Transpiration and Water Movement 89 



role, as later developed. They usually open into a sub- 

 stomatal cavity which, in turn, is in communication with 

 the intercellular spaces, or aeriferous system. Since the 

 leaves are the organs commonly active in transpiration, 

 it is necessary to note the structure in a typical case. 



In Figure 26 there is shown a cross-section of the leaf of 

 tomato. There is a single epidermal layer (e) on each 

 surface, a single palisade layer (p), and the mesophyll or 

 leaf parenchyma. Small veins, or fibrovascular bundles, 

 in cross and longitudinal section are also shown. In the 

 lower epidermis there are several stomata. Many leaves 

 show a multiple palisade, and there is considerable diver- 

 sity generally in the form and compactness of the tissues. 



Each cell of the leaf is directly or indirectly in contact 

 with the air spaces, and ultimately with the substomatal 

 cavities, so that the mechanism is a physical system per- 

 mitting diffusion. The protoplasm of each cell is thor- 

 oughly penetrated with water ; it is in contact with the 

 penetrable cell- wall, an imbibition membrane, which is 

 therefore moist. From such moist membranes water- 

 vapor passes into the intercellular spaces, which have a 

 tendency to become saturated. Under external conditions 

 favorable for evaporation there is a high gradient with 

 respect to the external air, so that water-vapor diffuses 

 rapidly from the substomatal cavit y through the stomata. 



As a result of the work of Brown and Escombe it is clear 

 that the stomatal system in such a plant as the sunflower, 

 for example, constitutes an extremely efficient multiper- 

 forate septum, the form and distance apart of the stomata 

 commonly permitting, when they are open, a diffusion 

 almost as rapid as though there were open space. This 



