136 PLANT PHYSIOLOGY 



some general macroscopic change, and sooner or later, by disturbing 

 pathological changes and subsequent death. It is reported, for ex- 

 ample, that the absence of iron prevents the development of a healthy 

 green color, and a scarcity of potassium is made evident, especially 

 in reduced photosynthesis. 



We are yet merely at the threshold of these problems. A cytological 

 and microchemical study of numerous plants in various conditions 

 of culture is needed. Loew has instituted some good work in this 

 direction. He attempted a careful microscopic study of Spirogyra 

 under the conditions indicated. Although well rewarded, he has not 

 followed up the result. The problem is, nevertheless, again under 

 serious investigation, and when much time and thought shall have 

 been devoted to it, with the utilization of the best cytological methods 

 available, important results may be anticipated. The possibilities 

 of the future are particularly dependent upon this, that investigation 

 must be made of all macroscopic changes as well as of all demon- 

 strable microscopic changes. 



The interrelations of parasites and hosts, or of symbionts, are of 

 such great physiological interest that some of the most significant 

 problems may not justly be omitted in this connection. It has long 

 been assumed that the conditions of nutrition of a host plant deter- 

 mine to a considerable extent its immunity to parasitic attack. Ward 

 was unable to detect in the bromes any modification of resistance 

 due to either high cultivation or to lack of sufficient mineral nutrients. 



The results which have been attained with the Uredinacese have 

 established the fact of the existence of " biologic forms." This opened 

 a new problem in the study of the Uredinaceae and it was later ascer- 

 tained that similar host-restricted forms are present in other groups 

 of the fungi, especially in the powdery mildew Erysiphe graminis. 

 Salmon has found bridging host species by means of which the para- 

 site may pass from one species or host to another; for example, the 

 form of E. graminis on Bromus racemosus is incapable of affecting 

 B. commutatus , but does not fail to affect B. hordeaceus; and the spores 

 produced on the latter will then affect the previously immune B. com- 

 mutatus. From infection studies it is further found that there are 

 biologic forms among the grass hosts as well. Salmon reports that this 

 restriction of the parasite to certain hosts may be broken down if 

 the vitality of the leaf has been lowered by traumatic means. In 

 this case penetration would result either in the injured area or cer- 

 tainly within the sphere of the traumatic influence. Spores produced 

 by such infections proved capable of infecting uninjured leaves. The 

 application of these results is certainly far-reaching; yet they must 

 be extended and confirmed before a conservative explanation may 

 be advanced. It is undoubtedly more or less in line with the well- 

 known capacity of such fungi as Botrytis, Nectria, and certain Basidio- 



