CONSOLIDATED I'ORMS OF VKGETATION. Ill 



fomied on the plan of the least possible amount of surface in pro- 

 portion to their bulk. A green rind serves the purpose of foliage ; 

 but the surface is as nothing compared with an ordinary leafy plant 

 of the same amount of vegetable material. This consolidation is 

 carried to the extreme in the Melon-Cactus, Mamillaria, and the 

 like, of globular or corm-like shapes ; their spherical ligure being 

 that which exposes the least possible part of the bulk to the air. 

 Such plants are evidently adapted and des'gned for very dry 

 regions ; and in such only are they naturally found. Similarly, 

 bulbous and corm-bearing plants, and the like, are a form of vege- 

 tation which in the growing season may in the foliage expand a 

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

 the living vegetable is reduced to a globular or other form of the 

 least surface ; and this is protected by its outer coats of dead and 

 dry scales, as well as by its subterranean situation ; — thus exhibits 

 ing another and very similar adaptation to a season of drought. 

 And such plants mainly belong to countries (such as Southern 

 Africa, and parts of the interior of Oregon and California) which 

 have a long hot season, during which little or no rain falls, when, 

 their stalks and fohage above and their roots beneath being early 

 cut off by drought, the plants rest securely in their compact bulbs, 

 filled with nourishment, and retain their moisture with great 

 tenacity, until the rainy season returns. Then they shoot forth 

 leaves and flowers with wonderful rapidity, and what was perhaps 

 a desert of arid sand becomes green with foliage and gay with blos- 

 soms, almost in a day. This will be more perfectly understood 

 when the nature and the use of foliage shall have been more fuUy 

 considered. 



Sect. IV. The Internal Structure of the Stem in 

 General. 



195. Having considered the various external forms and appear- 

 ances which the stem exhibits, and its mode of increase in length, 

 our attention may now be directed to its internal structure, and its 

 mode of increase in diameter. 



196. The stem embraces in its composition the various forms of 

 elementary tissue that have already been described (Chap. I., Sect. 

 II., in.) ; namely, ordinary cells, woody fibre, and vessels. At 



