152 TIIK LEAVES. 



same space of the lower. The leaf of the Coltsfoot has 12,000 

 stomata to a square inch of the lower epidermis, and only 1,200 in 

 the upper. That of the White Lily has from 20,000 to G0,000 to 

 the square inch on the lower surface, and perhaps 3,000 on the up- 

 per. In this plant, and in other true Lilies, they are so remarkably 

 large (Fig. 221, 22G - 228) that they may be discerned by a simple 

 lens of an inch focus. In most plants they ai'e very much smaller 

 than this. 



271. Succulent or fleshy plants, such as those of the Cactus tribe, 

 Mesembryanthemums, Sedums, Aloes, &c.,are remarkable for holding 

 the water they imbibe with great tenacity, rather in consequence of 

 the thickness of the epidermis, or from the deposit which early ac- 

 cumulates in the superficial cells of the parenchyma (26G), than 

 from the want of stomata. The latter are usually, abundant,* but 

 they seem to open less than in ordinary plants, except in young and 

 growing parts. Hence the tissue becomes gorged as it were with 

 fluid, which is retained Avith great tenacity, especially during the 

 hot season. They are evidently constructed for enduring severe 

 droughts ; and are accordingly found to inhabit dry and sunburnt 

 places, such as the arid plains of Africa, — the principal home of the 

 Stapelias, Aloes, succulent Euphorbias, &c., — or the hottest and 

 driest parts of our o\vn continent, to which the whole Cactus family 

 is indigenous. Or, A\hen such plants inhabit the cooler temperate 

 regions, like the Sedums and the common Houseleek, &c., they are 

 commonly found In the most arid situations, on naked rocks, old 

 walls, or sandy plains, exposed to the fiercest rays of the noonday 

 sun, and thriving where ordinary plants would speedily perish. The 

 drier the atmosphere, the greater their apparent reluctance to part 

 with the fluid they have accumulated, and upon which they live 

 during the long period when little or no moisture is yielded by the 

 soil or the air. Their structure and economy fully explain tneir 

 tolerance of the very diy air of our houses in midwinter, when or- 

 dinary thin-leaved plants become unhealthy or perish. 



272. Sometimes the leaves of succulent plants merely become 

 obese or misshapen, hke those of the Ice-plant and other species 



* The thickened epidermis of the fleshy leaves of the Sea-Sand wort (Hon- 

 kenya) is provided with an abundance of large stomata, on the upper as well 

 as the lower face. But this plant, though very fleshy, grows in situations where 

 its roots are always supplied with moisture. 



