PLANT BIOLOGY 105 



most purposes to rely upon morphology as the basis of 

 classification. Our flowering plants, for example, are 

 divided into groups, called Natural Orders, which are 

 based entirely on morphological relations. Generally the 

 form and characters of the flowers are taken, the vegeta- 

 tive bodies of plants varying too much to enable us to 

 make much use of them as a means of identification. 



Biologically, however, other classifications are possible. 

 Plants may be grouped together according as they re- 

 semble one another biologically, and it is the biological 

 groupings of plants and parts of plants that are important 

 in ecology. 



As an example, we have already in Part I. classified 

 plants according to their relations to water, dividing 

 them first into two main groups, Water-Plants and Land- 

 Plants, and then the land-plants further into Hygro- 

 phytes, Mesophytes, and Xerophytes, according as the 

 amount of water available is abundant, adequate, or 

 small. In this classification form and structure were 

 regarded merely as the expression of physiological need, 

 and we selected ivater since it is the most important of all 

 the ecological factors and lies at the base of nutrition. 

 This division takes no account of relationships. Plants 

 which are widely separated in descent may fall together 

 in the same ecological grouping. Thus the Cacti and 

 Euphorbias are plants which have no relationship at all 

 to one another, but when growing in deserts they ap- 

 proach one another so closely in form and characters that 

 it is difficult to distinguish between them apart from 

 their flowers. It is a case of parallel development. 

 Growing under similar conditions, surrounded by the 

 same environment, these two races have, in the course of 

 ages, succeeded in adapting themselves to the same 

 conditions in the same way. On the other hand, two 

 closely - related species may be widely separated eco- 

 logically, one being adapted to one mode of life, the other 

 to another. One, for example, may be a hygrophyte and 

 the other a xerophyte. The genus Senecio, to which the 

 groundsel and ragwort belong, is remarkable in this 

 respect. It is of world-wide distribution, and contains 

 a huge number of species. But it includes plants of the 

 most diverse habit. Some are annuals, others perennials ; 

 some are alpines, some marsh-plants, some pronounced 



