of hair may consist of a single series of cells, terminat- 

 ing in an acute terminal cell or a (jlandular cell; or the hair may 

 be branched in various ways. In some jilants, the hairs are 

 composed of several layers of cells and are termed shag-haivs. 

 Such multiseriate hairs are often borne upon an emergence 

 (De Bary, pp. 64-6."), and Pig. 21c). Two general regions may 

 be distinguished in a hair, viz.: (1) the foot, which is the por- 

 tion lying within the epidermal surface and Avhich is often dif- 

 ferent in form from the adjacent epidermal cells, and (2) the 

 ho(h) which is the portion extending away from the epidermal 

 surface. Occasionally, a given epidermal surface develops but a 

 single type of hair (e.g., root hairs). ]More commonly, especially 

 in leaves, several different morphological types of hairs occur 

 side by side on the same ei:)idermal area. 



2. Scales. These trichomes consist of a plale of cells and arc 

 eithei- pelf ale (as in certain angiosi)ernis ) or are attached to llic 

 epidei'mis only a1 one edge (e.g., tlic chaffy scales or /xthai so 

 characterisi ic of numy ferns). Scales in the gcnci-a SJieplu rdia 

 and ElaeagnuH ai'e so closely crowded on the surface of the young 

 stem and 1h(> leaves tlial Ihcy i)ro(]ucc a lypical "scurfy" 

 appearance. 



3. Collefers. On many foliar oi-gans, ])ailicnlai-ly on bnd 

 scales and stipules and on Ihc foliage leaves of certain genera 

 (e.g.. Rho(l(Hl( n(lro)i, Carifa) ghnidnlai- Iriclionics occur. These 

 structures W(>re oi-iginally termed collcters by Ilanstein (1868). 

 Colleters consist of a short, often niulticellnhir stalk bearing an 

 expanded disc oi- knob of secretory cells. The characteristic 



