103 



nutrition differences. It is clear that conidia developed in the open on 

 leaves or stems some of the conidia in humid conditions, some in dry; 

 some on leaves of rich nutrient value, others on those less nutritious may 

 differ materially in measurement owing to environmental differences; and 

 similarly, it is clear that sclerotium-development is markedly influenced by 

 slight changes in humidity. 



Ravn (91) states that conidia measured from one plant were thicker 

 than those from another, but whether this was due to environment or to 

 actual differences in the fungi was not stated. He does, however, state 

 definitely that in his groups conidia collected under different conditions 

 showed different lengths; and that their length depends upon the conditions 

 under which they developed. Examples of extensive modifications due 

 to substratum are mentioned by Edgerton (50), Coons (33), Duggar (45), 

 Moreau (84), and Burger (32). Variations similar to those herein noted 

 as due to substratum, especially those due to the relation of carbohydrate 

 nutriment to color and alteration of conidial mode were noted by me in 

 1909 (Stevens and Hall, 118). Brierley (26) denominates such variation 

 "modal variation." 



MORPHOLOGY OF THE FOOT-ROT FUNGUS 



The parts chiefly to be considered, in the absence of knowledge of the 

 ascigerous stage, are the mycelium, aerial, surface, and submerged, conid- 

 iophores, and conidia. Discussion of the relation of the mycelium in and 

 to host-tissue will be given under the heading "Infection phenomena 

 on wheat" (page 128). 



This morphological study was made in part on the various media here- 

 tofore mentioned and on diseased plants in moist chamber. It was impor- 

 tant, however, to have some means of securing the conidia, and other or- 

 gans above specified, in large quantity, in form readily and conveniently 

 available for examination. Moreover, the variability of the fungus under 

 slight environmental changes emphasized the necessity of securing such 

 morphological units as had developed under conditions as nearly identical 

 and uniform as possible. This led to search for some standard means of 

 culturing the conidia. Evidently corn-meal agar was unsatisfactory, since 

 very slight changes in quality due to mode of preparation led to great mor- 

 phological changes. Green-wheat agar and bean agar, both open to the 

 same objection, also gave too much vegetative growth and too few and often 

 abnormal conidia. Wheat in moist chamber, or wheat leaves or shoots in 

 test-tubes varied so much in humidity that they could not give constant 



