686 



PART IV. THE PHYSIOLOGY OF PLANTS. 



by the following considerations. The palisade-layers occur 

 always, beneath 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 imperfectly 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 promotes 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 dis- 

 posed on the surf ace- walls, both upper and lower, of the palisade- 

 cells (epistrophe). When, however, diffuse daylight is replaced 



by direct sunlight, the 

 position of the corpuscles 

 is changed (see Fig. 469) 

 so that their margin, and 

 not their surface, is pre- 

 sented 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 in- 

 tense 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 

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

 readily take place, is brought into direct relation with the external 

 air. Transpiration takes place from the cells of the spongy 



FIG. 469. (After Stahl). Sections of the phylloid 

 stem of Lemna trisulca, illustrating epistrophe and 

 apostrophe of the chloroplastids : A position in dif- 

 fuse daylight (epistrophe); B position in intense 

 sunlight (apostrophe). 



