196 THE BIOLOGY OF FLOWERING PLANTS 



combined with the evolution of a xerophytic leaf- like stem, 

 has been a modification easier, in some cases, than an 

 alteration in the structure of the leaf itself. The change 

 may have originated as a chance mutation. 



Succulents. — The leaf function has been taken over by 

 the stem in a great many succulents. The finest examples 

 of this are offered by the succulent cactuses of America, 

 and euphorbias and stapelias of Africa. The leaves of the 

 cactuses are to be seen as small blunt projections on the 

 young shoots, e.g. of Opuntia. These soon fall off, and the 

 spines, which may also be modified leaves of side shoots, 

 appear. There is no approach to leaf form in the stems, 

 except in the flattened segments of Opuntia. Our native 

 herbaceous Salicornias or glassworts are also stem succu- 

 lents with small leaves. 



§ 30. Transpiration of Succulents 



More familiar in temperate climates are leaf succulents, 

 such as the stonecrops and house-leeks, in which the suc- 

 culent habit is sometimes associated with a reduction of leaf 

 surface. The leaf succulent Mesembryanthemums are 

 frequently grown in gardens. 



Succulents are highly characteristic of two types of 

 station — desert and salt marsh. There seems, also, to be a 

 general tendency for seaside plants to become more fleshy 

 than individuals of the same species growing inland, or for 

 fleshy varieties of a species to occur near the coast ; Matri- 

 caria inodora, var. salina, is for example distinguished from 

 the type by its more fleshy habit. Such British seaside 

 plants as Salicornia herhacea, Suaeda maritirna, Salsola 

 Kali, Cochlearia officinalis, Glaux maritima, Arenaria pep- 

 loides, Cakile maritima, Mertensia maritima, Plantago 

 maritima, Aster Tripolium, and Statice Limonitim are all 

 more or less markedly fleshy. Saline soils were dis- 

 tinguished by Schimper as physiologically dry. As far 

 as absorption goes we have already seen that the develop- 

 ment of an osmotic pressure sufficient to withdraw water 



