SOLUBILITIES 435 



tance to the well-being of the plant of factors which have a 

 bearing on these properties. Thus any cause which removes 

 water, not immediately replaceable, from the cell, and so leads 

 to a concentration of the cell sap, may be a determining factor 

 in the existence of a plant. Cold is one such factor ; * a fall 

 in the temperature may cause the water to crystallize, so that 

 the salt solutions in the cell become stronger, with the result 

 that some of the proteins of the protoplasm may be dissolved 

 and other proteins in solution may be precipitated. The im- 

 portance of soluble carbohydrates and of oils in the cell sap in 

 this connection has already been pointed out. 



It is unnecessary to remark that this effect of cold must 

 vary pretty considerably in different plants, and depends upon 

 the nature of the salts dissolved in the cell sap and the proteins 

 upon which they can act. To take a few examples : It was 

 found that in Begonia, soluble proteins were precipitated when 

 the temperature reached — 3° C. ; on the other hand, in the 

 leaves of Pinus, a temperature of —40° C. was required to 

 obtain a similar result. f This may, in part, be due to the 

 paucity of crystalloids in the cell sap, for it is stated that 

 plants which are subject to periodic drought possess only 

 small amounts of soluble crystalloids in the cell sap. 



In the case of the barley, it was observed that an exposure 

 for one night to a temperature of —7° C. reduced the yield of 

 soluble proteins by about one-third as compared with a control 

 experiment in which the temperature was not so lowered. This 

 salting-out effect is much increased if the cell sap becomes acid 

 on cooling, as is not infrequently the case. 



If the low temperature be long continued, the precipitated 

 proteins will not again enter into solution when the amount of 

 water is increased by raising the temperature ; on the other 

 hand, if the temperature be suddenly raised, the precipitated 

 proteins will re-dissolve, provided that they have not stood too 

 long, and thus the plant will not be greatly harmed. 



* See Blackman : " New Phytol.," 1909, 8, 354. 



f Gorke : " Landwirth. Versuchs. Stat.," 1906, 65, 149. 



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