NEGATIVE HELIOTROPISM OF UREDINIOSPORE GERM-TUBES 83 
The following method of germination was chiefly used. Dry 
urediniospores were dusted over the surface of a drop of a 5 per cent 
non-nutrient gelatin placed on a glass slide in a Petri dish containing 
a moist filter paper. The Petri dish was then placed in a dark box on 
a window ledge with an aperture, 2.5 cm. in diameter, towards the 
window. All tests were made with diffused daylight. Controls were 
maintained in darkness. 
In all of the ten or more tests that were made with an exposure of 
four or five hours to a unilateral illumination more than four-fifths of 
the germ-tubes responded negatively to the light stimulus. A count of 
some 200 germ-tubes in three tests gave an average of 86 per cent that 
had grown away from the light {fig. la). A small part of the remainder 
Fig. I. Germinating urediniospores of Puccinia Rhamni; a and h, exposed for 
three hours to a unilateral illumination; c, germinated in darkness. The arrows 
indicate the direction from which light was admitted. 
had grown towards the light and a large part were recorded as transverse 
to the incidence of light. The germ-tubes of the controls in darkness 
grew in all directions {fig. ic). 
Urediniospores of Puccinia Rhamni have from six to eight germ- 
pores that are distributed at approximately equal distances apart 
over the spore surface. A study of the germ-tubes that had grown 
away from the light showed that the large majority of them has issued 
from pores located on the part of the spore wall farthest from the light 
{fig. 2). Others that had issued from pores on the part of the wall 
towards the light or transverse to it had soon changed their direction 
