INSIDE THE LEAF 



85 



in the palisades which surrounded the blockhouses of the 

 early settlers. This palisade, tissue is firm, but the 

 mesophyll beneath it is soft and spongy. In the spongy 

 mesophyll the cells are 

 loosely arranged ; there 

 are open spaces between 

 them. The stomates of 

 the under epidermis open 

 directly into large inter- 

 cellular chambers. (See 

 page S4 ; ) 



Air passes through the 

 many stomates of the 

 under epidermis. It dif- 

 fuses freely among the in- 

 tercellular spaces. From 

 it the cells of the meso- 

 phyll absorb the gases 

 that they need : oxygen 

 for respiration and carbon 

 dioxide forphotosyn thesis. 

 To it they yield other gases, chiefly water vapor, for the 

 water brought from the soil evaporates. The substances 

 which came with it do not evaporate. They remain and 

 are used by the leaf in its work. 



Thus together in the leaf we find these three things 

 necessary to its work — the soil materials, the air materials, 

 and the chloroplasts. And through the leaf the sunlight 

 strikes, yielding to it that energy whereby its work is done, 

 that force which is itself transformed and stored in the 

 finished product. For from sunlight green plants derive 

 energy which is stored in the food they make. 



Fig. 36. — Cross section of a leaf of a lily; 

 e, upper epidermis ; p, palisade tissue ; v, 

 vascular bundle or vein, showing xylem and 

 phloem ; i, an intercellular space with loose 

 mesophyll above it ; e f , lower epidermis 

 showing three stomates opening into inter- 

 cellular spaces. 



