WATER REQUIREMENTS FOR PLANT GROWTH 



189 



are the most important agricultural plants among the hydro- 

 phytes (Figs. 102 and 103). 



Plants may also be grouped with reference to tempera- 

 ture, soil, fresh or salt water and many other environmental 

 factors. In nature we always find the character of the plants 

 dependent on a combination of factors which may be considered 



FIG. 103. Zone formation of vegetation dependent upon depth of water. 



as physiological. The study cf the grouping of plants in nature 

 and their relationship to their environment is usually known 

 as Ecology. 



It will be readily seen that plants which are widely sepa- 

 rated so far as phylogenetic relationships are concerned, fre- 

 quently grow under the same environmental conditions. This 

 is especially noticeable among the hydrophytes, in which, as 



