52 BOTANY 



map of England or of the world in which the different 

 areas covered by heath, moorland, woodland, marsh, 

 and so on were coloured in different colours, then we 

 can recognise at once that though the extent of the 

 different patches would not entirely coincide with the 

 different physical characters of the ground, yet there 

 would be a distinct tendency for them to coincide, 

 except where cultivation has seriously interfered. 



Rich, warm soil, with a sufficiency of water which is 

 well drained off, yields most of the " normal " plants, 

 while difficulties of any kind, such as the want of water 

 on a high sandy soil, or the extreme scarcity of water 

 combined with a troublesome shifty soil in the sand- 

 dunes, tend to produce plants with organs specialised 

 to meet the peculiarity of the environment. Such 

 specialised plants are among the most interesting and 

 curious, for one organ is often elaborately developed, 

 apparently out of all proportion to the others, as in 

 the case of the little tufted plants, where there may be 

 a root many feet long to provide a visible plant only 

 an inch high above ground. 



As a general rule, the strange modifications and elabo- 

 rate devices in plant organs have taken place in relation 

 to the water supply. Hence the study of those which 

 live under desert and other drought conditions has 

 been one of the most attractive and obvious fields 

 of ecological work. The Cactus, with its leaves all 

 turned into spines, and the fleshy-leaved Stonecrop, 

 the plant with dry, rolled-up leaves or those thickly 

 covered with woolly hairs, each finds these peculiarities 

 an aid to retaining the scanty water which would not 

 suffice to supply ordinary broad soft leaves, from which 

 water evaporates rapidly. The Cactus and the leaves 

 of the fleshy-leaved Stonecrop, by becoming cylindrical 



