28 BRITISH PLANTS 



the tips of the teeth or in the grasses balanced upon the 

 apex of the leaf e.g., hogweed, garden-nasturtium, 

 creeping buttercup, etc. 



The Supply of Water in the Soil. This depends upon : 



(a) The rainfall that is, upon the water which enters 

 the soil from above ; and, as we have shown in Chapter I., 

 this has to be considered in relation to (1) its amount, 

 (2) its frequency, and (3) the season of maximum fall. 



(6) The nature of the soil which receives the rain. 

 This is described in detail in Chapter IX. 



(c) The water which enters the surface-soil from below. 

 This depends upon the existence and availability of 

 ground-water. The ultimate source of all the water in 

 the soil is rain ; but rain, sinking through the ground, 

 sooner or later reaches a layer of clay or hard rock, 

 through which it cannot penetrate. Upon this imperme- 

 able bed it settles, and forms a supply of underground, or 

 telluric (Lat. tellus, the ground), water. The presence 

 or absence of this underground water, its depth below 

 the surface, the power of the soil above it to suck it up, 

 and its availability to the surf ace- vegetation, constitute 

 a factor of such ecological importance that we shall deal 

 with it at length in a later chapter (Chapter IX.). 



So important is water, that plants may be divided 

 into two groups, according to whether they live in water 

 or upon land : 



1. Aquatics (Lat. aqua, water), or Water-Plants, 

 adapted to life in water. Their vegetative parts are 

 partly or entirely surrounded by liquid water (Chapter V.). 



2. Terrestrial, or Land-Plants, adapted to existence on 

 land. Their vegetative organs are surrounded by air 

 (Chapters IV. and VI.). 



The division between the two groups is not well marked. 

 On the border lie plants which can live either in water 

 or air. Many marsh-plants are amphibious in this way 

 e.g., Polygonum amphibium, Nasturtium amphibium, 

 and Pilularia. The land-form, however, differs more 

 or less from the water-form, the latter generally having 

 weaker stems and narrower leaves. Polygonum am- 

 phibium in water has floating leaves ; on muddy soil the 

 stems creep at the base, and its leaves are often downy. 



Land-plants are usually divided into three classes, 

 according to the nature of their water environment : 



