FOLIAGE LEAVES. 



397 



position prevents excessive radiation of heat. The vertical or 

 profile position of the leaves of the compass plant already re- 

 ferred to not only lessens transpiration, but the intense heat and 

 light of the midday sun is avoided. This profile position is 

 characteristic of certain plants in the dry regions of Australia, 

 and the topmost leaves of tropical forests. 



IV. Relation of Leaves to Light. 



768. It is very obvious from our study of the function of the 

 foliage leaf that its most important relation to environment is 

 that which brings it in touch with light and air. It is necessary 

 that light penetrate the leaf tissue that the gases of the air and 



Fig. 437 

 Mosaic form by trailing shoots of Panicum variegatum, "ribbon grass." 



plant may readily diffuse and that water vapor may pass out 

 of the leaf. The thin expanded leaf-blade is the most economi- 

 cal and efficient organ for leaf work. We have seen that leaves 

 respond to light stimulus in such a way as to bring their upper 

 sides usually to face the source of light, at right angles to it or 

 nearly so (heliotropism, see Chapter XIII). How fully this is 

 brought about depends on the kind of plant, as well as on other 

 elements of the environment, for as we have seen in our study of 

 leaf protection there is danger to some plants in any region, 



