420 BULLETIN OF THE BUSSEY INSTITUTION. 
and, finally, darted off with great rapidity a full-fledged zoéspore 
furnished with two cilia. The number of zodspores produced in a 
conidia is very variable. The most frequent number is five or six. 
Sometimes there are not more than three, and we counted in one case 
seventeen. Not unfrequently one or two of the zodspores do not 
succeed in escaping from the mother-cell, but they are seen to move 
about inside. Where two remain, it often happens that, by trying to 
move in opposite directions, they prevent one another from passing 
out at the open mouth of the conidia. The shape of the zodspores can 
hardly be described, as it is continually changing. Plate IIL., Figs. 6, 7, 
show two of the most common forms which they assume. At times, 
they are irregularly oval, flattened on one side, and more acute at one 
end than at the other. There are always two bright spots near the flat 
edge, from which two cilia project in opposite directions. In other 
cases, the zodspore rolls itself up into a ball, the two bright spots are 
brought almost in contact with one another, and the cilia project 
nearly parallel to one another. Plate III., Fig. 7. The length of the 
zoospores varies from .008 mm. to .010 mm. ‘They move about for 
from fifteen to twenty minutes, the motion growing gradually slower. 
At the end of that time they come to rest, the cilia drop off, they 
assume a spherical shape; and, in about a quarter of an hour, an out- 
growth appears on one side which develops rapidly into the 
mycelium of a new plant. Under no circumstances have we seen any 
direct production of a hypha from the conidia itself as sometimes 
happens in Peronospora infestans. 
The regularity and punctuality with which germination takes place, 
notwithstanding the variations in light, heat, and other external con- 
ditions, is quite surprising. It is so regular that by properly 
arranging the time of sowing, and first making sure that the conidia 
used are quite ripe,* we are able to be tolerably certain of a crop of 
zoospores for class demonstration at any given hour. At the end of 
an hour, or an hour and a quarter, the conidia will have begun to 
swell; a quarter of an hour later, the zodspores will have been dis- 
charged; in another quarter of an hour they will have come to rest; 
and in another quarter of an hour they will have begun to germinate. 
Time and time again have we sat with watch open before us to observe 
* To make sure of this, it is better to shake a piece of a diseased leaf over 
the water to be used, rather than to plunge it into the water. 
