no THE BIOLOGY OF FLOWERING PLANTS 



marked off not only from the other epidermal cells, but also 

 from the mesophyll. It may be that special permeability 

 relations enable it to obtain and retain an excessive amount of 

 carbohydrates ; the chloroplast may function primarily as 

 a starch-building and not as an assimilating organ. In light 

 the equilibrium between starch and sugar is pushed much 

 further towards the sugar side than is usual. This may 

 be due to changes in the acidity of the cell induced by light. 

 The precise effect of the chlorophyll content is obscure. 



When we ask of what use the stomatal movements are 

 to the plant, we find ourselves in a region where opinions 

 have differed greatly. Formerly stress was laid on the 

 stoma's ready reaction to changes of atmospheric humidity, 

 or to changes in the moisture content of the plant ; the 

 apparatus was looked on as providing a delicate means of 

 cutting down transpiration when too great a loss of water 

 was threatened, and so averting the danger of v/ilting. 

 Recently this view has been combated. We will deal with 

 the evidence later. It may be pointed out here, however, 

 that the stoma responds most readily to changes of illumina- 

 tion, not of humidity, and that its normal condition is open 

 through the day and closed at night. Both facts emphasise 

 its close relation to the supply of the raw material of photo- 

 synthesis, and tend to discount a primary connection with 

 limitation of transpiration. Nor does behaviour on wilting 

 favour the viev/ that the stomata can save the plant from 

 excessive transpiration. The meaning of the closure at 

 night is not clear. It might lead to an accumulation of 

 respiratory carbon dioxide which would then be available 

 for assimilation in light. 



§ 7. Gaseous Exchange of Aquatics 



Submerged water plants draw their supply of carbon 

 dioxide from gas dissolved in the water. As they have a 

 very fine and permeable cuticle, water and dissolved sub- 

 stances can pass freely through the external cell walls over 

 their whole surface. Diffusion is the easier as the leaves are 



