TRANSPIRATION & ABSORPTION OF SALTS 209 



was very considerable even in the shade plants, and in all 

 probability the supply of salts was sufficient in both sets 

 and did not limit growth. The experiment really throws 

 little light on the question at issue. 



Muenscher (1922) grew barley in water culture and 

 compared the behaviour of plants in different atmospheric 

 humidities. With transpiration cut down to 50 per cent, 

 of the maximum, the fresh weight attained was somewhat 

 greater and the ash content 10 per cent, less ; calculated in 

 terms of the dry weight the ash content was about equal. 

 Much more ash was absorbed per 1000 c.c. of tran- 

 spired water by the plant with lower transpiration, e.g. 

 0*795 compared with 0*42 grm. Mendiola (1922) found 

 that diminished transpiration in the tobacco resulted in 

 increased dry weight and decreased ash content. These 

 three modern investigations agree in one point — that there 

 is no proportionality between the amount of water transpired 

 and the amount of ash absorbed. They disagree on the 

 question as to whether more ash is absorbed under conditions 

 of higher transpiration. The subject evidently requires 

 further investigation. We may, however, note four points, 

 (i) It would be surprising if there were a direct relation 

 between transpiration and salt absorption ; the actual rate 

 of entrance of salts into the plant can be affected to a slight 

 degree only by the rate of entry of water. But if salts are 

 swept up the wood vessels in the transpiration current 

 diffusion from the parenchyma cells of the roots into the 

 vessels must be accelerated. (2) The function of the 

 transpiration stream, if it has one in this connection, 

 must lie in the transport of the salts from the root to the 

 shoot ; it is likely that quite a slow rate of transpiration 

 would be sufficient to supply enough salts to the leaves and 

 growing points to secure normal growth. (3) The experi- 

 ments quoted all deal with what are really high transpiration 

 rates. The effect of transpiration could only become 

 evident if it were a limiting factor, and there is no evidence 

 whatever that it is a limiting factor in these experiments. 

 (4) What is required is a series of graded experiments which 



p 



