Junes, 1916 Siher-Scurj of Irish Potato 341 



From this it is apparent that, even though considerable variation in 

 spore dimensions occurred on infected tubers from different locaHties, 

 nevertheless an even greater variation resulted in the case of spores from 

 a single spore strain. This shows that normally a wide variation exists, 

 and consequently it does not appear necessary to form small-spore and 

 large-spore species. 



REACTION OF THE FUNGUS TO LIGHT 



In order to secure a better knowledge of the relation of 5. atrovirens to 

 its environment so that its life history might be better understood, 

 experiments on some of the physiological characteristics of this organism 

 were conducted. 



The reaction to Hght is of special interest in connection with the effect 

 of storage conditions upon the development of the fungus on potatoes. 



In this study the writer used the plate-dilution method, the conidia 

 being sufficiently diluted on Lima-bean agar plates to be observed 

 individually. Immediately after the plates were poured, each was 

 wrapped in carbon paper, the entire dish being covered except an aperture 

 from I to 2 cm. in diameter at the side, and the plates were then arranged 

 with the apertures facing the light from the window. 



The plates were examined at the end of three days and it was found 

 that the mycelial branches developed on the side of the hyphas farthest 

 away from the window and that the majority of these grew in the opposite 

 direction from the source of the light. The position of germ-tube forma- 

 tion does not appear to be influenced by the light, germination sometimes 

 taking place from the side closest to the source of light; but as soon as the 

 germ tube receives the heliotropic stimulus — that is, when it is a few 

 millimeters long — it invariably turns away from the light, and subsequent 

 mycelial development is formed on the side of the conidium farthest 

 from the source of light. Instead of appearing at the center of the colony, 

 therefore, the conidia are found at the margin exposed to the light, and at 

 the end of 5 to 10 days the entire colony appears as if a gentle breeze had 

 blown the hypha in one general direction away from the light (PI. XLVI, 

 fig. 3). These results also confirm Eichinger's (5) observations. 



The reaction of this fungus to light in culture media demonstrated 

 that it is negatively heliotropic. In view of the fact that infection 

 of the tubers in the field takes place in the dark, negative heliotropism 

 here does not obtain. In order to determine whether this heliotropic 

 property favored tuber infection, artificial inoculations were made on 

 tubers in the light. In this case no perceptible difference occurred, 

 since infection appeared on all parts of the tubers alike. 



REACTION OF THE FUNGUS TO MOISTURE 



Like most fungi, S. atrovirens requires considerable moisture for 

 development; but, owing to the absence of accurate instruments for 



