48 



STEMS. 



[section 6. 



same view, an Opuntia or Prickly-Pear Cactus, its stem and branches 

 formed of a succession of thick and flattened joints (Fig. Ill, a), -vrhicb 

 may be likened to tubers, or an Epipliyllum (d), having short and flat 

 joints, with an ordinary leafy slirub or herb of equal size. And finally, 

 in Melon-Cactuses, Echinocactus (c), or other globose forms (which may 

 be likened to permanent corlns), with their globular or bulb-like shapes, 

 we have plants in the compactest shape ; their spherical figure being such 

 as to expose the least possible amount of substance to the air. These are 

 adaptations to climates which are very dry, either throughout or for a part 

 of the year. Similarly, bulbous and corm-bearing plants, and the like, are 

 examples of a form of vegetation which in the growing season may expand a 

 large surface to the air and light, while during the period of rest the living 

 vegetable is reduced to a globe, or solid form of the least possible surface; 

 and this protected by its outer coats of dead and dry scales, as well as by 

 its situation under ground. Such are also adapted to a season of drought. 

 They largely belong to countries which have a long hot season of little or 

 no rain, when, their stalks and foliage above and their roots beneath early 

 perishing, the plants rest securely in their compact bulbs, filled with 

 nourishment and retaining their moisture with great tenacity, until the 

 rainy season comes round. Then they shoot forth leaves and flowers with 

 wonderful rapidity, and what was perhaps a desert of arid sand becomeL 

 green with foliage and gay with blossoms, almost in a day. 



