16 



THE FUNGI WHICH CAUSE PLANT DISEASE 



to the peculiar and characteristic form knovra as "Nail Head" or 

 "Drum Stick" bacteria. In nearly all species of the Eubacteria 

 the spores are solitary. 



There are three modes of spore germination. The most com- 

 mon, polar germination, consists in a rupture of one pole of the 

 spore and the development of a normal vegetative cell through 

 the opening. The second mode, equatorial, Fig. 9, consists in a 

 rupture in the side instead of the end of the spore. The third mode, 



FiQ. 9. — Spores of bacteria, showing bispored cells, spore formation and spore 

 germination. After Frazmowski, De Bary and Koch. 



absorption, consists in a direct development of the whole spore 

 into a vegetative cell. In suitable environment germination may 

 occur immediately after spore formation; if conditions be unsuit- 

 able it may be delayed for many years. 



Under certain conditions most bacteria undergo abnormal 

 changes in form becoming elongated, branched, swollen, bulged, 

 curved, or variously, usually irregularly, distorted. Such are 

 termed involution forms. They are in most cases due to unfavor- 

 able conditions of temperature and nutriment, and the bacteria 

 resume their normal form when again in normal environment. 



The branched forms found in root tubercles after the period of 

 luxuriant growth has passed, and the branched thread-like growth 

 of the bacterium of human tuberculosis upon artificial media, are 

 by many regarded as involution forms. 



