172 



PART III. PHYSIOLOGY. 



[41 



The internal structure of the leaf is in direct relation to these 

 two functions (see p. 114). The particular significance of the form 

 and arrangement of the cells of the mesophyll is made clear by the 

 following considerations. The palisade-layers occur always, be- 

 neath the epidermis, at those surfaces which are directly exposed 

 to the sun's rays. Further, if a plant which, when grown exposed 

 to sunlight, has well-marked palisade-layers in its leaves, be grown 

 in the shade, it will be found that the palisade-layers are imper- 

 fectly differentiated, even if they can be detected at all. The 

 development of the palisade-layers is clearly a peculiarity of leaves 

 which are exposed to sunlight. One explanation is this, that bright 

 light not only promotes the assimilatory function, but also pro- 

 motes the oxidation and decomposition of the chlorophyll. The 

 palisade-tissue affords a means of protection from the latter effect. 

 When a leaf-surface is exposed to diffuse daylight, the position of 



the chlorophyll-corpuscles 

 in the palisade-cells is such 

 as to expose them as fully 

 as possible to the light ; 

 they are disposed on the 

 surface-w T alls, both upper 

 and lower, of the palisade- 

 cells (epistrophe). W T hen, 

 however, diffuse daylight 

 is replaced by direct sun- 

 light, the position of the 

 corpuscles is changed (see 

 Fig. 124) so that their 

 margin, and not their sur- 

 face, is presented to the 

 sun's rays ; they are removed to the lateral walls and towards the 

 inner end of the cell (apostrophe). It is clear that the elongated 

 form of the cells facilitates this withdrawal of the corpuscles from 

 too intense light, to light of a degree of intensity which promotes 

 the assimilatory function to the utmost extent compatible with a 

 due economy of the chlorophyll. 



The spongy portion of the mesophyll is the tissue especially 

 adapted to the transpiratory function. By means of the large 

 intercellular spaces which form a system of channels throughout 

 this tissue communicating with the external air by means of the 

 stomata, a very large cell-surface, from which transpiration can 



FIG. 121. (After Stahl). Sections of the thalloid 

 stem of LenvL tnsulca, illustrating epistrophe and 

 apostrophe of the chloroplastids : A position in dif- 



B position in intense 



