6 STUDIES IN THE VEGETATION OF THE STATE 



standpoint, it is difficult to decide whether the results are of 

 most importance to plant physiology or to plant pathology, 

 since both are concerned either directly or indirectly in the 

 problems involved. The whole question of resistance to 

 drought is intimately connected with the ability of proto- 

 plasm to withstand the effects of unfavorable stimuli. Fur- 

 thermore the problem resolves itself into inquiries as to the 

 proper amount of light, heat, water, etc., necessary for the 

 natural development of the plant. When a plant does not 

 receive the stimuli necessary for its growth and maturity, it 

 soon passe.s over into a condition which may or may not be 

 pathological, but which directly concerns the plant pathol- 

 ogist, as well as the plant physiologist. The former is con- 

 cerned with conditions which lower the vitality of the plant, 

 the latter, with those which are necessary to growth and 

 maturity. It is hardly possible to draw the line between 

 health and disease, or to establish the proper amount of 

 light, heat, moisture, etc., requisite for a healthy plant, 

 since the quantity of each may vary not only with the inher- 

 ent nature of the species, but also with its adaptability. 



When a mesophyte dies from drought, there is always some 

 water remaining in the soil which is not available for the 

 use of the plant. This has been called "non-available" 

 water, in distinction to the physiological water of the soil 

 which is available for the needs of the plant. There are so 

 many factors and combinations of factors, both physical and 

 biological, that may vary the limit of the amount of non-avail- 

 able soil water that only a few of tLese can be considered in 

 one series of experiments. The work that has been done is 

 included in the following outline. No attempt will be made 

 to enter into a complete discussion of any one topic, for that 

 would require the results of many years of investigation. 

 The following outline contains the subjects investigated: 



I. Water in Different Soils. 



1. Physical Water., 



2. Physiological Water. 



3. Non-available Water. 



