PLANT DISEASES 247 



the higher plants. They consist of thread-like filaments 

 which may occur singly or in tangled masses, or organized 

 into more or less definite forms. The fungi have many dif- 

 ferent degrees of complexity, from the exceedingly simple 

 bacteria, consisting of only one cell with its contents, to the 

 very complicated mushroom with its thousands of cells and 

 intricate structure. But even the complexity of structure 

 in the mushroom does not compare with the differentiation 

 of tissues in the simplest of the flowering plants. These 

 fungi have no flowers and produce no seeds in the botanical 

 sense. Instead, they have reproductive bodies called spores, 

 which are usually exceedingly small, and are easily dis- 

 tributed from place to place. Some idea of their prevalence 

 may be gained from the fact that ordinary bread placed under 

 conditions of warmth and moisture will invariably develop 

 bread mold; and this mold is merely one among thousands 

 of similar fungi. 



358. How fungi grow. Every green leaf contains thou- 

 sands of very minute green particles called chlorophyll bodies. 

 These are the active workers which enable these plants to 

 change the crude sap received from the ground into manu- 

 factured food such as starch, sugars, fats, and oils. 1 Fungi, 

 however, possess no chlorophyll; and since they, like all 

 other plants, must have these manufactured foods, it follows 

 that they can live only on the food manufactured by the 

 higher organisms. All fungi are, therefore, either parasites 

 or saprophytes; that is, they live either on living plants or 

 animals, or on dead material. It is the former class that is 

 especially active in producing plant diseases. 



The spores (Fig. 126) of the fungus are carried in one way 

 or another to the surface of the leaf or stem. If conditions are 

 correct (in general, if the air is warm and humid), these 

 spores germinate and penetrate the surface tissue of the plant. 

 The new fungi then grow down through the cells of the host 



See Chapter II. 



