42 BOTANY AND PHARMACOGNOSY. 



tissues of the host, as in tuberculosis, and in the other they pro- 

 duce powerful poisonous substances, or toxins, as in diphtheria. 



Classes of Bacteria. In order to study Bacteria they are 

 grown upon nutrient media, such as sterile bouillon, potato, milk, 

 etc. They are divided into a number of classes, depending for 

 the most part on the shape of the cell : ( i ) The Sphserobacteria, 

 or Cocci, are those whose cells are spherical or spheroid, and in 

 which division takes place in one, two or three directions of space. 

 Very few of the group are provided with cilia. According to the 

 number of cells in a colony they are distinguished as Micrococci, 

 Diplococci, etc. (2) Bacteria proper are elongated, . rod-shaped 

 organisms in which division occurs in only one direction, namely, 

 transversely to the long axis, and only after a preliminary elong- 

 ation of the bacterium. The Bacteria are subdivided into two 

 important groups, namely. Bacterium and Bacillus. The Bacilli 

 are motile organisms and produce endospores (Fig. 26a), whereas 

 the Bacteria are non-motile and do not usually produce endospores. 

 (3) Spiral bacteria constitute the third principal group and are 

 characterized by the cells being spirally coiled. Division is in 

 only one direction. These bacteria are usually motile, and seldom 

 produce endospores. (4) There is another important group 

 which includes the Sulphur Bacteria, of which the most common 

 one is Beggiafoa. These occur in long threads, and move in an 

 undulating manner much like Oscillaria, one of the Blue-green 

 Algse. They are found in sulphur waters, as in sulphur springs, 

 and contain sulphur granules. 



Bacteriological Technique. Principally because of the 

 minuteness in size of micro-organisms a different technique is 

 required in their study from that required in the study of the 

 higher plants. In the first place it is difficult to isolate them 

 so as to be able to study individual forms. Another difficulty is 

 to prevent contamination after they are isolated. And even 

 though a pure culture is obtained it is difficult on purely morpho- 

 logical grounds to differentiate the various forms, as they are all 

 so much alike. 



I. While it is comparatively easy to prepare a sterile solution, 

 that is, one in which all life is absent, it is very difficult to prevent 



