64 



drouth conditions. Volkens (f) found that salt is secreted by 

 various desert Tamaricaceae. An analysis of the exudate 

 showed that the chief constituents are the chlorides of sodium, 

 calcium and magnesium. As these salts are more or less deli- 

 quescent, Volkens thinks that they serve to gather water at 

 night from the atmosphere, so that the plant is enabled to ac- 

 quire moisture which would otherwise be unavailable. Haber- 

 h;ndt thinks, and doubtless rightly, that the plant would not be 

 able to extract water from solutions as concentrated as those in 

 \>hich the salts are dissolved. 



IMarloth (g) thinks that the white crusts prevent the" too 

 rapid cooling by evaporation of water from the leaf surface at 

 night. Haberlandt (h) offers the most plausible theory when 

 he states that the salt secretion is to reduce the accumulation of 

 inorganic material. Such material might arise from the kata- 

 bolism of the plant, or might be absorbed with the water of 

 transpiration in amounts harmful to the plant tissues. In 

 either case, direct excretion would prove of value. Especially 

 would this be the case in plants living in salt oi- brackish water, 

 where the concentration of the water forbids abundant absorp- 

 tion. If a plant like Spartina can get rid of a portion of the 

 salt absorbed it would be enabled to absorb just that more water, 

 and with it the salts necessary in the synthetic processes. 



The manner in which secretion takes place in Spartina is 

 a-' yet undetermined; though from appearances, it would seem 

 to be due to exudation pressure in the epidermal cells. 



(f) Die Flora der aegyptisch-arabischen Wuste. 



(g) Zur Bedeutung der Sa.lz abscheidenden Drusen der Tamaras- 

 cineen. 



(h) loc. citt. 



