240 MISSOURI STATE HORTICULTURAL SOCIETY. 



nourishment from the cells by means of sucker-like branches called 

 Jiastoina. 



As stated above, the conidia serves to propagate the fungus dur- 

 ing the summer. The germination of the conidia is a very interesting 

 process, and may be observed by any one having a compound micros- 

 cope. When sown in a drop of water they first absorb some of the 

 water and swell; soon lines of demarcation appear, and shortly after- 

 wards the walls of the conidia are ruptured, and a number of small 

 motile bodies escape. These little bodies called zoospores remain mo- 

 tionless for a few moments; soon, however, they begin moving, and by 

 means of two hairs or cilia^ with which each zoospore is provided, 

 they swim rapidly about. After moving about for some time, the cilia 

 drop off and the zoospore becomes motionless. Each zoospore then 

 sends out a tube, and if this is in the proper position, it enters one of 

 the breathing pores of the leaf, and gives rise to another crop of 

 conidia' Water is absolutely necessary for the germination of the 

 spores, and this is one reason why the mildew is more abundant dur- 

 ing a wet season. 



The conidia or reproductive bodies are formed at intervals from 

 the middle of July until frost. The first frost destroys them, but be- 

 fore they are destroyed another kind of reproductive body is formed 

 on the internal mycelium. These bodies called oospores., fall to the 

 ground with the leaf^ remain dormant during the winter, and the fol- 

 lowing spring germinate. They are much larger than the conidia., and 

 are hidden in the tissue of the leaf. There are two investigating 

 membranes; the outer is thick, hard and colored, while the inner is 

 nearly transparent. 



As stated above, they remain dormant during the winter and when 

 the leaf decomposes in the spring they germinate. In germinating 

 the oospore absorbs matter, swells and finally ruptures the outer mem- 

 brane allowing the inner membrane to escape. Enclosed within this 

 thin membrane is a number of zoospores and they are set free by the 

 bursting of the thin membrane. The zoospores are active for a time, 

 but they finally settle down and send out a tube the same as those 

 derived from the conidia- We have then the life history (beginning 

 with the winter spores in the tissues of the decaying leaf) about as 

 follows: The winter spores when they germinate in the spring, give 

 rise to a mycelium which in turn gives rise to the conidia or summer 

 spores. These conidia are blown about by the wind and when they 

 fall upon the proper host plant and there is moisture enough present, 

 they germinate, producing the internal mycelium which may in turn 



