NITROGEN SUPPLY 51 



acid, are better than amino acids as a source of nitrogen in 

 protein synthesis. He points out that such feeding experi- 

 ments should be brief, in order to avoid secondary, especially 

 proteolytic, changes, and that the reaction of the culture 

 medium is all-important. Acidity may inhibit the growth of 

 many fungi, so that if ammonium salts of inorganic acids be 

 used as a source of nitrogen, the observed results may be due 

 not to the inability of the plant to employ the particular salt, 

 but to the acidity which develops as the nitrogen is as- 

 similated. 



For ordinary green plants the supply of nitrogen is found 

 in the simple nitrogen-containing salts of the soil water. Thus 

 the fertility of the soil, not only with respect to nitrates but 

 also in regard to other substances,* is an important factor, 

 conditioning the amount of protein found in the plant. In 

 addition to nitrates, some plants can make use of ammonium 

 salts. Hutchinson and Miller f found this to be true under 

 conditions of culture which precluded the presence of nitrates 

 in the soil. In this respect, however, all plants do not be- 

 have alike ; whilst some will grow equally well whether 

 supplied with nitrates or ammonium salts, others flourish 

 best when supplied with the former, and others seemingly 

 prefer ammonium salts to begin with and then nitrates. 



Mention has just been made of the importance of soil 

 substances other than nitrates in the protein synthesis of 

 plants : potassium may be taken in illustration. Species of 

 bacteria grown in the dark in culture media containing the 

 requisite organic food materials and salts but lacking potas- 

 sium show but poor development and no protein synthesis. 

 Under similar cultural conditions seedlings of the beet showed 

 seven times less protein and eighteen times less sugar, when 

 grown without potassium, as compared with the controls ; 

 when grown under sterile conditions in a culture medium 

 containing a sugar and supplied with known amounts of 

 carbon dioxide, it was found that those grown in the light 

 were independent of potassium as regards the synthesis of 

 protein, and that the addition of sugar to the culture medium 



* Whitson and Stoddart : "Ann. Rep. Wisconsin Exp. Sta.," 1904, 193. 

 t Hutchinson and Miller: " Journ. Agric. Sci.," 1909, 3, 179. 



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