MORPHOLOGY 



105 



Fid. 118. Water-pore from the margin of a leaf of 

 Tropaeolum majus, with surrounding epidermal 

 cells, (x 240.) 



Although submerged leaves of aquatic plants are devoid of air- 



stomata, water-stomata often occur on them. 



Hairs or TRICHOMES and EMERGENCES are characteristic of the 



tegumentary system. The cells of the hairs may retain their living 



contents, or die and become 



filled with air ; in the latter 



case the hairs appear white. 



The simplest form of hairs are 



the PAPILLAE, which are merely 



epidermal cells, the external 



walls of which have protruded 



in a conical form. Papillae are 



often developed on the petals 



of flowers, and are the cause 



of their velvety appearance 



(Fig. 1 14). Longer hairs, such 



as the root hairs (Fig. 170 r), 



are also prolongations of single 



epidermal cells ; these are 



characteristic of definite 



regions of the root, and only 



a limited portion of the outer 



wall of the epidermal cell 



protrudes as a hair. The woolly hairs found in young buds are 



generally similarly prolonged . epidermal cells which, as a protec- 



tive covering, surround the young growing tissues and some- 



times remain on fully developed parts of plants to protect them 



against too rapid evaporation 

 and direct insolation. The 

 hairs developed from some of 

 the epidermal cells of the seed 

 coats of various species of Gossy- 

 pium attain an unusual length, 

 and supply the cotton of com- 

 merce (Fig. 115). These cotton 



hairs are Sometimes 6 Cm. long, 

 an( j in ^^ f u |i y developed State 

 . . i n n 



contain only air ; their cell walls 

 are thicker than those of ordinary 

 hairs, and covered with a delicate cuticle. They are usually some- 

 what flattened and at the same time twisted ; and are wider in the 

 middle than at either end. Only a certain number of the epidermal 

 cells of the seed grow out as cotton hairs (Fig. 115 BJ. BRISTLES 

 are short, pointed hairs, in the thickened cell walls of which calcium 

 carbonate or silica has been deposited (Fig. 116, below, to the right). 

 The STINGING HAIRS (Fig. 116), such as those of Nettles (Urtica) 



FIG. 114. Surface of the upper epidermis of a petal 

 of Viola tricolor, showing ridge-like projections 

 from the lateral walls, and protruding papillae. 



(x 250 ) 



