II.] DEVICES TO REDUCE TRANSPIRATION 41 



rainfall of two years can thus be stored up for a single 

 crop, or sometimes if three seasons' rainfall can be 

 utilised by two crops only. 



It should not be supposed that all plants demand such 

 great quantities of water for transpiration as do our 

 ordinary field crops, because they are plants which have 

 been selected for their rapid growth under favourable 

 conditions. But in nature we always find that plants 

 which have adjusted themselves to live either in very 

 dry situations or in places where it would be injurious 

 to take in too much water, have always modified them- 

 selves in some way so as to reduce transpiration. 

 Whereas in temperate climates the leaves of a plant 

 arrange themselves to wave in the air and to present as 

 great a surface as possible to the light, the leaves of the 

 Eucalyptus and most of the other Australian trees so 

 dispose themselves as to present only an edge to the 

 sun, thus reducing both the transpiration and the heat- 

 ing effect of its rays. Very generally, in such circum- 

 stances the plant reduces the size of its leaves or even 

 replaces them entirely by spines, gorse or whin being a 

 case in point ; sometimes, as in the stonecrops and 

 houseleeks or the cacti, the leaf is made very thick, so 

 that its storage capacity is great as compared with 

 its evaporating surface. A hairy or a waxy glaucous 

 surface, or the presence of resins and essential oils 

 (among the plants growing on dry banks there is a 

 notable proportion of aromatic ones), are all regarded as 

 mechanisms by means of which the plant has learnt to 

 reduce transpiration. 



Various other consequences also follow from the 

 continued loss of water at the leaf surface by transpira- 

 tion, such as the flow of sap in the stem ; but first it will 

 be necessary to consider at some length how the water 

 enters the plant. 



