1886.] NATURAL SCIENCES OP PHILADELPHIA. 339 



These tests do not cover the possibility of the ptomaine being 

 volatile, which is really not very great. It is yet necessary to 

 make tests of freshly blighted tissues from the tree, which can 

 only be done during the hot months. 



Action of the Organism in the Living Plant. The bacteria of 

 blight have the power of growth and multiplication in the pres- 

 ence of the living cells of the pear, and in this one important 

 respect differ essentially from other species of bacteria. By arti- 

 ficial inoculation into growing unripe pears, which give most 

 marked and certain results, it is found that other bacteria are 

 entirely innocuous, at once disappearing without having made 

 any growth or induced any changes in the tissue of the pear. If 

 blight bacteria in active condition are intermixed with the other 

 forms, they penetrate the cells, multiply, and finally bring about 

 the disorganization and death of the tissues which marks the pro- 

 gress of the disease, but the associated forms disappear the same 

 as when introduced alone, and the product is a mass of practically 

 pure blight bacteria. 



This result is rendered possible on account of the fact already 

 stated, that the blight bacteria penetrate the tissues, and main- 

 tain their normal growth for some time (days or weeks), before 

 the life of the cells is sufficiently interfered with to permit the 

 growth of other forms. The bacteria always extend beyond the 

 visible location of the disease in small branches, often to the 

 distance of a third of a meter or more. 



One of the properties which enables this species to successfully 

 penetrate the pear-tree is evidently its unusual indifference to 

 acids, which prevents most other forms from making any 

 growth; the juices of the pear give a strong acid reaction with 

 test paper. 



What chemical changes are brought about by its activity in 

 the plant cannot be definitely stated, further than to say that a 

 mucilage or gum, which is soluble in water, is produced in abun- 

 dance, with the disengagement of carbon dioxide. The contents 

 of the cells, together with the cell- walls which have not been 

 liquefied or changed into stony tissue, pass over into this viscous 

 product.^ 



^ Rep. N. T. Agric. Exper. Station for 1885, p. 248 ; Araer. Nat., vol, 

 xix, 1885, p. 1181. 



