64 BOTANY OF THE LIVING PLANT 



outline (Fig. 46). The stomata form part of the epidermal layer, 

 but their cells differ in form from the rest. Each stoma is composed 

 of two guard-cells attached by their ends, so that between them there 

 is a pore that may be either open or closed. In a microscopic pre- 

 paration of a living epidermis taken on a sunny day, and mounted 

 in water, the pores, governed by the still living cells, will be seen to 



FIG. 46. 



Part of the lamina of Tropaeolum, seen as a transparency, in surface view from 

 above ; showing the sinuous outline of the epidermal cells, with stomata. Below 

 the palisade-cells are seen end-on, with large intercellular spaces, especially below the 

 stomata. The vascular veins are more deeply shaded. { x 175.) 



be widely open, as they are represented to be in the drawing of 

 the leaf of Tropaeolum. Access is thus readily given to the inter- 

 cellular spaces within. Fig. 46 further shows the mesophyll visible 

 through the transparent epidermis. The palisade cells are here 

 seen end-on; and it is more apparent than in transverse section 

 how well ventilated this tissue actually is. Almost all its cells touch 

 another cell laterally ; but in 1 all of them a very considerable propor- 

 tion of the wall-surface is freely exposed to the gases contained in 

 the intercellular spaces. These spaces are specially large in the 



