RELATION OF LEAF TO PHOTOSYNTHESIS 



147 



rage tissue 



parts of the leaf the carbon dioxide that has entered through 

 the lower surface. The border parenchyma cells deliver water 

 from the veins to the rest of the mesophyll, and receive food from 

 palisade and spongy cells and together with the phloem part of 

 the veins transport it out of the leaf. The conditions necessary 

 to photosynthesis are these: The photosynthetic apparatus 

 'must be present in working order; there must be light; the sto- 

 mata must stand open and admit 

 COj; the veins must bring water; the 

 veins must carry away the food or 

 its accumulation will hinder its 

 further construction. 



Fig. 78 represents in a general 

 way the cellular architecture of a 

 leaf. The leaves of different plants 

 of course vary from this in certain 

 details. , 



In the leaf of the India rubber 

 tree, Ficus elastica (Fig. 79), at the 

 upper surface the epidermis is suc- 

 ceeded by a clear tissue two to three 

 cell-layers deep which serves as a 

 water-storage tissue. Then follow 

 two to four layers of palisade cells, 

 several layers of spongy cells, two 

 to three layers of relatively small 



water-storage cells, and last the lower epidermis. In the spaces 

 between the veins where the palisade cells cannot communicate 

 directly with the food-conducting cells the spongy cells collect 

 the products of the palisade cells and deliver them to the veins 

 by the indirect route shown in Fig. 79. This device is by no 

 means peculiar to the rubber leaf, but is very generally employed. 



In Indian corn each of the veinlets is surrounded by a sheath 

 of border parenchyma cells, and these in turn by palisade cells 

 radiating from them, as shown in Fig. 80. In corn, as indeed 

 in many grasses, starch is not formed in the palisade cells at all. 



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Fig. 79. — Cross section through 

 a portion of rubber leaf, showing the 

 large percentage of water-storage 

 tissues on both sides of the leaf, 

 and the relation of the palisade and 

 spongy parenchyma to the lateral 

 veins. 



