484 RELATION TO ENVIRONMENT. 



flora. In subarctic and alpine regions, as well as in rocky 

 places or in sand-hills, mesophytic plants are often intermingled, 

 because the conditions of environment are not so austere. Xero- 

 phytic situations are as follows: 



1. Deserts, sand and gravel hills, sand-dunes, rocky places, 

 steppes and some prairies, high arctic and alpine districts. 



2. Soils or waters with large quantities of acids (as humic acid 

 in certain marshes or woods), or of salts (brackish or salt marshes 

 or bodies of water, and alkaline soils). 



945. Hydrophytes. Plants growing in "fresh" water, or in 

 land where the soil is very wet, or the air very humid throughout 

 the season, are hydrophytes. They have a water environment, or 

 one in which the air is so moist that loss of water from the aerial 

 organs is hindered. The leaves, and often the stems, even of 

 land hydrophytes, are soft and watery. They favor rapid loss of 

 water, but the moist environment checks the too rapid transpira- 

 tion. 



946. Halophytes. It is customary to restrict the term hydro- 

 phytes to plants growing in bodies of "fresh" water. Plants in 

 salt or brackish water are halophytes. Halophytes differ from 

 hydrophytes in the fact that the protoplasm of the former has a 

 higher osmotic tension than the latter. Halophytes are there- 

 fore enabled to live in saline water and to absorb liquid food from 

 the same. The usual hydrophytes, placed in the same environ- 

 ment, would collapse, because not only could they not absorb 

 water, they would lose their turgor and collapse because of the 

 higher osmotic tension of the water environment. Those halo- 

 phytes which are rooted in soil saturated or covered with salt 

 water (salt-marsh plants), and have aerial organs for transpira- 

 tion, have a xerophytic habit so far as their leaves or aerial parts 

 are concerned. In fact they are by some classed with xero- 

 phytes. This is because the salinity of the water retards root 

 absorption, and, in correlation with this, the aerial parts have 



'taken on a xerophytic habit in order to retard transpiration. 

 Halophytes, however, which are habitually immersed in water, 

 as the seaweeds for example, cannot be considered as xerophytes. 



