Communications. 261 



edges. They pjather in a cluster aud remain motionless for a 

 short time, but soon they separate. Two cilia, or fine threads 

 like hairs, start out from the same point on the plane or under- 

 side of the spore. These cilia are of unequal length. The 

 shorter one soon straightens out towards what is called the front, 

 the other extends in the opposite direction, and commences to 

 move about with a quivering, undulatory motion, and the spore 

 sails off through the water or drop on the leaf as though en- 

 dowed with life. On account of this power to move from place 

 to place as though possessed of independent life, they are called 

 zoospores. 



At times these conidia, instead of directly producing zoospores, 

 are seen to send out a tube with an enlargement of an oval form 

 at the end; part of the contents of the original conidium passes 

 into this vesicle and it soon separates itself from the original 

 germ tube and forms an independent conidium, which in turn 

 may develop in the same way into a third, and each one, if 

 planted in favorable conditions, will produce zoospores. 



The number of living germs that an be produced by these va- 

 rious methods on a single plant is incalculable. It has been esti- 

 mated that a single square line on the underside of a leaf can 

 produce over three thousand conidia, each one of which may con- 

 tain or develop at least six zoospores, and possible more than 

 double this number. Multiply these by the number of times 

 this can be repeated on one plant with its hundreds of leaves, and 

 it cepses to be a wonder that the fungus is so abundant, or spreads 

 so rapidly. Had it the power to lay hold of and destroy the 

 vitality of healthy plants, the great wonder would be that a single 

 plant should escape destruction. 



The rapidity of the germination of the spores, and the growth 

 of the fungus, will but serve to increase this wonder. Where the 

 atmospheric conditions are favorable and the fungus finds a suit- 

 able soil in the plant, the spores will mature in a few hours after 

 the appearance of the threads through the stomata. They soon 

 sever their connection with the stem and fall to the leaves below, 

 or are borne by the winds to other plants or fields. If they meet 

 the requisite conditions for rapid development on the plants on 



