EFFECTS ON PLANTS 13 



normal, while the protoplasts differ chiefly in the character of the inclusions and 

 the composition of the vacuolar fluids." For further details, MacDougal's 

 monograph may be consulted; also Palladin ('18). The results of etiolation, 

 where there is a carbohydrate deficiency, may be contrasted with the converse 

 experiments of Urbain (previously mentioned), in which the reserve food material 

 (protein, fats, etc. of the endosperm) was removed and other nutrients fully 

 supplied. 



Vitamins 



The vitamins, or accessory food factors (of unknown composition) so essen- 

 tial in animal nutrition are, as is well known, derived directly or indirectly from 

 plants. Funk ('22) and Sherman and Smith ('22) have recently reviewed at 

 length the literature covering numerous researches which indicate the necessity, 

 or at least the advantage, of the vitamins or apparently similar growth-promoting 

 substances in the development of plants themselves, including yeast, bacteria, 

 fungi and higher plants. It is quite possible, for instance, that the remarkable 

 changes which Urbain ('20) effected by removal of the endosperm are at least 

 partly due to the loss of vitamins stored to supply the needs of the plant during 

 development. The fully-grown plants are apparently able to synthesize these 

 vitamins (whatever they may be), possibly in some cases by the symbiotic aid 

 of nitrifying bacteria. Possibly some of the growth-promoting effects of minute 

 quantities of certain substances may belong in a similar category, as shown, for 

 example, in the experiments of Bottomley ('14) on wheat seedlings. It remains 

 for the future to reveal the chemical nature and mode of action of these obscure 

 substances. Thjotta and Avery ('21) concluded that two distinct vitamin-like 

 substances are required for the growth of the hemophilic bacteria. Ellis and 

 Macleod ('22) have recently reviewed the literature on the relation of vitamins 

 to the growth of yeast. 



Liebig's Law of the Minimum 



In connection with the inanition of plants, it seems desirable to consider 

 what is generally referred to as Liebig's "law of the minimum" or the "limiting 

 factor," because of its fundamental importance in animal as well as plant 

 nutrition under conditions of partial inanition. Since this doctrine has occa- 

 sioned much controversy, it is advisable to examine somewhat carefully Liebig's 

 original ideas, which have been frequently misunderstood and greatly modified 

 by subsequent investigators. 



In the edition which I have consulted (Die Chemie in ihrer Anwendung auf 

 Agricultur and Physiologie, 9. Aufl. Braunschweig, 1876), v. Liebig approaches 

 the question from the point of view of fertilizers and their application to obtain 

 the maximum harvest of crops. Under "Lehre (Gesetz) des Minimums" 

 (p. 330 ff .) he explains how soils vary in composition and how successive crops 

 may exhaust the soil by removing certain of its constituents which form the 

 essential nutrients for plants. In the application of manures or fertilizers to 

 enrich an impoverished soil, he emphasizes the importance of a knowledge of 

 its content in the various essential plant nutrients, all of which are indispensable 



