[Vol. 2 



384 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



bacterium, and we are forced to the hypothesis of chemical 

 changes within the fruits to account for the failure of inocula- 

 tions. As a rule (there are striking exceptions), parasitic 

 micro-organisms are rather sensitive to changes in their en- 

 vironment, e. g., to drying, exhaustion of food supplies, multi- 

 plication of their own by-products, conversion of an easily 

 assimilable substance into one less assimilable or actually 

 harmful, appearance of esters, new acids, etc. But why 

 speculate! Much additional experimenting must be under- 

 taken before we shall have precise and full data. We are still 

 largely in the observational stage. 



The parasites of ripened tissues do not require so much 

 water, are able to convert starch into sugar, or have a special 

 liking for some other element of the plant tissue. 



Externally, a number of factors favor infection. One of 

 these is excessive shade, either of clouds or of foliage. An- 

 other is high temperature. When these two factors arc; ac- 

 companied by excessive rainfall, wet earth, and heavy dews, 

 the conditions are ideal for the rapid dissemination and the 

 destructive prevalence of a variety of bacterial diseases of 

 cultivated plants. The bean spot due to Bacterium Phaseoli, 

 the black spot of plum due to Bacterium Pruni, and the lark- 

 spur disease due to Bacterium Delpliinii, are all favored by 

 heavy dews and by shade. In hot, wet weather in duly pear 

 blight due to Bacillus amylovorus often bursts out like a con- 

 flagration and sweeps over whole orchards. In warm, moist 

 autumns bacterial diseases of the potato may destroy almost 

 or quite the entire crop over extensive districts. 



HOW INFECTION OCCURS 



As I have already described elsewhere how infection oc- 

 curs, 1 I will only dwell for a moment on it here, offering a few 

 examples. 



The commonest way of infection is probably through 

 wounds. 



1 Smith, E. F. Bacteria in relation to plant diseases. Carnegie Inst. Wash- 

 ington, Publ. 27* : pp. 51-C4. 1911. 



