66 CARNEGIE INSTITUTION OF WASHINGTON. 



under controlled conditions, prove that the relation is not so simple 

 as the influence of mere changes in H-ion concentration. The regular 

 accumulation and disappearance of many substances within the plant 

 must be considered not only in the light of their individual influence 

 on the water-holding capacity, but in that of their combined effect. 



(8) In former experiments the author has found well-defined auto- 

 nomic movements in cacti which could be traced to changes in the 

 water-content of the plant. These changes in water-content are of 

 such a nature that the difference between absorption and transpi- 

 ration is positive for the day and negative for the night. The dis- 

 covery of corresponding variations in the water-holding capacity of 

 internal tissue traces the causes of the autonomic movements one step 

 further. 



(9) As desiccation progresses a cactus loses less and less water by 

 transpiration until a point is reached where the total loss for 24 hours 

 is almost zero. There is a small loss occurring during the daylight 

 hours which is frequently entirely replaced by a gain at night. Only 

 about one-tenth of this gain can be accounted for by hygroscopicity of 

 the spines. 



(10) The water-absorbing capacity of internal tissue from plants 

 which have been without water for 6 months is about 5 times as great 

 as of tissue from plants which have had sufficient water. This is 

 true only if the absorbing capacity be based upon wet weight. 



(11) The interpretation of paragraphs (9) and (10) seems to be as 

 follows: As the plant loses water during a drought period the total 

 mass has an increasing hold upon its water, until a balance is reached 

 where the vapor pressures within and without the plant are nearly 

 equal, even a gain at night taking place when the relative humidity 

 is higher than during the preceding day. The increased absorbing 

 capacity of the tissue would tend also to increase the pull of the roots 

 on the soil-water, and thus perhaps change the amount of available 

 soil-water, even though the water-content of the soil is not raised. 



(12) By the above, the ability of the cactus to withstand long periods 

 of drought is traced to its power to hold water within its tissue 

 against the evaporative force of the air. 



Experimental evidence is now being sought for the theory mentioned 

 above, i. e., that the same internal agencies influence both the trans- 

 piring capacity and the water-absorbing capacity of cacti. Joints 

 have been taken from plants at various times of the day and night 

 and sunultaneous measurements made of their evaporating capacity 

 and water-holding capacity. The experiments thus far show promising 

 results, but the work has not progressed far enough to pennit of the 

 statement of a definite law. Expermients are also under way for the 

 investigation of the relation of evaporation-rate and water-absorbing 

 rate in non-living colloidal gels. 



