332 PROCEEDINGS OF THE ACADEMY Olr [1886. 



(PI. Ill, figs. 1, 2, 6). They are 1 to 1;^ ,a long, by i to | ,a broad, 

 and quite colorless. For the most part, they exist as single inde- 

 pendent cells, but may often be found in pairs, especially when 

 still multiplying, and in rare instances are united into a series of 

 four or even more, but never extend into chains. 



During rapid vegetation, in rich nutritive media, the move- 

 ments reach a stage of extreme activity. The appearance is 

 what is termed swarming, in which the bacteria move rapidly 

 back and forth, in and out among each other, but never in a 

 straight line to any distance. As the rate of growth becomes 

 less from any cause the movements are retarded. Taken directly 

 from the tissues of a blighting tree, the movements of transla- 

 tion are usually sluggish or imperceptible, although the universal 

 Brownian movement is likelj' to give a misleading appearance of 

 activity. Under speciall}'^ favorable conditions, as when grown 

 during hot weather in very succulent shoots, or from artificial 

 inoculation in unripe fruit, the movements are much increased 

 and may become quite rapid. When taken from the tree in 

 winter, or when grown in solutions that are too acid or too alka- 

 line, or which are deficient in the proper nutritive substances, 

 there is no perceptible locomotion. 



When in active growth, the cells present a uniformly dull 

 appearance. By conditions which are unfavorable to normal 

 growth, yet do not entirely check it, such as strongly acid or 

 alkaline solutions, deficient nutriment, or exhaustion b}'^ keeping 

 the cultures several months, the cells become highl}^ refractive, 

 and to some extent take on the appearance of the spores of 

 other species of bacteria. Whether in this state the}^ possess 

 any of the characteristic powers of resistance which belo^ig to 

 spores, has not been ascertained. 



Formation of Zoogloea. By far the most characteristic feature 

 in the life history of Micrococcus amylovorus is the formation of 

 zoogloea (figs. 2, 3, 5). These have never been observed in the 

 tissues of the tree under an 3- conditions, or in or upon any sort 

 of solid media, but they occur with much regularity in fluid 

 cultures, when placed under favorable conditions for rapid 

 growth. 



Thej'^ are produced to some extent throughout the fluid, but 

 are most abundant in the thin pellicle which forms upon the 

 surface, appearing within forty-eight hours from the beginning 



