no BIOLOGY AND HUMAN LIFE 



teria, molds, and yeasts and probably wheat and other green plants are 

 capable of making proteins by using nitrogen from the atmosphere ; this 

 may prove to be a matter of great practical importance. 



87. The leaf. The common green plants carry on photo- 

 synthesis in special organs, the leaves. The most common fact 

 about a leaf is that it is flat and comparatively thin/ Some 

 leaves have stalks, or pedicels, and all have veins running 

 through the flat portion, or blade. They differ as to the charac- 

 ter of the edge. Some are smooth, whereas others have wrinkled, 

 uneven surfaces. Some kinds are hairy, while others are quite 

 bald. Even the color of leaves is not uniform, for the chloro- 

 phyl varies in density, and in some plants the appearance is 

 modified by other coloring matters, the hair, etc. (Fig. 63). 



88. Work of the leaf. The structure of a leaf is shown in 

 Fig. 61. The oxygen given off by the cells passes into the air 

 spaces and diffuses from these to the exterior by way of the 

 stomates (see Fig. 64). The skin cells are not directly con- 

 cerned in the w'ork of starch-making. Their function may be de- 

 scribed as protective. They protect the delicate pulp cells against 

 mechanical injuries and the whole plant against the loss of water. 



89. Transpiration. The loss of water is perhaps the most se- 

 rious danger to which most plants are exposed, since more 

 plants die from the results of wilting than from any other one 

 cause. And yet transpiration, as this evaporation from the 

 leaves is called, may be of use to plants indirectly. 



The rapid evaporation of water results in lowering the temperature of 

 the plant. If conditions interfere with transpiration, the temperature of 

 leaves exposed to sunshine increases so rapidly that the protoplasm is 

 sometimes killed. This kind of occurrence may be observed in the sum- 

 mer time, when the sun comes out quickly after a shower that has left a 

 great deal of moisture in the air. The moisture in the air prevents tran- 

 spiration ; the sunshine is largely converted into heat inside the leaves, 

 and as a consequence the protoplasm is injured. 



iJn some plants leaves depart considerably from this model. Some leaves 

 are nothing more than fine hairs, as on certain cactuses; others have exten- 

 sions that behave like tendrils; and some are spines. Certain plants have leaves 

 that are more or less active in capturing animal food. 



