66 PRACTICAL BOTANY. 



injury. Some putrefactive Bacteria act as organized fer- 

 ments. The conversion of the sugar of milk into lactic 

 acid by Bacterium lactis in the souring of milk, and the 

 production of acetic acid from alcohol and oxygen by 

 Bacterium aceti in the souring of wine, are examples. 

 The nitrification of the soil already mentioned is still an- 

 other example of the action of a bacterial ferment. 



Various classifications of Bacteria have been proposed, 

 but none are very satisfactory on account of the difficulties 

 in the way of their study, and because of the still unde- 

 veloped state of bacteriology as a science. It is not yet 

 settled that forms which are now generally considered as 

 genera may not be different stages in the life history of 

 the same species, so that the study of the morphology of 

 Bacteria gives very indefinite results. It is, however, 

 convenient to distinguish the different forms in a rough 

 way, and the following names have been adopted : Bacterium 

 denotes a short, thick, straight rod; Bacillus, a longer, 

 slimmer, straight rod ; Leptothrix, .unbranched filaments 

 in which the segments can usually be distinguished only 

 after treatment with stains ; Micrococcus, a small sphere ; 

 Vibrio, Spirochceta, Spirillum, coiled forms. De Bary 

 illustrates the three principal forms by " a billiard ball, a 

 lead pencil, and a corkscrew." 



BACTERIA. PRACTICAL STUDIES. 



1. Mount a little of the fur from your teeth in a drop 

 of water. Scrape the teeth just under the gums with a 

 knife point. Place a little of the scrapings in a drop of 

 water on a cover glass. Put another cover glass upon it, 

 press and rub the two cover glasses together until there 

 is a thin film on each. Mount by inverting one of the 

 cover glasses on a small drop of water on a microscope 

 slide. Examine with high power and find Micrococci, 



