160 THE pla:!^t world. 



the transpiration of equal areas of leaf surface and cortical sur- 

 face the former was from 214 to 3 times as great. In such forms 

 as the Scotch broom, Cytisus scoparius, the total transpiration 

 per hour of tlie leaves during the leafy season is more than three 

 times as great as the total cortical transpiration. It is a signifi- 

 cant fact that thrifty plants of Spartium junceuni which in any 

 given season do not produce leaves, grow hardly at all, though 

 they usually blossom and fruit. 



Cliffside plants were found to be in many cases ill provided 

 with means of resisting the extreme conditions to which they are 

 exposed during the summer drought when the total soil moisture 

 may fall as low as 2 per cent. Some of these plants seemed to 

 be occupying almost bare rock surfaces, simply because the coin- 

 petition there was less severe. When found growing in ordinary 

 soil these species were much larger and more vigorous. Import- 

 ant means of protection from dessication are shedding the leaves 

 or even sacrificing a large portion of the younger shoots toward 

 the end of the summer. The value of parting with the leaves as 

 a means of reducing transpiration is well shown by the fact that 

 a very characteristic cliff-side shrub, Medicago arhorescens, in its 

 leafless and nearly dried condition loses less than 3 per cent, of 

 the water per hour that it lost when fully leaf clad in early spring. 



The leaves of evergreen trees and shrubs were found to 

 differ greatly in longevity, their duration ranging from about 15 

 months in some species to more than 30 months in others. The 

 extreme age limit for those which last more than two years was 

 not ascertained. Those leaves which last more than 15 months 

 were found to transpire much faster when 15 to 18 months old 

 than at 3 or 4 months, when they have just reached their maxi- 

 mum area. The average for six species gave a transpiration 

 twice as rapid for the older leaves. 



Sun leaves and shade leaves on the same individual were 

 found to differ much in shape, thickness and area. Sun leaves 

 are narrower in proportion to their length, thicker and smaller 

 than shade leaves. When the transpiration of sun leaves placed 

 in sunshine is compared with that of shade leaves in the shade. 



