Palisade layer 



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Fibrovascular 

 bundle 



Guard cell 

 of stoma 



Epidermis 



Air space in 

 spongy tissue 



Stoma 



STRUCTURE OF LEAF 



Vessels of the fibrovascular bundles, the air spaces among the cells, and the stomata 

 in the epidermis act as channels through which the living cells inside the leaf com- 

 municate with lower parts of the plant and with the surrounding atmosphere 



Transpiration may also be of use to the plant indirectly, for the rapid 

 evaporation of water lowers the temperature of the plant. Sometimes in 

 the summer the sun comes out quickly after a shower. Then the moisture 

 left in the air may prevent transpiration, and as a result the sunlight is 

 converted into heat inside the leaf so rapidly that the protoplasm is injured. 



Both "breathing", or gas exchange, and transpiration appear to be regu- 

 lated by the guard cells of the stomata (see illustration, p. 143). 



Our Dependence upon Chlorophyl From careful chemical studies it 

 appears that plant cells make proteins when they receive, in addition to 

 carbohydrates, salts containing certain elements. Nitrates, for example, 

 contain nitrogen; phosphates contain phosphorus; sulfates contain sulfur; 

 and so on. A green plant can therefore produce its own food if it receives, 

 in addition to the water and carbon dioxide, a suitable supply of minerals 

 from the soil. Many plants without chlorophyl, such as molds and yeasts, 

 are also able to make proteins when supplied with carbohydrates and 

 suitable minerals. And we know that our own bodies as well as those of 

 other animals and of plants can transform starches and sugars into fats. 



The parts of a plant that have no chlorophyl (for example, the root or 

 the stem of a tree) are unable to make food substances out of inorganic 

 materials. They are nourished by materials obtained from the leaves. But 



141 



