102 PLANT MORPHOLOGY 



tion and great extremes of temperature. In some bacilli the resting cell 

 becomes an endospore. Here the protoplast rounds up inside the cell 

 cavity and invests itself with a new cell wall, the old wall eventually 

 disappearing. Endospores are extremely resistant. With the return of 

 favorable conditions, they again become active vegetative cells. Thus 

 "spore formation" in bacteria does not result in reproduction but merely 

 in survival during a period of stress. 



Although the bacteria are said to be without sexual reproduction, 

 there is some evidence that it may occur at least in certain bacteria, since 

 there seems to be Mendelian segregation resulting from mixtures of 

 different types. 



Activities. Most bacteria are either saprophytic or parasitic and in 

 both cases food is absorbed through the cell wall. Some can live either 

 as saprophytes or as parasites, while a few can make their own food 

 without the aid of chlorophyll or light. Aerobic bacteria require free 

 oxygen in respiration, while anaerobic bacteria obtain oxygen from organic 

 compounds. 



Most diseases of animals and many diseases of plants are caused by 

 parasitic (pathogenic) bacteria, the disease itself being merely a response 

 on the part of the host to the presence of the parasite. A disease mani- 

 fests itself by symptoms, which are abnormalities in structure or function. 

 Well-known human diseases of bacterial origin are typhoid fever, tuber- 

 culosis, diphtheria, pneumonia, cholera, and tetanus. Some bacterial 

 diseases of plants are pear blight, cabbage rot, cucurbit wilt, and crown 

 gall. The disease may be caused by direct attack of the bacteria on the 

 host tissues, by the liberation of toxins, or by both. 



The decomposition of dead organic matter is accomplished chiefly 

 by saprophytic bacteria. They break up organic compounds into sim- 

 pler substances through a series of intermediate steps, a succession of 

 different bacteria being involved. The ultimate products of decom- 

 position are such simple substances as water, carbon dioxide, ammonia, 

 hydrogen sulphide, etc. Bacteria of decay cause fermentation and 

 putrefaction. They play an important part in the economy of nature 

 by returning to the air and soil substances that may again be used by 

 other organisms. 



All plants require nitrogen in order to synthesize proteins, but only 

 the nitrogen-fixing bacteria and a few other forms are able to use the 

 nitrogen of the air directly. Practically all green plants are dependent 

 for nitrogen upon its compovnids, particularly nitrates. Some of the 

 nitrogen-fixing bacteria, such as Clostridium and Azotobacter, live free in 

 the soil and are saprophytic on organic matter, while Rhizobium is para- 

 sitic in the roots of various Leguminosae, such as clover, alfalfa, peas, 

 beans, etc. These bacteria combine atmospheric nitrogen with oxygen 



