48 BACTERIA IN RELATION TO PLANT DISEASES. 



were cut out on March 21, and the central or lower portions saved for planting. The juice 

 titrated was taken from canes belonging to these cuttings. Between the date of cutting 

 and the time of extraction of the juice for titration a period of 17 days elapsed, during which 

 the canes were kept in the ice-box, the cut ends sealed with paraffin. The titration was 

 performed in the same manner as previously. Two titrations were made of each variety. 

 The results were as follows (Fuller's scale) : 



Cinta +25 Crystalina +33-5 Caledonia Queen + 24 



Striped Green +25 Blanca +30.5 Common Green +40 



Jamaica + 19 5 A Cuban variety +20 Louisiana No. 74 +35 



The writer has had no opportunity to repeat these tests on fresh canes. 



A sugar chemist, with whom the writer discussed the matter after the first titration, 

 did not believe that well-grounded conclusions could be drawn from single canes ; the next 

 titration might give very different results, as, in fact, proved to be the case (but so also did 

 the inoculations). He said that in order to determine the comparative acidity of different 

 varieties of sugar-cane, the mixed sap of many canes of each variety must be titrated. This 

 I am ready to concede. So much depends on varying factors, e. g., soil, manure, sunshine, 

 rain, time of year, degree of maturity, lodging, etc. The writer does not know whether any 

 chemist has tested the comparative acidity of the sap of different varieties of sugar-cane. 

 From a pathological standpoint, he believes that it would be very interesting. Many bac- 

 teria do not thrive in acid culture-media and often a strong antiseptic influence is exerted. 

 That there should be an average higher degree of acidity in some varieties of cane than in 

 others is in accord with many facts in plant physiology, and perhaps the great susceptibility 

 of some varieties is due to weak acidity, i. e., low total acidity, or else to a smaller amount of 

 some particular acid unfavorable to the growth of the organism and more abundant in resist- 

 ant varieties. It is conceivable that the cane may contain mixed acids in varying propor- 

 tions, one more antiseptic than the other, and this fact would not be brought out by ordinary 

 titrations, but might be fundamental. This might explain why Jamaica was resistant to 

 the disease when from its low total acidity I had anticipated that it would be sensitive. The 

 difference in immunity is evident, whatever the explanation may be. The acidity of the 

 cane-juice explains, furthermore, to the writer at least, why the bacteria tend to avoid the 

 parenchyma and inhabit by preference the vascular system, the juices of which (Greig 

 Smith) are only slightly acid or neutral. They do finally make cavities in the parenchyma, 

 especially in late stages of the disease and in the upper part of the stem, but in this part of 

 the plant there is an excess of water, favoring rapid growth of the bacteria, and with this 

 more vigorous growth there would be produced sufficient alkali to neutralize the inhibiting 

 acids. Another reason, perhaps, why the bacteria are more abundant in the vessels is the 

 better aeration, i. e., presence of more free oxygen in the vessels than in the parenchyma. 



CONDITIONS FAVORING THE SPREAD OF THE DISEASE. 



The writer has seen nothing to indicate that the disease is very infectious. In very bad 

 cases, younger canes from the same stools sometimes showed etiolation, which the writer 

 believed to be due to the secondary effect of the bacteria in the mother stem underground, 

 but no signs were observed among canes growing from stools bearing no inoculated canes, 

 although often these were for months in close proximity to infected canes. So far as it goes, 

 and it must be admitted this is not a very great way, since insect enemies of the cane were 

 absent and water did not rest long on the foliage, this indicates that the disease generally 

 originates with the sets. We can well imagine, however, that under ordinary field condi- 

 tions, with an abundance of dew or rainfall, and plenty of insect depredators, diseased plants 

 might readily infect neighboring healthy ones, especially when young. Wet soil would tend 

 to waterlog the plant, and would also favor the deposit of dew, in both ways favoring 

 infection. 



