THE LEAF 



7? 



outline (Fig. 49). 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. 49. 



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 epidermis 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. 49 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 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 



