97 
reduced by an environment of relatively dry air; also that the relative vari- 
ability is much increased by these conditions (cf. data of Fig. F with data 
of Fig. A and Fig. G). The great influence of humidity on the morphology 
of conidia and conidiophores is discussed by Wartenweiler (124). Pammel, 
King, and Bakke show by respective illustrations (90, PI. I, figs. 1, 2, 3, and 
13-16) large variation in shape and size, and apparently in mode, of conidia 
of Helminthosporium as grown in the field and in the greenhouse. Similar 
effects from humidity were noted by Beach (12) in Septoria. The relative 
paucity of sporulation and the tendency of H. No. 1 to turn to the produc- 
tion of aerial mycelium unless the air-humidity was very high, and to pro- 
ceed to profuse conidia-formation only when the relative humidity was 
above 90%, explains the failure to find Helminthosporium conidia on dis- 
eased wheat-stems in ordinary moist-chamber conditions, though these 
same stems, when given very moist conditions, invariably became covered 
with Helminthosporium conidia. 
The six strains of Helminthosporium subjected to the humidity tests 
all agreed in essential behavior and characters with the reaction of H. No. 1 
to the same, as given above; but there is apparent disagreement here with 
the conclusions of Ravn (91) who says that he placed “‘no water in the damp 
room (Boettcher chamber) since the moisture would be so great that it 
would prevent the development of conidia.” 
Test 3. Under a bell jar lined with filter-paper dipping in water— 
thus securing a close approximation to a saturated atmosphere—open 
Petri-dishes of inoculated corn-meal agar were placed. Under these con- 
ditions H. No. 3 made much more white aerial mycelium than did H. 
No. 1, and also grew faster, whether on thin (30 c.c.) or thick (60 c.c.) 
layers of agar. 
It here appears that a condition of excessive humidity best develops the 
differential characters of the aerial mycelium of these closely related races. 
Test 4. H. No. 1 was grown on corn-meal agar, and when the colony 
was well developed it was dried slowly until growth completely stopped. 
‘This resulted in a dense black band of conidia near the edge of the colony. 
There were many conidia upon each conidiophore, as many as thirteen being 
counted in one instance, and the conidia clusters resembled bunches of 
grapes. Only the oldest conidia of a cluster were at all large and these fell 
far below the mean as grown under standard conditions, while the other 
conidia were much below the usual size (cf. Graphs 62-64 [Fig. O] and 39-42 
[Fig. K]) and extremely variable, with coefficient of variability 29.99 as 
compared with 12.22 under standard conditions (Graph 42, Fig. K). It 
