128 MICROSCOPIC FUNGI. 



point of the margin which during the movement 

 of the zoospore is foremost, is a disk-shaped 

 vacuole, with two cilias of unequal length attached 

 to its margin; the shorter cilia is directed forwards, 

 and the longer in the opposite direction, during the 

 evolutions of the zoospores. 



The zoospores are produced within from an hour 

 and a half to three hours after the sowing of the 

 conidia in water. They are never absent if the 

 conidia are fresh, or even a month old, but beyond 

 this period their artificial generation is very un- 

 certain. This little experiment is a very simple 

 and interesting one, and may be performed by any 

 one who will take the trouble to follow out these 

 instructions. 



From this simple experiment, let us turn for a 

 moment to the plant in its natural condition when 

 affected by the white rust. If, after rain or dew, 

 when the little drops of moisture hang like pearls 

 about the sickly pallid leaves of the shepherd's- 

 purse, bespattered with the white pustules of the 

 rust, we collect and examine a drop of water from 

 the immediate neighbourhood of one of the pustules, 

 we shall commonly find empty conidia and zoospores 

 in different stages of development. 



Water alone seems to be essential to them, and 

 for this the conidia may remain unchanged for a 

 month, and literally burst into activity at the first 

 gentle shower, till the whole surface of the plant is 

 swarming with zoospores. We may no lon^ei 



