1] UPON THE RATE OF GROWTH 333 



faster than one under similar conditions but without ether. If 

 the plant is subjected to an increased quantity of ether, growth 

 is retarded. The effect of these poisons is thus very different 

 from that of nutritive substances ; it is due to the irritating 

 properties of the poison. 



b. The ^Election of Organic Food. Not all of the food-stuff 

 presented to the organism is utilized by it neither, on the 

 one hand, all of the kinds of food, nor, on the other, all of the 

 food of the most acceptable kind if offered beyond a certain 

 amount. There is an election of kind and of amount. A 

 study of an election of kind has been made by DUCLAUX ('89), 

 and, especially, PFEFFER ('95). The method employed was 

 this : To the organisms (various molds, Aspergillus, Penicil- 

 lium, etc.) were offered two compounds ; one more nutritious, 

 the other less so. Under these circumstances, the more nutri- 

 tious compound was usually taken by the organism, while the 

 less nutritious was often left entirely alone ; thus, dextrose 

 was preferred to glycerine, peptone to glycerine, and dextrose 

 to lactic acid, even when, in each case, the quantity of the 

 latter substance was in excess of the former. The election 

 was not, however, always of the more nutritious material, so 

 far as we can judge of relative nutritiveness. Thus, when 

 Aspergillus was sown on a nutritive fluid, containing 8% dex- 

 trose and ~L% acetic acid, proportionally far more of the latter 

 was assimilated than of the former, although the latter is of 

 less value as food, as was shown by the fact that the plant had 

 also to devour a considerable quantity of the dextrose. This 

 extensive assimilation of a slightly nutritive substance is not, 

 so far as we can see, an adaptive process. 



The character of the election may change with age, so that 

 what is favorable for growth at one time is not at another. 

 Thus DUCLAUX ('89) found that alcohol restrains or arrests 

 the germination of the spores of molds, whereas it is made use 

 of almost as abundantly as sugar by the adult plant ; so, like- 

 wise, lactose and mannit cannot nourish young plants when 

 they replace sugar in the nutritive solution, whereas they are 

 a good food for the older plants. 



So, also, among vertebrates, the food of the young, supplied 

 in the egg or in the milk of the parent, is very unlike that 



