PRECIPITATION 305 



inasmuch as the proteins are precipitated unchanged, and 

 retain all their original properties and solubilities. Absolute 

 alcohol, also, as mentioned above, precipitates the proteins 

 unchanged, though the precipitate must not be left in contact 

 with the alcohol, or else it will become coagulated. 



With regard to the precipitating power of these various 

 salts, it should be mentioned that saturated ammonium sul- 

 phate will precipitate all proteins except peptones, and con- 

 sequently a solution which on saturation with ammonium 

 sulphate remains clear, can be regarded as free from protein. 



Furthermore, zinc sulphate is approximately 



equivalent to ammonium sulphate, 



saturated sodium chloride is 



approximately equivalent to saturated magnesium sul- 

 phate, or 

 1/2 saturated ammonium 

 sulphate. 



In view of the number of proteins in the plant and their 

 different characteristic solubilities, it is easy to see the import- 

 ance 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 connexion 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 Pmus, 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 



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

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

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