ADAPTATION TO WATER 155 



a habitat-form, or ecad. Thus, there are water ecads, and Ught 

 ecads, i.e., plants whose character has been determined by adapta- 

 tion to water, or to hght. The same species may show several 

 ecads, in case it grows in habitats sufficiently different, a fact well 

 illustrated by the variovis forms of Ranunculus sceleratus. In 

 addition to the impress which a species owes to the water or light 

 of its habitat, it possesses other characteristic features, which can 

 be referred only indirectly or not at all to these. Such are the 

 forms termed trees, shrubs, grasses, etc. These are characteristic of 

 great areas of vegetation, and are hence termed vegetation forms 

 or phyads. The causes that produce them lie hidden in the history 

 of each species, and at present they can only be grouped with 

 respect to form. 



174. Types produced by adaptation to water. Plants which 

 grow habitually where the water supply is low show one or more 

 of the characteristic modifications due to the latter. They are 

 consequently termed dry-land plants or xerophytes. Those found 

 in habitats with an excessive water supply show corresponding 

 modifications, and are called water plants or hydrophytes. Xero- 

 phytes and hydrophytes represent more or less extreme conditions 

 of habitat and structure. Habitats which are neither dry nor 

 wet produce plants intermediate between these two types. Such 

 intermediate plants, or mesophytes, show no characteristic modi- 

 fications. As a rule, definite structures for increasing water 

 supply or decreasing water loss are either slightly developed or 

 completely absent. As would be expected, certain mesophytes 

 approach the xerophytes, while others are more or less hydrophytic 

 in nature. The plants of forests, meadows, prairies, and cultivated 

 fields are usually mesophytes. Those of high prairies, tablelands, 

 plains, sandhills, deserts, alpine peaks, etc., are xerophytes, and 

 the dwellers in wet meadows, swamps, ponds, streams, and other 

 bodies of water are hydrophytes. Generally speaking, xerophytes 

 grow in dry soils, mesophytes in moist soils, and hydrophytes in 

 wet soil or in water. Partly on account of the influence of humidity, 

 and partly because many habitats shade very gradually into each 

 other, it is impossible to establish an absolute correspondence 

 between each group and the water content. Xerophytes commonly 

 grow in soils whose holard is less than 15%, and with a chrcsard 

 of 5-10%, while hydrophytes grow in saturated soils or in water. 

 Sands and gravels are saturated at about 20%, and hence contain 



