68 CARNEGIE INSTITUTION OF WASHINGTON. 



The chance for the greatest error in the use of the cobalt sUps now 

 rests in the assumption that the leaf and air temperatures are the same 

 at the time of the tests. A new method for the determination of leaf 

 temperature is being developed which it is hoped will reduce this danger 

 of error. Further search for the exact value of the constant will await 

 the developments of the leaf-temperature method. 



The experiments mentioned above have brought out very clearly 

 that when it is desired to make a comparison of the transpiring powers 

 of plants of different species or strains, it is necessary to obtain the 

 daily march of transpiring power, and that little reliance can be placed 

 upon a comparison of isolated readings, this being true no matter 

 which method is used. Furthermore, when potted plants are used it 

 is never safe to draw conclusions from the behavior of one or two plants. 

 When the cobalt slips are used care must be taken to test several leaves 

 of all the ages and sizes that exist on the plant. 



The Relation between Water Loss by Evaporation and Water Gain by Absorption 

 in Colloidal Gels, by Edith B. Shreve. 



As was noted in the report of last year,^ preliminary experiments 

 on the tissue of Opuntia versicolor showed the probability of a direct 

 relation between the water-absorbing power and the power to withhold 

 water against the evaporative forces of the air. Further experimenta- 

 tion has now confirmed this for tissues of Opuntia versicolor and 0. 

 blakeana (?) . It was also mentioned last year that work had been begun 

 on the testing of this relation for non-hving colloidal gels. Plans are 

 laid for the use of a number of gels, particularly of carbohydrate con- 

 stitution, but the work has not yet progressed beyond tests with 

 gelatine, owing to the unexpected necessity of investigating the nature 

 of the absorbing power of gelatine itself. 



It was easily shown that the greater the water-content of the gela- 

 tine the less the absorbing power and the greater the evaporating power 

 until a concentration is reached where the gelatine loses water at the 

 same rate as a similar water-surface. 



The work of Hofmeister and others has shown that the presence of 

 acids and certain salts in a solution siu-rounding gelatine increases the 

 absorbing capacity of the gelatine, while that of certain other salts 

 decreases it. The plan was to make up gelatine in various concen- 

 trations of these salts, i. e., including the salts within the gel, and 

 ascertain if those salts which increase absorption decrease evapora- 

 tion, and vice versa. The results show conclusively that a large increase 

 in the absorbing power is accompanied by a small decrease in the evap- 

 orating power, ^. e., by an increased ability of the gelatine to resist the 

 evaporative force of the air. The presence of N/100 HCl reduced the 



^Carnegie Inst. Wash. Year Book 1916, p. 66. 



