THEIR ANATOMY. 159 



271. At the points on the surface of the developing leaf where 

 stomates are about to be formed, one of the epidermal cells early 

 ceases to enlarge and thicken with the rest, but divides into two (in 

 the manner formerly described, 32), forming the two guardian 

 cells : as they grow, the two constituent portions of their common 

 partition separate, leaving an interspace or orifice between. In 

 some cases, each new cell divides again, when the stomate is 

 formed of four cells in place of two. 



272. 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 accumulates in the superficial cells of the parenchyma 

 (265), than from the want of stomata. The latter are usually 

 abundant,* but they seem to remain closed, or 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 with 

 great tenacity, especially during the hot season. They are evi- 

 dently constructed for enduring severe droughts ; and are accord- 

 ingly found to inhabit dry and sunburnt places, such as the arid 

 plains of Africa, the principal home of the Stapelias, Aloes, suc- 

 culent Euphorbias, &c., or the hottest and driest parts of our 

 own continent, to which the whole Cactus Family is indigenous. 

 Or, when 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 under conditions which would insure the speedy destruc- 

 tion of ordinary plants. The drier the atmosphere, the greater 

 their apparent reluctance to part with the fluid they have accumu- 

 lated, 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 their tolerance of the very dry air of 

 our houses in mid-winter, when ordinary thin-leaved plants become 

 unhealthy or perish. 



273. Sometimes the leaves of succulent plants merely become 



* The thickened epidermis of the fleshy leaves of the Sea-Sandwort (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. 



