250 LEAVES 



transpiration than the crosswise position. The direct ex- 

 posure of the leaf blade to the warmest and brightest rays 

 of the sun results in a higher rate of evaporation than if 

 the edge is turned to these rays. In many situations this 

 advantage is probably more important to edgewise leaves 

 than the advantage of protection from excessive light. 

 In some dry regions the leaves of many forest trees and 

 shrubs have the edgewise position. This is particularly 

 true in certain regions of Australia. 



Apart from protection from excessive light and excessive 

 transpiration, there is another distinct advantage to be 

 noted in connection with the edgewise position of leaves. 

 This is an advantage, not to one plant, but to a great 

 many plants taken together ; it is a collective advantage 

 rather than an individual advantage. Though the edge- 

 wise position results in minimum lighting for any one 

 leaf, it results in maximum lighting for the vegetation as a 

 whole. The more vertical the leaves, the more numerous 

 may they be in any given space and yet have sufficient 

 light to live. This position, which may seem bad in some 

 respects for one leaf, seems very good for all the leaves; 

 it results in " the greatest food production possible within 

 a given volume of leafage." (See Figure 88.) 



Certain plants with edgewise leaves are called compass 

 plants, for the reason that when they grow in the open 

 their leaves tend to point north and south. It is evident 

 that this position is more favorable for receiving the morn- 

 ing and afternoon rays of the sun than if the leaves pointed 

 east and west. The rosin-weed of the prairies is the best- 

 known compass plant of the United States, and the same 

 tendency in a less degree may be noted in the edgewise 

 leaves of the common wild or prickly lettuce. 



