BOTANICAL INTRODUCTION 41 



described, there are many others in whicli the leaf is more or 

 less modified to suit its special environment. 



The anatomy and form of the leaf depend, to a very large 

 extent, upon the reaction between the plant and its surroundings, 

 as regards transpiration and assimilation. The object of the 

 plant is to expose as large a leaf surface as possible to the 

 action of the light ; wlrile at the same time a balance must be 

 maintained between the loss of water and its supply. If 

 more water leaves the plant than enters it, the leaves must 

 inevitabl>- wither, and this is a danger that the plant has to 

 guard agamst at all hazards. Plants living m dry situations 

 have adopted most ingenious devices to reduce transpiration to 

 a minimum, wliilst obtaining a maximum of assimilatory 

 surface. Some of these devices will be considered in connection 

 with file various species exhibiting them. Plants living in 

 wet situations recjuire a different structure from those that 

 grow on bare rocky cliffs or shmgle-slips. Hence, recent 

 writers have found it convenient and helpful to group plants 

 together, according to the modifications of stem and leaf 

 structure that they exhibit, in response to their special 

 environments. As the water supply is the chief factor m 

 moulding the form of the plant, this classification has reference 

 chiefiv to it. Dry soils, such as those of desert regions, 

 soils at times subject to a low temperature, or jjogs abounding 

 in humic and other acids, are generally inhabited by plants 

 which have contrivances to check transpiration. Such plants 

 are termed Xero])Jujtes. In New Zealand this group is 

 exceptionally well developed. Indeed, Dr. Diels, no mean 

 autliority upon the subject, speaking of the shrubs of the open 

 plains, states " Their xerophytic structure is of striking 

 intensitv, and difficult to understand m comparison witli otlrer 

 floras." Again, in discussing the plants of the sub-alpine 

 pastures, he tells us their xero[)hytisin is S(j extreme that then- 

 physiognomy can be compared only with that of the 

 almost rainless Persian steppes. (For a fuller discussion 

 ■of this anomaly, see Discaria, Plagidiitliu^i betiiliiiK.s, etc.) 



