FOLIAGE LEAVES. 



397 



position prevents excessive radiation of heat. The vertical or 

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

 ferred to not onlv 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 regi(jns of Austraha, 

 and the topmost leaves of tropical forests. 



IV. Relation of Leaves to Light. 



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



foHage leaf that its most important relation to en\'ironment is 



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



that light penetrate the leaf ti>sue that the gases of the air and 



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



plant mav readily diffuse and that water vapor may pass out 

 of the leaf. The thin e.xpanded leaf-ljlade is the most economi- 

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

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

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

 nearlv so Qieliotropism, 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 studv of 

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



