20 DISEASES OF TROPICAL PLANTS CH . 



and have the power of motion, while others cannot 

 move except when carried by other agencies. In re- 

 production they divide, each cell forming two, each of 

 which divides, etc., at such a rate as to result in the 

 formation of many millions in a few hours. With the 

 reduction of the food supply the individuals form heavy 

 membranes and become extremely resistant to external 

 influences until conditions are again favourable for their 

 growth and reproduction. In this condition they are 

 known as spores. 



Bacteria are of the very greatest importance in the 

 economy of nature. Some of them cause the decay of 

 dead animals and plants and thus make the materials 

 which have been stored up in these bodies available for 

 plant food ; others live in the soil and in the tubercles 

 on the roots of leguminous plants and are able to take 

 the free nitrogen from the air and form the definite 

 ammonium compounds which are so important for the 

 growing plants ; while still others are the causes of 

 many diseases of both animals and plants, such as 

 tetanus, diphtheria, tuberculosis, typhoid and other 

 fevers, cholera, crown galls, olive knot, and cabbage rot. 



FUNGI 



The fungi do not contain chlorophyll and therefore 

 cannot make use of the energy of sunlight and manu- 

 facture food from the gases of the air, the water, and 

 salts of the earth. They must make use of food which 

 'has already been manufactured by the chlorophyll- 

 bearing plants and must therefore live upon other 

 plants or animals. Many of them live upon living 

 animals or plants, reducing their vitality and in some 

 cases causing death. They are known as parasites or 

 parasitic fungi. Many other species of fungi live upon 

 the dead and decaying animals and plants, and are 

 known as saprophytes or saprophytic fungi. Some 

 fungi have the power of living both parasitically and 

 saprophytically. 



