LEAF STRUCTURE AND ASSIMILATION 127 



intercellular space it also shows chloroplasts, but the cross 

 walls between neighbouring cells may be bare of chloro- 

 plasts ; this is the case for the walls marching with the upper 

 epiderm. We see in this an arrangement which brings the 

 chloroplasts into the most favourable position for gas supply, 

 lying as they do against the walls through which they may 

 receive carbon dioxide by direct diffusion from the inter- 

 cellular spaces. The same features may be seen on the long 

 walls of the palisade cells. There is a distinct tendency 

 for the chloroplasts to be most abundant on the walls 

 bounding the air spaces. In Sempervivum the paHsade is 

 arranged in plates, with long narrow spaces between, and 

 it is on the walls next these spaces that the chloroplasts are 

 most abundant. 



In relation to light absorption this chloroplast distribu- 

 tion does not appear so favourable at first sight. It might 

 be thought that if the chloroplasts lay on the transverse walls, 

 a better utilisation of light would be obtained. We must, 

 however, look at the leaf as a whole. With such an arrange- 

 ment the upper cells would seriously shade the lower, and 

 they would also absorb far more light than, with the very 

 limited supply of carbon dioxide at their disposal, they 

 could utilise ; the actual arrangement permits a much 

 fuller utilisation of light. We must also remember that 

 light does not strike straight through the leaf, even in the 

 case of a leaf at right angles to the direct rays of the sun. 

 As it passes through cells, with walls lying in various 

 directions, and with heterogeneous contents of varying 

 refractive powers, it is reflected and scattered in all direc- 

 tions. In actual fact the chloroplasts do not really lie 

 edge-on to the light ; they receive the scattered rays from 

 all directions. At the same time any injurious effect of 

 direct insolation is minimised. The palisade arrangement 

 is therefore favourable for the individual chloroplasts, 

 and for the leaf as a whole, while at the same time the supply 

 of carbon dioxide is also dealt with advantageously. The 

 absence of chloroplasts from the epiderm may well be 

 connected with carbon dioxide supply, since the cuticularised 



