12 BACTERIAL DISEASES OF PLANTS 



find in the maize stems that extensive breaking down of the 

 pith and bark into vast cavities which is so common, for example, 

 in tobacco and tomato stems. 



WHAT GOVERNS INFECTION 



Within the plant we may suppose, from certain indications, 

 that abundant juiciness is one of the factors governing the in- 

 fection of immature tissues. To this may be added an abun- 

 dant supply of well-adapted food and, in some cases, probably 

 the absence of inhibiting substances, which may appear later. 

 As parts approach maturity, the intercellular air-spaces become 

 much larger and the water content becomes relatively less. 

 Along with this, acids, sugars, proteids, amino-acids, etc., are 

 consumed and converted into substances less well adapted to 

 the needs of the meristem-parasites, if not wholly inimical. 

 In young shoots of potato and tomato, or of pear and apple, 

 as contrasted with old ones, or in the roots of carrots as com- 

 pared with the leaves, or in juicy carrots as compared with 

 flabby ones, or in rapidly growing cabbages as compared with 

 slow-growing ones, we know that there is an excess of water, and 

 this alone appears to be sufficient to explain the difference in 

 behavior of their respective parasites in old versus young parts. 

 When, however, we come to ripening fruits, such as the pear 

 and the plum, it would seem that they are still juicy enough to 

 favor the growth of almost any bacterium ; we are forced, there- 

 fore, to the hypothesis of chemical changes within the fruits 

 to account for the failure of inoculations, and this throws some 

 doubt on the preceding hypothesis. As a rule (there are striking 

 exceptions), parasitic micro-organisms are rather sensitive to 

 changes in their environment, e.g., to drying, exhaustion of food- 

 supplies, multiplication 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 and experiments are needed.^ 



1 In the above connection the following list of fruit acids may be of some use: 



