146 



CONSTRUCTION OF PLANT'S FOOD 



are not easily made out in cross sections of leaves, but in sec- 

 tions parallel to the leaf surface (tangential sections), or when 

 looking through a bleached leaf with a microscope, they appear 

 as in Fig. 77, where every palisade cell borders on one or more 

 of them for a part of its surface. This general view of leaf anat- 

 omy shows how all the parts are related in the interest of 

 photosynthesis. 



Palisade parenchyma 



"II 



»1 ^ 



Fig. 78. — Diagram to show the architecture of a typical leaf in the region of one of the 

 lateral veins. The shaded parts amongst the palisade and spongy parenchyma represent 

 intercellular spaces. 



It will be well now to recapitulate briefly the main facts in 

 what has thus far been told about the leaf: The epidermis is 

 transparent and lets the light through. The chloroplasts in 

 the palisade cells absorb most of the light and use approxi- 

 mately 2 per cent, of its energy in carrying on food synthesis. 

 Light that escapes through the palisade parenchyma is arrested 

 so completely by the spongy tissue that not enough goes on 

 through the leaf to be useful to other leaves. The intercellular 

 spaces of the spongy parenchyma receive and distribute to all 



