8 



PHOTOSYNTHESIS 



air spaces which penetrate throughout the leaf and provide 

 an internal air space with a consequent internal surface 

 many times greater than the external. The resulting struc- 

 ture allows gaseous diffusion (for which the coefficient of 

 diffusion is some 10,000 times greater than for water) up to 

 the wall of the assimilating cell itself. The structure of the 



FIG. 2.1. Transverse sections of an aerial (upper) and a completely 

 submerged (lower) leaf of Sagittaria sagittifolia. Note the 

 system of air spaces connecting with the external air via the 

 stomata in the aerial leaf; in the submerged leaf the absence 

 of stomata, the presence of chloroplasts in the epidermis and 

 the absence of cuticle. 



land leaf, with its development of an internal system of air 

 spaces, bears an obvious relation to the supply of gases by 

 diffusion. Fig. 2.1 illustrates transverse sections of a sub- 

 merged and an aerial leaf taken from a single plant of Sagit- 

 taria sagittifolia and illustrates some of the characteristic 

 differences in structure to which we have referred. 



