ADAPTATION TO WATER 165 



often found themselves in conditions that have changed them 

 into mesophytes. Many of the latter have in consequence retained 

 characters of leaf, stem, or root which are to be regarded as ances- 

 tral rather than as the result of adaptation to the present habitat. 

 All of these facts make it unprofitable if not impossible to arrange 

 mesophytes under various types. Although they show a large 

 variety of forms, these are likewise found in the other two groups. 

 Such vegetation forms cannot serve as a basis for separating 

 mesophytes into groups based upon the kind or amount of adapta- 

 tion to water. 



iMesophytic species fall naturally into the two groups, sun 

 plants and shade plants. This is due chiefly to the fact that 

 shade in large measure offsets xerophytic conditions, and also 

 generally retards the development of hydrophytes. The factor 

 concerned here is no longer water content, but light. The char- 

 acteristic changes are due to the latter, and sun and shade forms 

 are types of adaptation to light. Consequently they are con- 

 sidered under the latter. 



Experiment 51. Comparison of mesophyte and xerophyte. Make a 

 careful comparison between the form and leaf structure of a mesophytic 

 and a xerophytic species of the same genus, e.g., Artemisia, Helianthus, 

 Muhlenbergia, Pentsternon, etc. 



184. Hydrophytes. The forms and structures of water plants 

 stand out in sharp contrast to those of xerophytes. On the other 

 hand, they grade insensibly into mesophytes, and it is impossible 

 to draw a sharp line between them. Typical hydrophytes grow 

 in water, in soil covered by it, or in saturated soil. With respect 

 to their relation to water and air they may be arranged in three 

 fairly natural groups, viz., amphibious, floating, and submerged 

 plants. In the amphibious form the leaves show the usual 

 relation. They grow in the air, while roots and stem are under 

 water to a greater or less degree. Floating plants have leaves 

 in which the upper surface is in contact with the air, and the 

 lower with water. In submerged forms the leaves are usually 

 below the surface of the water, i.e., carbon dioxide and oxygen are 

 obtained from the water and not from the air. Both surfaces of 

 the amphibious leaf and the upper surface of the floating leaf are 

 of such a nature as to permit as much transpiration as possible. 

 This function is entirely lacking in the leaves of submorgcd plants. 



