EFFECT OF CERTAIN ECOLOGICAL FACTORS ON THE 

 MORPHOLOGY OF THE UREDINIOSPORES OF PUC- 

 CINIA GRAMINIS ' 



By E. C. Stakman, Head of the Section of Plant Pathology, Division of Plant Pathology, 

 and Botany, Department of Agriculture, University of Minnesota, and M. N. Levine, 

 Field Assistant, Cereal Investigations, Bureau of Plant Industry, United States 

 Department of Agriculture 



COOPERATIVE INVESTIGATIONS BETWEEN THE AGRICULTURAL EXPERIMENT 

 STATION OF THE UNIVERSITY OF MINNESOTA AND THE BUREAU OF PLANT INDUS- 

 TRY OF THE UNITED STATES DEPARTMENT OF AGRICULTURE 



INTRODUCTION 



Extensive studies have been made of biologic forms of Puccinia 

 gratninis Pers., but these studies have been mainly on the physiological 

 rather than on the morphological phase of the problem. The effect of 

 host plants and other factors on the parasitic capabilities of biologic 

 forms has been quite thoroughly investigated. Some work has also been 

 done on the effect of host plants on the morphology of the fungus, but 

 hardly as much as the importance of the problem warrants. 



The question whether biologic forms change readily in response to 

 environmental conditions is important practically and scientifically. 

 The measure of plasticity has usually been the parasitic performance 

 of the rust. But if there is a tendency for biologic forms to change 

 rather quickly, it is reasonable to expect that the morphology might 

 change also. The object of this work, therefore, was to determine the 

 effect of hosts and of physical factors such as heat, light, and humidity 

 on the morphology or urediniospores. It would be desirable to include 

 a study of teliospores and aeciospores also, but the difficulties are obvious. 



Since the effect of physical factors on the morphology of the uredini- 

 ospores may be indirect — by affecting the vigor of the rust — the virulence 

 of the rust under different conditions was also studied. 



Although it has been generally believed that the various biologic 

 forms of P. graminis differ only functionally, yet as early as 1902, Ward 

 {15, p. 2^6)^ suggested that each specialized form — 



is in coiirse of becoming a species and may during the lapse of time actually become 

 a species of Puccinia, which will eventually show morphological differences in 

 addition to the physiological ones it already shows. 



Freeman and Johnson (5, p. 14) expressed a similar opinion in 191 1. 

 Stakman also (11) obtained some evidence that long association with 



* Published, with the approval of the Director, as Paper No. 143 of the Journal Series of the Minnesota 

 Agricultural Experiment Station. 

 2 Reference is made by number (italic) to "Literature cited," p. 77. 



Journal of Agricultural Research, Vol. XVI. No. 2 



Washington, D. C. ' Jan. 13. 1919 



rb Key No. Minn.-as 



(43) 



