SOIL MOISTURE AND TO EVAPORATION. 43 



any way, since the effect of wound stimulus is sometimes great and is 

 always an unknown factor until carefully studied. Thus the potometer 

 commonly used in transpiration measurements is at least of doubtful 

 value until it is tested for each form experimented upon by some other 

 method which does not involve mutilating the plant. On this point see 

 also Curtis (1902). 



Furthermore, if the subject of stomatal or other physiological regu- 

 lation of water loss is to be studied, it is essential that the rate of 

 merely physical evaporation from a uniform water surface be ascer- 

 tained simultaneously and for the same place with the transpiration 

 measurements. The evaporimeter devised for this purpose has already 

 been described. 



The only method which fulfills all the conditions is that of weighing 

 potted plants, the soil of which is so inclosed as to lose no water except- 

 ing through transpiration. This method was adopted for the work. 

 Plants which had been lifted from the ground or had come from seed 

 sown in the pots were allowed to grow in the plant cage already 

 described, the soil being kept moist by frequent waterings, until they 

 appeared perfectly healthy and vigorous and had attained a convenient 

 size. Then the pots were sealed up so as to prevent water loss except- 

 ing through the plant, and the cultures thus treated were weighed at 

 intervals, readings on an evaporimeter which stood beside them being 

 made simultaneously with the weighings. 



For sealing up the pots the composite modeling clay used by sculp- 

 tors was found to answer very well. It is of about the consistency of 

 putty, adheres with an air-tight joint to all dry solids, hardens very 

 little with age, is readily removed with a knife or spatula when the 

 experiment is finished, and, most important of all, can be applied cold to 

 plant surfaces and has no injurious effect. In short, it is an ideal soft 

 sealing-wax for use in all cases where air-tight and water-tight joints 

 of any kind are to be made and where it is not necessary that the 

 joint bear much pressure. Its cheapness and the fact that it can be 

 obtained from any dealer in artists' supplies, together with the ease 

 with which it can be removed when it has served its purpose, make it 

 much more satisfactory than any of the soft waxes prepared with 

 Venice turpentine, beeswax, etc., with which the author is acquainted. 



During the time of the experiment the plant received no addition 

 of water. The soil, of course, became gradually drier and many of 

 the plants finally wilted or their leaves began to wither, showing that 

 they were suffering from lack of water. 



At the end of most of the experiments the leaves were removed from 

 the stems and dried in a press. After the writer's return to Chicago the 



