128 Plant Physiology 



practicable, but it serves to emphasize the fact that alter- 

 nate flooding and drying is not necessarily ideal. The 

 latter is far better than stagnation. When in vigorous 

 growth, the plant suffers from drying-out, and some part 

 of the absorbing surface is killed every time the soil be- 

 comes air-dry. 



In general, there is a certain relation between abundant 

 water-supply and vegetative growth, so that it may be 

 necessary to check watering somewhat to induce more 

 abundant flowering. Again, in the case of plants which 

 flower periodically, it may be desirable, or even imperative, 

 to permit the plant to pass into a resting or semidormant 

 condition. If the plant as a whole is to remain alive, water 

 may not be entirely withheld, but in the case of many 

 bulbous and fleshy-rooted plants it may be highly desirable 

 that all other vegetative organs disappear, and coinci- 

 dently it may be desirable that all other conditions favor- 

 ing metabolism (such as high temperature) may be reduced. 



The cultivation of plants whose peculiar growth-forms 

 are dependent upon dryness of habitat is a special case, 

 just as is the cultivation of water plants, and some of the 

 general relations of these types are subsequently treated. 



70. Ecological classification based upon the water 

 relation. In the previous paragraphs of this chapter 

 plants of the most diverse water relations have been dis- 

 cussed; those of the desert represent one extreme and 

 those of ponds and water-courses the other, between which 

 extremes falls the great majority of plants. The water 

 relation was recognized by Warming to be most important 

 in attempting a habitat or ecological classification of forms. 

 With respect to this factor he has made from the natural 



