48 THE ECOLOGY OF LEAVES 



The arrangement of the stomata seems to be largely a matter of 

 inheritance rather than environment. In the majority of mono- 

 cotyledonous plants the stomata are arranged on the leaves in more 

 or less definite rows and have their long axes extending lengthwise 

 of the leaves. This is true also of some dicotyledonous plants that 

 have long narrow leaves. In most dicotyledonous plants, however, 

 the stomata are scattered irregularly and have their long axes 

 extending in various directions. 



The question as to whether stomata shall develop at all, however, 

 is, to a certain extent at least, a matter of environment. The 

 majority of plants that have their leaves fully exposed to the sun 

 have stomata on the lower surfaces of their leaves only. This is 

 true of most trees and many herbaceous plants although there are 

 exceptions. Most grasses that grow in exposed places, for instance, 

 have stomata only on the upper surface, but this is the surface that 

 is most protected in dry weather when the leaves of these plants 

 roll. Plants that grow in the shade, on the other hand, usually 

 have stomata on both surfaces. In water plants, leaves that are sub- 

 mersed are without stomata while leaves of the same plant that are 

 above the water may have stomata on both sides. Floating leaves 

 commonly have stomata on the upper surface only. 



29. Protection from Excessive Transpiration.— It used to be said 

 frequently that transpiration is a necessary evil. Certainly excessive 

 transpiration is the greatest danger to which most plants are sub- 

 jected. Its necessity has been considered as due to the fact that 

 there must be openings for the interchange of gases in order that 

 respiration and photosynthesis may be carried on and if there are 

 openings of any sort it is not possible to entirely prevent transpira- 

 tion. In other words, it would be a simple matter to construct a 

 leaf that would not allow any transpiration but such a leaf would be 

 useless as a photosynthetic organ. It is now believed, however, that 

 transpiration is not entirely an evil but is often of considerable 

 importance to the plant because of its cooling effect. It takes a con- 

 siderable amount of heat to evaporate water and, just as evaporation 

 of water from a canvas water-!)ag keeps the water inside the bag 

 cool, so transpiration from a leaf prevents the internal temperature 

 from going so high as to prove disastrous to the protoplasm. This 

 is, of course, especially true t)f leaves that are ex-posed to the full 

 light and heat of the sim since a great deal of radiant energy is ab- 

 sorbed under such conditions. In spite of the obvious necessity of 

 transpiration, however, the presence of an adequate amount of water 



