20 STUDIES IN THE VEGETATION OF THE STATE 



It seems almost impossible to determine when plants are 

 wilting in the same degre, that is, when they have lost an 

 equal per cent of water from their weight. Until there is 

 some accurate way of determining this, the results obtained 

 are mere approximations. The whole subject of plant water 

 is a very complicated one since the water content of the 

 plant is varied by so many factors. 



The amount of water in the soil non-available for plants 

 in time of drought appears to be affected by the same physi- 

 cal conditions that tend to vary the amount of water in the 

 plant, with the exception that in the soil the variation is 

 more uniform. The tenacity with which the protoplasm of 

 the plant may retain water probably varies not only in spe- 

 cies but also in individuals depending primarily upon devices 

 protective against evaporation, and secondarily upon the 

 vigor of the plant. In times of drought, if the process of 

 evaporation be a slow one, the amount of water is increased 

 since the plant has a longer time to adapt itself to the 

 changed condition. On the other hand, a rapid evaporation 

 hastens the death of the plant and tends to raise the limit. 

 The physical factors which increase evaporation are in- 

 creased heat and light, decreased humidity, and winds. The 

 amount of non-available water is increased by shade, humid 

 conditions and abundant physical water. The effect of shade 

 upon plant structures is quite noticeable. Down to a certain 

 point of light diminution the size of the leaves is increased, 

 but beyond this point the effect is to decrease the siae ; at 

 the same time the root system is retarded in its development. 



As a result shade conditions affect directly the ability of 

 the plant to exist in times of drought, and directly increase 

 the amount of non-available water. Humid conditions also 

 tend to increase the size of the leaves and at the same time 

 to retard the development of the root system, the effect be- 

 ing almost identical with that of shade. 



