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 water 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 ease 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 
