PLANTS OF HABITATS THAT ARE PHYSIOLOGICALLY DRY 173 



humid atmosphere they may transpire at identical rates but as the 

 aridity of the atmosphere is increased the xerophyte will lose more 

 water than the mesophyte because its stomata will remain open 

 longer, but it will suffer less because it can endure wilting so much 

 better and after its stomata are closed it will lose almost no water 

 because of its protective features. INIaximov also believes that 

 intense insolation and water deficiency are the chief factors that 

 lead to higher osmotic pressures in xerophytes and that the high 

 osmotic pressures are in some way related to the power of resistance 

 of the protoplasm to injury during wilting. 



Whether Maximov is entirely correct in his conclusions remains to 

 be either proven or disproven by other workers but, at least, we can 

 say with considerable assurance that xerophytes, for the most part, 

 are plants that have been derived from mesophytes through re- 

 sponses to adverse environmental conditions. Most of them will 

 grow better if their water supply is increased, within certain limits, 

 and their xerophytism is measured by their ability to resist drought 

 and to endure wilting rather than by their morphological structures. 



109. Plants of Habitats that are Physiologically Dry.— Plants 

 that grow in saline soil or in salty water are called halophytes and 

 they are strikingly xeric. Perhaps the most characteristic feature of 

 halophytes, as a group, is their succulence which is accompanied 

 by very high osmotic pressure. We have already spoken of alkali 

 plains, which are usually physically as well as physiologically dry, 

 and we are concerned now onl}- with those habitats which contain 

 an abundance of water. The oceans are, of course, halic (char- 

 acterized by physiological dryness) habitats but the majority of 

 plants in them are algse which are for the most part submerged and, 

 therefore, not subject to transpiration. Because of this protection 

 from transpiration these algse do not show xeric features. 



Much more interesting from our point of view are the salt marshes 

 which contain strikingly xeric plants comparable to those occurring 

 on saline soils. In temperate regions the most representative salt- 

 marsh plants are herbaceous, but in tropical and subtropical regions 

 there are extensive mangrove forests occupying halic habitats. 

 Few plants show more marked xeric features than the mangroves 

 (Fig. 78). They have evergreen leaves which have water storage 

 tissue, prominent palisade cells, and thick cutin. Often there is a 

 network of roots extending from the branches down through the 

 water and into the soil below. Frequently, also, there are ascending 

 "knees" comparable to those found in cypress swamps. 



