64 STUDIES IN THE VEGETATION OF THE STATE 



moisture than under normal conditions. Such plants reduce 

 the size of their leaves when grown in dry soil. 



The influence of shade upon plants accustomed to full sun- 

 light is such that they die at a higher limit in times of 

 drought. This fact was shown by the results from both 

 plant house and fields. It cannot be decided as to whether 

 any of the plants in the experiment had fully adapted them- 

 selves to shade conditions and whether the consequent 

 change in function and structure was the cause of the in- 

 creased amount of non-available water in the soil. Since the 

 change of habitat was unfavorable to the plants, there is 

 the possibility that most of them were still suffering from 

 the lack of sufBcient light stimuli and the protoplasm as a 

 consequence was unable to function normally. Since shade 

 plants contain a higher per cent of water in their tissues, 

 it is possible that the lessened density of the solutions in the 

 protoplasm of the root hairs lowers the ability of the plant 

 to take water from the soil solution. This tends to raise the 

 limit of non-available water. 



The limit of physiological water is varied by several fac- 

 tors involved in the composition and texture of the soil. The 

 great difference in the limit of sand and clay might be attrib- 

 uted to the increasing negative effect of capillarity and sur- 

 face tension in the latter This would tend to reduce the 

 absorption of water by the root hairs. In case of humus, loess, 

 saline and loam soil there are very evidently other disturbing 

 factors. In these soils a very important factor must be that 

 of the variable density of the soil solution. Diosmotic inter- 

 change, aside from other forces, varies directly with the 

 difference in density between the soil solution and the solu- 

 tion in the protoplasm of the root hairs ; as a result the plant 

 may obtain less and less water from the soil as it dries out. 

 Finally the solution in the soil may become so concentrated 

 that it actually extracts water from the roots of the plant. 



Plants wilting excessively from drought when slightly re- 

 vived by watering are not able before they die to remove as 

 much water from the soil as plants dying without ever hav- 

 ing been revived. This is apparently due to the loss of the 



