DEVELOPMENT OF PLANTS 145 



the entire body. Gradually the system forms a substance called 

 an antitoxine that stops the growth and eventually kills the bac- 

 teria. The vigor of the system and the virulence of the bac- 

 teria decide whether the disease shall prove fatal before the 

 antitoxines are formed in sufficient quantity to destroy the bac- 

 teria. Note that these disease-producing bacteria vary in their 

 power to produce toxine. Sometimes, as in epidemics, they are 

 so virulent as to attack with fatal results the strongest individuals. 

 In other cases the bacteria appear to be degenerate and only pro- 

 duce mild symptoms of the disease. This is the principle of 

 vaccination. The little animals causing smallpox are cultivated 

 under conditions that weaken them and render them less virile. 

 Consequently when they are introduced into the system they pro- 

 duce only mild symptoms of the disease. The history of the 

 study of bacteria constitutes one of the most interesting and fasci- 

 nating pages in science. Pasteur was the first to demonstrate 

 that fermentations and putrefactions, and later that certain con- 

 tagious diseases, were due to bacteria and animal organisms. 

 His work is now commemorated by a tablet on the rue Pasteur 

 in Paris with the inscription: Here stood Pasteur's laboratory. 

 1857 Fermentations; 1860 Spontaneous Generation; 1865 Dis- 

 eases of Wines and Beers; 1881 Virus and Vaccini; 1888 Silk- 

 worm Distempers; 1864-1888 Hydrophobic Remedies. To-day 

 every state and government has its corps of workers and every 

 city its expert bacteriologists who are studying the nature of 

 bacteria and their relation to plants and animals. 



CLASS B. BLUE GREEN ALGAE OR CYANOPHYCEAE 



63. The Structure and Nature of the Cyanophyceae. These 

 plants are very simple organisms that have some features in com- 

 mon with the bacteria. They are unicellular plants, although the 

 cells are more commonly joined into rows, forming a thread or 

 filament (Fig. 92, A, B). In some cases these filaments may be 

 regarded as multicellular plants since they branch and because 

 the various cells perform different functions. They form slimy, 

 blue-green, black, yellow or violet masses and, like the bacteria, 

 1 1 



