PROTECTIVE TISSUES. 



129 



Fig. 30.— Simple papillae on petals of Viola tri- 

 color. (Wiesner). 



i 



occur as short unbranclied or branched, 

 conical hairs. In the leaf of Cheiran- 

 thes the trichoines are flattened, one- 

 celled or many-celled, scale-like hairs. 

 For pharmacognostical purposes plant 



hairs are of great importance, as they 



£ 



^ 



4 *\ 



^_l * 





ii 



4 .. 



i « 



form the diagnostic features by which 

 certain ground drugs, especially those 

 made from leaves, may be recognized, (a) 

 Glands are epidermal growths, gener- 

 ally various in shape, few to many- 

 celled, and containing excretions. They 

 will be discussed under the head of Sec- 

 retory Tissues. 



Fig. 32— Leaf of Mentha piperita showing up- 

 per surface. /^, simple hairi^. of, glandular hairs. 

 (Voffl). 



Water Pores and Stomata are modifi- 



L 



cations of the epidermis, but as they 

 make such a large part of the leaf 

 structure and have special physiological 

 significance they will be discussed under 

 the subject of Transpiration and Assim- 

 ilation. 



Secondary growths of the Epidermis. 

 In the stems of most plants the epider- 

 mis serves as a means of protection un- 

 til about the end of the first year's 



u 



in 



Fier. 33.— Upper 11 and under III, sides of leaf of 

 Altliffa showing star shaped hairs. (Vogl). 



growth; after this the increasing diam- 

 eter of the stem causes it to crack off in 

 varying degrees, thus leaving the tissues 

 beneath unprotected. Hence a secon- 

 dary growth, called the Periderm, takes 

 the place of the primary epidermis. This 

 periderm varies greatly, but in the main 

 it consists of two parts, meristematic 



Fig. 31.-Variou:sly shaped hairs. 



(a> Weiss. Die Pflanzen haare. 

 Botanische Untei-suchungeu vol. I. 

 Companitive anatomy. 



Kai'stens 

 DelJary, 



