FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 39 



foliage leaves and other green parts of plants which live 

 submerged in water, we find that the epidermis contains 

 no stomata. Therefore, stomata hold a definite relation 

 to green parts covered by epidermis only when this epider- 

 mis is exposed to the air. 



It would seem that the stomata supply open passage- 

 ways for material from the green tissue through the epider- 

 mis to the air, or from the air to the green tissue, or both. 

 It will be remembered, however, that quite a number of 

 substances are taken into the leaf and given out from it, 

 so that it is hard to determine whether the stomata are 

 specially for any one of these movements. For instance, 

 the leaf gives out moisture in transpiration, oxygen in 

 photosynthesis, and carbon dioxide in respiration ; while it 

 takes in carbon dioxide in photosynthesis, and oxygen in 

 respiration. It is thought that stomata specially favor 

 transpiration, and that they also much facilitate the en- 

 trance of carbon dioxide. 



3.2. Mesophyll. — If a cross-section be made of an ordi- 

 nary foliage leaf, such as that of a lily, the three leaf 

 regions can be seen in their proper relation to each other. 

 Bounding the section above and below is the layer of trans- 

 parent epidermal cells, pierced here and there by stomata, 

 marked by their peculiar guard-cells. Between the epi- 

 dermal layers is the green tissue, known as the mesophyll, 

 made up of cells which contain numerous small green 

 bodies which give color to the whole leaf, and are known as 

 chlorophyll bodies or chloroplasts. 



The mesophyll cells are usually arranged differently in 

 the upper and lower regions of the leaf. In the upper 

 region the cells are elongated and stand upright, present- 

 ing their narrow ends to the upper leaf surface, forming 

 the palisade tissue. In the lower region the cells are irreg- 

 ular, and so loosely arranged as to leave passageways for air 

 between, forming the spongy tissue. The air spaces among 

 the cells communicate with one another, so that a system of 

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