176 BOTANY OF THE LIVING PLANT 



water-storage, as in the Stonecrop, Aloe, or Onion. In other cases 

 leaf-reduction may go along with a corresponding distension of the 

 stem, which becomes green, and takes over the function of Photo- 

 Synthesis. This is seen in the tropical Euphorbias, and Cactaceae, 

 the former especially on the dry areas of the Old, the latter of the New 

 World (Fig. 132). In some cases the stem swells to an almost spherical 

 form, by which means the greatest possible proportion of bulk to 



FIG. 132. 

 Succulent stem and flowers of a Cactus. (After Fignier.) 



surface is attained. Succulence with a distended form is thus a fre- 

 quent character in Xerophytes. With succulence often goes a spinous 

 development, as Fig. 132 shows. It may doubtless be effective as a 

 defence against herbivorous animals. A thorny character is common 

 in Xerophyte vegetation, and is a marked feature in dry districts such 

 as the Veldt of South Africa. Finally, many Xerophytes are very 

 deep-rooted. As special instances of this Acanthosicyos and Welwits- 

 chia of south-west Africa may be quoted. 



Along with succulence and reduction of surface go various other structural 

 modifications. In xerophytic plants there is usually a special dermal develop- 

 ment, with thickened cuticle. It is often covered with wax, giving a glaucous 

 surface. The stomata may be protected by being sunk in deep pits (Fig. 133). 



