18S6.] NATURAL SCIENCES OF PHILADELPHIA. 335 



one part of glucose to fifty of water gave no growth of the 

 bacteria. 



In testing the effect of acids upon the development of blight 

 bacteria, a ^ per cent, of malic acid was added to the usual infu- 

 sion of potato. This prevented the formation of a i>ellicle, tur- 

 bidit}^ or zoogla3a, but gave a very considerable cloudj^ sediment,, 

 largely made up of loosely aggregated groups of blight bacteria, 

 which were brilliantly^ refractive. A similar solution with 2 per 

 cent, of malic acid gave a slightly less abundant sediment, but with 

 otherwise the same results. Some of the latter was transferred 

 to a corn-meal solution, producing the characteristics of a pear- 

 blight culture, except the formation of zooglcea. After some 

 days this was introduced into a pear tree, which in due course of 

 time gave the true blight, showing that the bacteria of the acid 

 solution were really blight bacteria. Attempts to grow them in 

 a nutrient 5 per cent, solution of citric and tartaric acids have 

 not been successful. 



Testing the nature of the bacteria in cultures producing 

 limited growth, by inoculating directly into the tree, has not, as a 

 rule, proved successful, as for some reason they seem unable to 

 gain a footing in the living tissues. It is therefore necessary to 

 transfer them first to richly nutrient cultures, from which, after 

 a time, they may be introduced into the tree, and, if the blight 

 bacteria are present, will start the disease. 



Cultivation in Solid Media. In test-tube cultures with nutrient 

 gelatine the most characteristic results have been obtained by 

 adding a drop containing blight bacteria to the gelatine while 

 liquid, and thoroughly distributing the germs by shaking the 

 tube. In from two to three days the gelatine contains numerous 

 small white dots, which, upon examination under the microscope, 

 prove to be a mass of bacteria of the usual appearance. The 

 dots are globular or oval, and increase to about .5 mm. in 

 diameter. No further growth or change takes place, and in this 

 condition they remain for weeks, without liquefying or otherwise 

 atlecting the gelatine. 



When sown upon the surface of the gelatine by drawing a 

 needle or glass rod over it, or by placing a drop on it, the growth 

 is feeble and does not amount to more than a slight shining 

 appearance of the surface. 



A nutrient solution made from an unripe pear, in which blight 



