326 



Introduction to Botany. 



or conservation. Plants in such situations can afford to be 

 indifferent to water in their form and construction, but 

 their attitude toward the atmosphere must be quite differ- 

 ent, for they are in danger of being cut off from it by the 

 water, and of dying from suspended respiration. 



Plants which are entirely submerged 

 and floating in the water must absorb 

 all needed substances from it. It is, 

 therefore, necessary for them to in- 

 crease the surfaces of their leaves and 

 stems by dividing them into thin rods 

 or plates, and to have no tissues very 

 remote from the absorbing surfaces. 

 For plants built after this plan, with 

 no parts extending into the air, sto- 

 mata and intercellular spaces lose their 

 importance, and are lacking or much 

 reduced in number and extent. 



The one most general characteris- 

 tic of the hydrophytes is their large 

 amount of free surface in proportion 

 to their volume. In this respect they 

 are in striking contrast to the xero- 

 phytes, which frequently have no more 

 than 3^0^ as much free surface for a given volume as the 

 hydrophytes. 



210. Origin of Xerophytes, Halophytes, and Hydro- 

 phytes. — The xerophytes, halophytes, and those hydro- 

 phytes which belong to the Phanerogams, ferns, and 

 mosses, have probably descended from mesophytes which 

 have been able to vary sufficiently to adapt themselves to 

 permanent extremes of moisture and dryness ; and so, 

 migrating into habitats of one extreme or the other, they 



FIG. 175. 



Cabomba Carolhiiana. The 

 lower dissected leaves 

 grow submerged in the 

 water, while the upper 

 entire leaves extend 

 above the surface. After 

 Belzung. 



