94 THE ELEMENTS OF VEGETABLE HISTOLOGY 



fruits and occasionally seeds. The cell walls are 

 cutinized and, in certain hairs, secretory organs or 

 glands are present. The absence or presence of glands 

 is the primary basis of classification into non-glandular 

 and glandular hairs. The cells of a trichome may 

 be living or dead. Living hairs are characterized by 

 possession of protoplasmic contents and inclusions 

 within the cell. In dead hairs the cell cavities are 

 filled with air. Although cutinized cell walls are the 

 usual rule, there are instances where the walls of hairs 

 consist partly of lignified tissue;' trichomes of this 

 sort are apt to be thicker and more like bristles. 



Non-glandular Trichomes. The functions of non- 

 glandular hairs include protection against temper- 

 ature changes, excessive evaporation of water and 

 attacks of insects. The simplest type of non-glandular 

 hair consists of a slight projection of the exposed 

 epidermal surface. This form of hair is termed a 

 papilla (plural, papillce) and appears as a wart or 

 blister upon the epidermal surface (Plates 35-36). 

 On surface view papilla appear as thick-walled circles, 

 one within each epidermal cell. On sectional view 

 they appear as a series of elevations on strips of 

 epidermal tissue. 



Classification. Non-glandular hairs may be classi- 

 fied according to the number of cells in the hair, the 

 number of hairs in the group, the presence or absence 

 of branches and the arrangement of the cells in each 

 hair. This classification may be tabulated as follows: 



(a) Number of cells in the hair; 



unicellular multicellular. 



(b) Number of hairs in the group; 



simple compound . 



