THE STEM AND BRANCHES. 173 



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 cov- 

 ered 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 acacias and other Australasian trees, 

 are well supplied with like pores for purposes of 

 evaporation. Again, while the pores are usually 

 found on the under surface of the leaf, they are 

 situated on the upper surface of leaves which float 

 on water, like the water-lily and the water-crow- 

 foot ; because in such plants they would be obvi- 

 ously useless for purposes of evaporation on the 

 lower side, which is in contact with the water. 

 Some leaves have the stomata on both sides alike, 

 especially when no one side is much more ex- 

 posed to sunlight than another. But wherever 

 they are found, they always lie above masses of 

 loose and spongy cell-tissue, in whose meshes and 

 air-spaces evaporation can go on readily. 



On the other hand, as I noted before, leaves 

 which grow in very dry or desert situations re- 

 quire as much as possible to curtail evaporation. 



