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. Ol and 39-42 

 [Fig. Kj) and extremely variable, with coefficient of variability 29.99 as 

 compared with 12.22 under standard conditions (Graph 42, Fig. K). It 



