TILLETIA TRITICI 



273 



bunt-balls over-wintering in the soil have their pericarps 

 destroyed in the same manner. 



Measurements of 75 dry and 75 

 wet chlamydospores of Tilletia tritici 

 showed that the average diameter of m 

 dry and of wet spores was 16-06 /x 

 and 19-16 /x respectively. There- 

 fore, the ratio of the volume of an 

 average dry spore to that of an 

 average wet spore is about 1 : 1-65. 

 When a bunt-ball swells up, there- 

 fore, the swelling is chiefly due to 

 the pressure exerted by the swelling 

 chlamydospores. 



The Reactions of the Pro- 

 mycelium to External Stimuli. — 

 As the chlamydospores of Tilletia 

 tritici must often germinate in the 

 interstices of the soil, it seemed p 

 worth while to determine whether 

 or not the promycelia are helio- 

 tropic. The following experiment 

 was therefore made. Chlamydo- 

 spores were sown on 2 per cent, 

 plain agar in a Petri dish and the 

 dish was completely covered with 

 black paper, except for an area 

 8 mm. square on one side through 

 which alone light could pass. The 

 Petri dish was then placed in a 

 black wooden box, 6 inches high, 

 6 inches wide, 15 inches long, with 

 one end open ; and the box was 

 then set on a table with its open 

 end toward a window. In the 



course of 3-5 days, the chlamydospores germinated and 

 produced numerous promycelia exposed to unilateral illumin- 



VOL. V. T 



Fig. 134. — Tilletia tritici. The 

 swelling of bunted wheat grains 

 in water and the emission of 

 their spores. Three bunted 

 wheat grains have been placed 

 in water contained in a glass 

 vessel. The grains float, ab- 

 sorb water, and increase in 

 volume. The swollen spores 

 press against one another, 

 escape en masse through a 

 crack in the pericarp, and sink 

 in the water. In the bunted 

 grain on the left the escaping 

 spores have been forced up into 

 the air like paint from a tube 

 of oil-paint. About one and 

 one-third the natural size. 



