PLANT PHYSIOLOGY PRESENT PROBLEMS 137 



mycetes to become parasitic under special conditions. Two leading 

 inquiries may be suggested: (1) What constitutes immunity or 

 resistance in the host? (2) What constitutes virulence or attenuation 

 in the parasite? 



As the result of practical experiments in cross-inoculation, on the 

 one hand, and of close morphological study, on the other, some in- 

 vestigators have long claimed that there are racial or specific differ- 

 ences between the organisms producing the tubercles on the roots of 

 certain leguminous plants. From the results obtained by Moore 

 (in the U. S. Department of Agriculture) which have been reported 

 but not yet published, I am permitted to recite a further interesting 

 fact of accommodation. When an organism isolated from one host 

 species is grown for a time artificially, under special conditions of 

 nutrition, its host limitations are in great measure broken down, 

 and it may produce tubercles on a variety of leguminous plants. 

 It is likewise conceivable that in the case of certain yeasts the tem- 

 peratures at which spores are formed, and the specific fermentative 

 activities, may be changed by special conditions of cultivation. 



In view, therefore, of the work already accomplished it is certainly 

 evident that the propriety of basing what are termed species upon 

 certain physiological characters has distinct limitations. I do not 

 intend to bring into this paper a discussion of the inadequacy of the 

 present nomenclature system from a physiological point of view. 

 It may be said, however, that it is scarcely possible for the system- 

 atist to consider all physiological characteristics or to appreciate 

 the confused ideals of the physiologist. 



Stimulated by the marked advancement which has been made in 

 physical chemistry, especially in the knowledge of electrolytic dis- 

 sociation, the past few years have added much to our fund of inform- 

 ation with relation to the toxic action upon plants of solutions of 

 both acids and salts, as well also as of certain non-electrolytes. The 

 work of Kahlenberg and True, Heald, Krdnig and Paul, Clark, and 

 others has contributed enough data for an appreciation of the lim- 

 itations of toxic action. Nevertheless, no broad generalizations are 

 as yet possible. Indeed, it is not generalizations which are wanted, 

 but further experimental data bearing upon the relation to the toxic- 

 ity of the ions and molecules and their respective interactions. 



Studies may well be made dealing with the relation of nutrition 

 to toxic agents, the effects of temperature and other conditions 

 upon such action, and the accommodation of organisms to increas- 

 ing strengths of deleterious agents. Naegeli's work on the oligo- 

 dynamic action of copper is beginning to be appreciated, and in one 

 way or another the results have in recent times been repeatedly 

 confirmed. In most cases, however, no allowance has been made 

 for the action of the nutrient salts which may be present in the 



