94 



they did not appear at all in the wetter ones (PI. XV). This was notably 

 true of H. Nos. 1 and 5. Even at the end of five weeks no sclerotia were 

 produced in the wet tube by H. No. 1. It is apparent that the smaller 

 amount of water is the more favorable for observation olsjclerotiu in- 

 formation, either at two or four weeks, and that observation of wet tubes 

 at the end of two weeks is sufficient for other characters. Ravn (91) has 

 suggested sclerotial formation as a character for the separation of certain 

 species. It is obvious that if this character is employed care must be given 

 to the humidity of the media. 



HUMIDITY OF AIR 



Test-tubes were prepared with water in the bottom and a glass -lip 

 inserted as is shown in Fig. 2, and autoclaved. If filter-paper or a wheat 

 leaf is laid on the glass slip the humidity is sustained throughout the length 

 of the tube by evaporation from the leaf or the paper, but if no such con- 

 ductor of water is used, and cereal (e.g. wheat) shoots are laid crosswise 

 on the glass slip (as indicated in Fig. 2) and inoculated, the growth of the 

 fungus may be observed under different conditions of air-humidity. 



Determinations kindly made for me by Dr. G. L. Peltier with espe- 

 cially accurate apparatus devised by Dr. C. F. Hottes, showed that moist 

 wheat sprouts became shriveled and apparently dry in 3 hours at a relative 

 humidity of to 60%; in 12 hours, at 70% and 80%; in 24 hours at 90%, 

 and that only in a relative humidity greater than 90% did the sprouts 

 remain apparently moist for a longer time. 



Test 1. Moist sterile wheat-shoots were placed on the glass slip (as 

 shown in Fig. 2) with centimeter intervals between them, and inoculated 

 with Helminthosporium No. 1. All shoots 3 cm. above the water-level 

 dried within 24 hours, indicating that a relative humidity as high as 90% 

 existed only in the region of the lowest shoot and that next above it. In the 

 region of approximately 90% relative humidity many of the conidiophores 

 became abnormally long (600 ju), and the basal part was of mycelial rather 

 than of conidiophore character (cf. Fig. 3) . It was apparent from observa- 

 tions on shoots in these various humidities that when the air was too dry 

 for conidia-production there was a considerable development of aerial my- 

 celium, which accounts for the fact that frequently the basal portion of a 

 vegetable stem (e. g., celery or wheat) may bear conidia, while the upper 

 part bears a tuft of woolly aerial mycelium. 



Test 2. Old, dry wheat-straw with water in test-tubes was autoclaved 

 and inoculated with H. No. 1. In the humid bottom the conidia-clusters 



