THE ANGIOSPERMAE : LEAVES 



1023 



Not only does the palisade layer possess the larger cell surface, but it also 

 contains a much greater number of chloroplasts than the spongy tissue. It 

 also occupies the better lighted side of the leaf, and for these reasons it must 

 be regarded as the principal photosynthetic tissue. 



At first sight the cylindrical form of the cells might not seem to be the 

 best fitted for the exposure of the maximal area of chlorophyll to the sun's 

 rays, towards which they present their narrow ends. \'ertical light, passing 

 through the epidermal cells is, however, refracted as we have seen above, 



Fig. 1012. — Helleborus niger. Epidermal view of the upper 

 surface of leaf showing the relationship of the ends of the 

 palisade cells to the epidermal cells. 



and passes obliquely into the cells below, thus ensuring the illumination of 

 the longitudinal walls of the palisade cells, and of the chloroplasts which 

 line them. The efficiency of illumination is thus high, and it is increased by 

 internal reflection from the cell walls, which carries the light still further 

 down in deep palisades. There is a general correlation between the depth 

 of the palisade layer and the intensity of the incident light. Leaves growing 

 under intense sunlight may have three or even more superimposed palisade 

 layers. Leaves in the shade have only one layer, and that of slight depth. 

 Indeed in deep shade the palisade may consist of funnel-shaped cells 

 with curved walls and with the apex downwards, the chloroplasts being 

 concentrated at the apex where the light is also concentrated bv internal 

 reflection from the cell walls, thus applying the principle of the searchlight 

 in reverse. 



We have described in Pimis (see p. 672) the flanged mesophyll cells 

 which are characteristic of that genus, and have suggested that they are due 

 to the constriction of the cells during growth by cuticularized rings on the 

 walls. Similar flanged or lobed palisade cells are not uncommon in Angio- 

 sperms. The H-shaped type is perhaps the commonest, and occurs widely 

 among the Ranunculaceae. Whether it originates in the same way as in 



