84 THE BIOLOGY OF FLOWERING PLANTS 



consists of irregular cells between which are large air 

 spaces — the spongy parenchyma (Fig. 14). The combined 

 volume of the air spaces — the internal atmosphere of the 

 leaf — may be as high as 77 per cent, of the total volume of 

 the leaf, or as low as 3*5 per cent. : commonly, it is about 25 

 per cent. Over all stretches the epiderm of shallow cells, 

 fitting together to form a continuous covering, their outer 

 walls waterproofed to a greater or less extent by an im- 

 pregnation of cutin, and an exterior delicate cuticle of the 

 same waxy nature. 



The Stomata. — In the developing epiderm some cells 

 undergo special, regular divisions ending in the production 

 of two equal sausage-shaped cells lying side by side, with a 

 slit between, due to the dissolution of the middle lamella 

 of the dividing wall. The sht provides an opening between 

 the atmosphere and the intercellular spaces of the leaf. 

 The whole apparatus is the stoma, the slit is the pore, the 

 two cells the guard-cells (Fig. 10), Through the stomata the 

 main interchange of gases takes place, relatively little going 

 on through the cuticle and wall of the epiderm. The stomata 

 are typically most abundant on the lower leaf surface, and 

 here they lead into the extensive air spaces of the spongy 

 parenchyma. From these diffusion goes on to the palisade 

 parenchyma in which assimilation is most active, partly 

 owing to the more abundant chloroplasts, partly owing to 

 the arrangement of the cells, and their more favourable 

 position for absorbing light. Water passes into the leaf 

 and is distributed along the network of veins, finally diffusing 

 a short distance through a few living cells. The products 

 of photosynthesis diffuse through living cells towards the 

 veins, and so to the general conducting system of petiole 

 and stem. 



§ 3. Relation of the Stomata to Gaseous Exchange 



In the intercellular spaces of the leaf the air differs from 

 that outside. It has no constant composition, but is always 

 tending towards equilibrium with the atmosphere on the 



