44 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 Beggiatoa. These occur in long threads, and move in an 

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

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

 and contain sulphur granules. 



ARCHEGONIATES. 



The two main features which distinguish the Archegoniates from 

 the Thallophytes are the structure of the sexual organs and the dis- 

 tinct manner in which the peculiar phases known as alternation of 

 generations is shown. The antheridium or male sexual organ is a 

 well differentiated multicellular body which is either sunk in the 

 adjacent tissues of the plant or is provided with a stalk. Within 

 it are organized the sperms or spermatozoids, which are ciliate 

 and swim freely in water. Corresponding to the oogonium of 

 the Thallophytes is the archegonium or female sexual organ 



