102 THE BIOLOGY OF FLOWERING PLANTS 



carbon dioxide, and are most important in the exchange of 

 water vapour ; and (2) that diffusion through the stomata 

 takes place at a rate sufficient to satisfy the maximum 

 needs of the plant in assimilation, and according to physical 

 laws capable of exact expression. 



It might be asked what advantage in the way of protec- 

 tion from desiccation the leaf gains from its cuticle, if 

 transpiration sometimes takes place in quantities approaching 

 the amount of evaporation from a free water surface. We 

 must take three points into account, (i) The cuticle on 

 the exposed surface may be continuous, so that no transpira- 

 tion takes place here. (2) The presence of the epiderm 

 with its cuticle and stomata permits of great variety of 

 specific and individual adjustments to special conditions of 

 transpiration. (3) The stomata are capable of closure. 



§ 6. Stomatal Movements and their Mechanism 



The pore of the stoma is not normally permanently 

 open. Changes in the size and form of the guard cells lead 

 to diminution in the size of the pore, or to complete closure. 

 The changes are possible because of the peculiar structure 

 and properties of the guard cells, and are carried out in 

 response to changes in external conditions and in the water 

 relations of the leaf cells. In surface view the guard cells 

 stand out sharply from the other epidermal cells by their 

 smaller size, their shape, and the fact that they contain 

 chloroplasts. In a transverse section through the stomata, 

 the guard cells are seen to be smaller than the other epidermal 

 cells, and to have walls thickened in a unique manner. 

 (See Figs. 10 and 19.) Typically, the wall next the 

 pore is thicker than that which separates the cell from 

 the neighbouring epidermal cell. At the point of 

 junction with the latter specially thin strips may be 

 present, forming a sort of hinge. The wall next the pore 

 has often a thickened projection above and below, and a 

 bulge in the middle, so that the pore is divided into outer 

 and inner " courts." Frequently the thickening of the 



