122 MASSACHUSETTS H<3RTICUL,TURAL SOCIETY. 



not easily wetted, a smootli covering from which the water rnns 

 off quickly ; or have interposed chemical obstacles, such as a sap 

 not adapted to the growth of the parasite. These then are some 

 of the reasons why all of our plants are not immediately attacked 

 and destroyed. To sum up or restate the case : First, the para- 

 site is very liable to be destroyed by adverse conditions before it 

 readies the plant. Second, it must gain an entrance into the 

 plant by its own exertions or by the help of otlier organisms, and 

 this may be difficult or impossible. Third, once in, its growth may 

 be effectually stopped by insuperable chemical or physical ob- 

 stacles. Not every potential parasite becomes an actual parasite. 

 The difficulties lying in the way are often too great for the organ- 

 ism to overcome and consequently a good portion of our crops 

 escape. The business of the pathologist is to find out just what 

 these difficulties are in case of each particular parasite and to 

 teach the grower how he may increase them. 



The most of this address will be devoted to showing just how 

 certain parasites succeed in getting from one plant to another. 

 It is commonly assumed, I know, that their reproductive bodies 

 float through the air and accidentally lodge on the plants, but 

 while not denying that this often occurs, and is perhaps in 

 many cases the exclusive means of infection, I am certain that 

 in some cases this rarely or never occurs, and I am inclined to 

 believe, for reasons which I will now explain, that in quite a large 

 number of diseases the chief danger of infection lies in other di- 

 rections. I shall occasionally refer to preventive measures, but 

 with the facts before you, you will in many cases be able to draw 

 your OAvn conclusions as to the best method of procedure. 



The first subject to which I invite your attention is the spread 

 of parasite organisms by means of insects. 



Spread by Insects. — This is a method of distribution which 

 has received little attention from the cultivator, but one which I 

 am persuaded is very common. The' gnawings, borings, and 

 punctures of insects, injurious as these often are, can by no means 

 be reckoned as the whole of the injury which they do, and some- 

 times they are the least part of it. As carriers of disease they 

 are no less potent in the vegetable than in the animal world. In 

 what I have to say under this head I shall deal only with three 

 diseases, but these are widespread, and their dissemination by 

 insects has i)een fairly well worked out : 



