946 



SCIENCE. 



[N. S. Vol. XXI. No. 547. 



potassium, Sterigmatocytis niger may pro- 

 duce no conidia or very curious modifica- 

 tions of tlie conidiophores. By far the most 

 interesting problems with, relation to the 

 mineral nutrients are those which have to 

 do with the roles of these elements in 

 metabolism. The effect of the lack of one 

 or another element is made manifest by 

 some general macroscopic change, and 

 sooner or later, by disturbing pathological 

 changes and subsequent death. It is re- 

 ported, for example, that the absence of 

 iron prevents the development of a healthy 

 green color, and a scarcity of potassium is 

 made evident, especially, in reduced pho- 

 tosynthesis. 



We are yet merely at the threshold of 

 these problems. A cytologieal and micro- 

 chemical study of numerous plants in vari- 

 ous conditions of culture is needed. Loew 

 has instituted some good work in this direc- 

 tion. He attempted a careful microscopic 

 study of Spirogyra under the conditions 

 indicated. Although well rewarded, he 

 has not followed up the result. The prob- 

 lem is, nevertheless, again under serious 

 investigation, and when much time and 

 thought shall have been devoted to it, with 

 the utilization of the best cytologieal meth- 

 ods available, important results may be 

 anticipated. The possibilities of the fix- 

 ture are particularly dependent upon this, 

 that investigation must be made of all 

 macroscopic changes as well as of all dem- 

 onstrable microscopic changes. 



The interrelations of parasites and hosts, 

 or of symbionts, are of such great physi- 

 ological 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 nutri- 

 tion of a host plant determine to a consid- 

 erable 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 Uredinaceffi have established the 

 fact of the existence of 'biologic forms.' 

 This opened a new problem in the study of 

 the Uredinacese and it was later ascertained 

 that similar host-restricted forms are pres- 

 ent in other groups of the fungi, especially 

 in the powdery mildew Erysiphe graminis. 

 Salmon has found bridging host species by 

 means of which the parasite may pass from 

 one species or host to another ; for example, 

 the form of E. graminis on Bromxis race- 

 mosus is incapable of afifecting B. comniu- 

 iatus, but does not fail to affect B. horde- 

 aceus; and the spores produced on the lat- 

 ter will then affect the previously immune 

 B. commiitatus. 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 in- 

 jured area or certainly within the sphere 

 of the traumatic influence. Spores pro- 

 duced by such infections proved capable 

 of infecting uninjured leaves. The appli- 

 cation of these results is certainly far- 

 reaching; yet they must be extended and 

 confirmed before a conservative explana- 

 tion may be advanced. It is undoubtedly 

 more or less in line with the well-known 

 capacity of such fungi as Botrytis, Nectria 

 and certain Basidiomycetes to become para- 

 sitic under special conditions. Two lead- 

 ing 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 moi'phological study, on the other, 



