BOTANICAL INTRODUCTION 41 



described, there are many others in which 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 ; while 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 

 inevitably wither, and this is a danger that the plant has to 

 guard against at all hazards. Plants living in dry situations 

 have adopted most ingenious devices to reduce transpiration to 

 a minimum, whilst obtaining a maximum of assimilatory 

 surface. Some of these devices will be considered in connection 

 with the various species exhibiting them. Plants living in 

 wet situations require a different structure from those that 

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

 writers have found it convenient and lielpful 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 in 

 moulding the form of the plant, this classification has reference 

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

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

 in humic and other acids, are generally inhabited by plants 

 which have contrivances to check transpiration. Such plants 

 are termed Xerophytes. In New Zealand this group is 

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

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

 plains, states " Their xerophytic structure is of striking 

 intensity, and difficult to understand in comparison with other 

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

 pastures, he tells us their xerophytism is so extreme that their 

 physiognomy can be compared only with that of the 

 almost rainless Persian steppes. (For a fuller discussion 

 •of this anomaly, see Discaria, Plagianthus hetulinus, etc.) 



