DAIRY AND SEED IMPROVEMENT MEETINGS. 359 



fungiis penetrate in various directions through it, killing the 

 cells and living on their contents. After a time certain threads 

 are pushed out through the little, lip-like breathing pores on 

 the under side of the leaves. These are to become the repro- 

 ductive organs. Then the end of one of these threads begins 

 to swell up and by and by becomes a little pear-shaped or 

 lemon-shaped body. This is pushed to one side and then the 

 thread goes on forming others. These bodies are the spores of 

 the fungus, corresponding to the seeds of higher plants. They 

 may germinate directly and infect new spots on the leaves, but 

 if the temperature conditions are right the interior contents of 

 these spore bodies split up into other small elements, on the 

 average about ten, which finally escape through the rupture of 

 the wall of the original spore. These are little, free-swimming 

 bodies which swim about in the drops of rain and dew and 

 finally come to rest and begin to germinate by throwing out n 

 tube which grows into the leaf and causes a new center of 

 infection. 



It will be seen that the fungus possesses almost unlimited 

 powers of reproduction. Each single disease spot on the leaf 

 produces innumerable threads which bear hundreds of spores, 

 and each one of these spores may split up into an average of 

 ten of the httle, free-swimming bodies, each capable of causing 

 a new center of infection. These are readily transported about 

 by means of wind, rain, dew, rubbing of the leaves together in 

 the wind, or in working among the plants, and probably by 

 various other agents. 



Tliis explains why it is that the disease which has passed 

 unnoticed, or has been seen but slightly on the field, may spread 

 like wildfire over it, destroying all the plants in a very short 

 time if weather conditions are right. It also explains why, 

 in spraying with bordeaux mixture, the most thorough and 

 painstaking w^ork is necessary, and why it is absolutely essen- 

 tial that spraying be begun and be thoroughly done before the 

 appearance of blight on the field. Bordeaux mixture is a pre- 

 ventive of late blight and not a cure. After the spores have 

 once germinated, and the germ tubes enter the leaves, spraying 

 would be somewhat comparable to attempting to put out a fire 

 in the interior partitions of a building by drenching the outside 

 of it with a fire hose. 



