PLANT DISEASES 



163 



behave in many respects like fungi and are regarded as such 

 by many botanists. Bacteria grow to a certain size and then 

 divide. This growth and division continues until a great 

 number of individuals is formed. Under certain conditions, 

 such as lack of moisture or food, they cease to develop, but 

 undergo changes which enable them to keep alive. When 

 in such a condition they may be carried about by the wind 

 or other means until con- 

 ditions favorable for re- 

 newed activity are met. 

 Some bacteria get their 

 food from other living 

 plants, thus becoming 

 parasites, and cause 

 disease. An example of 

 this type is the organism 

 causing pear blight. 



In order to recognize 

 the way in which fungi 



Diagrams showing essential parts of the 

 black mold. 



A. Showing how spore cases are related 



grow, it might be profita- to other parts of the mold. 



ble to study the common . J de Single spore case showing sp01 

 black mold as it is seen on 



bread left in damp places. The black mold does not ordin- 

 arily injure plants by causing disease, but a study of its man- 

 ner of producing spores will aid in the understanding of par- 

 asitic fungi which are harder to study. At first the black 

 mold consists of a net work of fine .threads spreading over 

 and extending into the bread. Later very small, dark specks 

 may be seen on the upper surface of the mass of threads. 

 On close inspection it will be seen that each black speck is a 

 tiny ball borne upon the end of an upright thread. When 

 examined with a lens, the ball will be seen to be filled with 



