170 PLANT PHYSIOLOGY AND ECOLOGY 



In consequence, it is incorrect to refer to bog plants as xerophytes. 

 In spite of the superficial evidence, they are hydrophytes. 



The presence in bog plants of modifications characteristic of 

 xerophytes seems to be explained by the stability of the species 

 concerned, i.e., by their ability to adjust themselves to changed 

 conditions without undergoing a corresponding change in structure. 

 It has recently been shown that certain sun plants undergo no 

 material change in structure when grown in the shade. This was 

 likewise found to be true of some species growing in two or more 

 habitats of very different water content. Hence it is probable 

 that the xerophytic features found in some amphibious plants are 

 due to the persistence of stable structures. The latter were 

 developed when these species were growing in xerophytic situa- 

 tions, and not by the hydrophytic habitat in which the plants are 

 found at present. The monocotyledons, and especially the grasses, 

 sedges, and rushes, are extremely slow in adapting themselves to 

 new conditions, i.e., they are very stable. Thus it is readily 

 seen how certain ancestral characters may have persisted in spite 

 of a striking change of habitat. 



Experiment 55. Study of water-content types. Classify the various 

 species found iu the field as xerophytes, mesophytes, and hydrophytes, 

 and arrange them under the proper tyfjes. 



