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.” 
MorpPHOLOGY OF THE Foot-RotT 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 
