PLANT STRUCTURE I3 



Some control of the diffusion of gases into and out of the 

 leaf is achieved by changes in the aperture of the stomatal 

 pore, the stomata generally tending to close when the plant 

 is placed in the dark or in very dry air. Stomatal closure can 

 of course only affect the stomatal portion of the diffusion 

 path. Since this represents a much greater proportion of 

 the total diffusion path in the case of transpiration, partial 

 closure of the stomata might be expected to result in a greater 

 decrease in the loss of water vapour than in the uptake of 

 carbon dioxide. Such a differentiation between the two pro- 

 cesses has been observed by a number of investigators. 

 When the stomata are completely closed there remains the 

 exchange of water and carbon dioxide through the cuticle. 

 The cuticle is a complex of fatty acids, alcohols, and hydro- 

 carbons. It has not yet been shown whether its chemical 

 nature favours the passage of carbon dioxide against that of 

 water (as, for example, would a polythene layer). 



DETAILED STRUCTURE OF THE ASSIMILATORY CELL AND 



CHLOROPLAST 



In the mesophyll cell the cytoplasm and its inclusions 

 must be regarded as the seat of photosynthesis. The chloro- 

 plasts are clearly defined structures within the cytoplasm 

 and the photosynthetic pigments are confined to them. 

 When removed from the cell and seen in a compact mass 

 they appear almost black. 



The chloroplasts of the leaf cells are ellipsoidal or have an 

 unsymmetrical lens shape. They vary in size, the longest 

 axis often being between 3 and 5 [x although it may reach 

 10 // or be as small as 2 /,<. The number of chloroplasts per 

 cell varies from several hundred down to only a few, in 

 general there being a greater number in the cells of the pali- 

 sade than in those of the spongy mesophyll. In the algae 

 frequently each cell contains only one chloroplast, the shape 

 of which varies, e.g. the stellate chloroplasts of Zygnema, 

 the cup-shape in Chlorella, and the long spiral of Spirogyra. 



Chloroplasts usually appear to consist of a membrane 

 containing a 'stroma' in which 'grana' are distributed. It 

 is thought that the photosynthetic pigments are largely 



