178 BOTANY OF THE LIVING PLANT 



Modifications like those shown by xerophytes are seen also in other 

 plants where water-supply is for other reasons difficult, as it is in those 

 which live attached to the branches or trunks of other plants {Epi- 

 phytes). Since they have no direct access to the soil, they must receive 

 and store the water from rainfall, or condense it from a moist atmo- 

 sphere. This is the condition of many tropical Orchids and Bromeliads. 

 In the latter a special surface-protection is afforded by scurfy peltate 

 hairs, while others serve for absorption of water. 



Again, in Arctic and Alpine plants many xerophytic characters are 

 presented, such as deep rooting, leaf-reduction, succulence, waxy 

 surface, or hairy coverings. These are probably related to the condi- 

 tion of physiological drought caused by the prevailing low temperature 

 of the soil, which checks the activity of root-absorption, while the 

 shoot, in clear weather and in a wind, may be exposed to conditions 

 which would stimulate transpiration to a dangerous degree. 



Halophytes living on a sea-shore, or in salt-marshes, also show 

 characters similar to xerophytes. The Glass-wort [Salicornia), with 

 its reduced leaves sunken into the succulent green stem, and having 

 a well-developed cuticle and glaucous surface, is a good example. 

 Some halophytes, such as Salsola and Eryngium, have also spinous 

 protections. These xerophytic characters are no doubt assumed in 

 relation to the fact that the water these plants absorb is impregnated 

 with salt, and restricted transpiration will avoid undue accumulation 

 of salt in their tissues. 



On the other hand. Hydrophytes, which grow in wet situations or 

 actually submerged, are independent of the risks of water-supply. 

 Their leaves are often finely divided, giving a large proportion of surface 

 to bulk, as in the water Buttercups. They are mostly perennials. 

 The Water-Lily [Nymphea) will serve as a good example. Its thick 

 stock is rooted in the mud, and throws up floating leaves with broad 

 lamina and smooth surface. Stomata are absent from the submerged 

 parts, but are present on the exposed upper surface (see p. 63). 

 The shoot contains large air-spaces, with little mechanical or woody 

 tissue. The large air-spaces provide an internal enclosed atmosphere, 

 where gaseous interchange can take place, thus helping to meet the 

 difficulty that diffusion at the surface is slower than is sometimes 

 required. The texture of such plants is limp. Their parts dry up 

 quickly in the air, owing to deficient cuticular protection. These 

 characters, which are common in the Hydrophytes, are in sharp 

 distinction to those of xerophytc types. 



The Mesophyte vegetation remains to be considered. Excepting in the 



