XEROPHYTES AND HALOPHYTES. 



243 



3. Other plants adapt aerial roots to the absorption of 

 moisture from the air, as well as falling water. (See ^ 165.) 



4. Many are surrounded by the bases of dead leaves, which 

 act as a sponge for absorbing water, and supply it gradually 

 to the stem or younger leaves. I^iving leaves, sometimes 

 singly, sometimes in clusters, form cuplike or tubular 

 structures, acting as water receptacles, from which it can be 

 absorbed as required. Such adaptations occur chiefly in 

 epiphytes. (See Tf 357- ) 5- Many xerophytes develo|5 

 exceedingly long tap roots, which penetrate the soil deeply 

 to a permanent water supply. 



C. Adaptations for storing water. 



341. I. Special cell contents. — The simplest of these 

 adaptations is the presence of mucilage. The presence of 

 acids, tannins, and certain salts perhaps aids in the retention 

 of water. 



342. 2. Water-storing tissues.— {a) Fleshy plants, or 

 succulents, are those which thicken their ' parts by the devel- 

 opment of cells, which contain a large quantity of water, and 

 usually much mucilage. 

 These mucilage-con- 

 taining parts form a 

 reservoir for the storing 

 of water. In such 

 plants the epidermis is 

 very strongly water- 

 proofed ; the stems are 

 thick, cylindrical, pris- 

 matic or spheroidal ; 

 the leaves are wanting, 

 or they are thick and 

 fleshy, cylindrical or broad (fig. 207), and arranged in 

 rosettes. 



Fig. 207. — A plant .of houseleek {Sejii^ervfz'um 

 tectorum), showing fleshy leaves arranged in a 

 rosette, with offsets formed at the ends of .special 

 branches. These become detached and form in- 

 dependent plants. About one half natural size. — 

 Aiter Gray. 



