40 PLANT RELATIONS. 



air chambers extends throughout the spongy mesophyll. 

 It is into this system of air chambers 'that the stomata 

 open, and so they are put into direct communication with 

 the mesophyll or working cells. The peculiar arrangement 

 of the upper mesophyll, to form the palisade tissue, has to 

 do with the fact that that surface of the leaf is exposed to 

 the direct rays of light. This light, so necessary to the 

 mesophyll, is also dangerous for at least two reasons. If 



St St 



Fig. 30. A section through the leaf of lily, showing upper epidermis (ue), lower epi- 

 dermis ile) with its stomata («0. mesophyll (dotted cells) composed of the palisade 

 region (p) and the spongy region (sp) with air spaces among the cells, and two 

 veins (v) cut across. 



the light is too intense it may destroy the chlorophyll, and 

 the heated air may dry out the cells. The narrow ends of 

 the cells present less exposure, and the depth of the cells 

 permits greater freedom of movement to the chloroplasts. 



33. Veins. — In the cross-section of the leaf there will 

 also be seen here and there, embedded in the mesophyll, 

 the cut ends of the veinlets, made up partly of thick- 

 walled cells, which hold the leaf in shape and conduct 

 material to and from the mesophyll (see Fig. 30). 



