50 WORCESTER COUNTY HORTICULTURAL SOCIETY. [1885. 



that the diseased germs enter the tree through the- tenderest tissues at 

 the ends of the branches and in the flowers, usually in early spring, 

 but where the germs come from had not yet been discovered. The 

 first trial was to learn if the germs would thrive outside the living 

 tissues of the tree. A nutritive solution was made by boiling some 

 corn meal in water and using the colorless liquid. A particle of 

 blighted pear twig was introduced into such a solution and in one to 

 two days at ordinary room temperature the whole liquid was filled 

 with the germs. It was also found that they would grow in hay tea, 

 made by boiling hay in water, and in various other things, including 

 barnyard manure, although not very vigorously in the last. It now 

 became evident that the course of the disease is usually something like 

 this : The germs in a diseased tree escape to the surface in the sticky 

 exudation, they are washed free of the gum surrounding them by the 

 rains, drop to the ground and multiply indefinitely in decaying vege- 

 table substances either beneath the trees or in marshy or wet places at 

 a distance. Here they may pass the winter and even live for several 

 years. They are borne into the air when dry by the winds, or are 

 carried up by evaporation. Being so extremely minute they may be 

 borne through the air for great distances and coming in contact with 

 the moist surface of tender twigs or the centre of the flower, find 

 entrance to the tree and again produce the disease. 



Proof that a specific germ causes the disease. — So far it has been 

 assumed that the germs accompanying the disease are the cause of it. 

 The attempt was now made to prove rigidly that such is accurately 

 true. It will be remembered that the disease was communicated by 

 inoculating with the diseased juices washed out with water. It is 

 evident therefore that the disease must be due to one of two things: 

 either to the germs or to the juices which accompany them. The first 

 attempt was to separate the germs from the juices about them. This 

 was done by a succession of cultures. A small drop of water con- 

 taining pear-blight germs was introduced into some corn meal solution 

 in which all life had been killed by boiling it. The germs increased 

 by growth and filled the liquid, the juices accompanying, not being 

 living matter, did not increase but were diflfused through the whole 

 liquid, and so greatly diluted. A small drop of this culture was intro- 

 duced into a fresh solution of corn meal ; the germs multiplied and 

 filled the liquid, while the very small part of the original diseased 

 juice, which had been introduced with the germs, was again greatly 

 diluted. Again, a drop from this culture was transferred to a fresh 

 preparation, and so on through six cultures. From the last culture a 

 drop was put into growing pear tissues, three months from the time 

 the germs were taken from the tree to start the first culture, and the 

 disease was produced in the usual form. By this means the germs 

 had been kept alive by constant growth, while the juices accompanying 

 them had been so enormously diluted as to be practically absent. 

 The attempt was now made to show that the juices of the disease by 

 themselves could not produce the disease. This was done by filtering 

 out all germs from an infusion made by slicing up diseased twigs in 

 water, by passing it through a fine porcelain jar, such as are used for 



