188 THE STORY OF THE PLANTS. 



on the upward path have a natural chemical 

 affinity for water, and will suck it up greedily 

 wherever they can get it. Thus each part, as 

 fast as it loses water by evaporation, takes up 

 more water in turn from its next neighbour 

 below; and that once more withdraws it from 

 the cell beneath it ; and so on step by step until 

 we reach the actual absorbent root-hairs. Root- 

 pressure by itself could not raise water as high 

 as we often see it raised in great forest trees and 

 tropical climbers ; it has not enough mechanical 

 motor power. But here evaporation comes in, 

 to aid it in its task ; and the real motor power 

 in this last case is the very potent force of 

 chemical attraction. 



What I have said here about evaporation, and 

 the way it is conducted by means of pores on the 

 surface of the leaves, is true of the vast majority 

 of green plants ; but considerable varieties and 

 modifications occur, of course, in accordance 

 with the necessities of various situations. For 

 example, the brooms and many other shrubs of 

 the same twiggy type have few green leaves, but 

 in their stead produce lithe green stems, filled 

 with active chlorophyll. These stems and 

 branches do all the work usually performed 

 by ordinary foliage. Stems and twigs of this 

 type are covered with mouth-like pores, or 

 stomata, in exactly the same way as the under 

 side of leaves in most other species. Similarly, 

 the very flattened leaf -like branches of the 

 butcher's broom, and of the Australian aca- 

 cias and other Australasian trees, are well 

 supplied with like pores for purposes of evaporq,» 



