ON WALLS, ROCKS AND STONY PLACES 310 



dry weather when the plant is in clanger of losing iniicli more 

 moisture than the roots can absorb. 



So far, however, wo have been dealing with a regulating process 

 that is common to green plants in general ; but we must look for 

 some additional protection against loss of moisture in tlie plants 

 which grow in such places that they have to live through longer 

 or shorter periods during which the roots have little or no moisture 

 within reach. 



From what has been said concerning the structure of the leaf 

 it will be understood that, as a rule, the larger the surface the 

 greater will be the loss of water in a given time. But when we 

 examine the leaves of the plants that grow on dry walls and rocks, 

 we frequently find that they are more or less thick and fleshy — that 

 the material of the leaf is disposed in such a manner as to reduce 

 the area of the surface as compared with other leaves made up 

 of the same amount of tissue. 



In some species this diminution of surface is carried to the 

 extreme, and the leaves have become very thick, assuming a 

 cylindrical or almost globular form ; and such leaves are capable of 

 absorbing and retaining large supplies of water that serve to main- 

 tain the plant during those periods in which the roots have no 

 moisture within their reacli. 



We also find that many of the plants in question are further 

 protected from a dangei'ous loss of moisture by the peculiar arrange- 

 ment of their leaves, which are often so closely applied to the stem, 

 or so closely overlapping one over another, that the total area of 

 exposed surface is considerably reduced ; and it frequently happens 

 that the stem of the plant becomes thick and succulent, as well 

 as the leaves, thus adding to the store of moistm-e kept in reserve 

 for the rainless days. 



While some plants are almost invariably found in dry, stony 

 places, others are very diverse in their habitats, sometimes growing in 

 moist and shady places, and sometimes on cliffs or other rocky 

 situations. In the latter we often hnd considerable modifications 

 of size, form and structure, the same species being more or less 

 luxuriant and thin-leaved when in damp soils, while in rocky 

 places it becomes more or less stunted, with a tendency to produce 

 thick and succulent leaves. 



A few of the plants that we include in the present chapter are 

 to be found only on wet rocks, and are therefore of a nature very 

 different from that of the species growing in dry places. They are 



