1902.] DISEASES AND INSECTS, ETC. I35 



to intelligently compreliend the philosophy of such treatment 

 it is necessary to know something of the nature of the para- 

 sitic fungi, their manner of living, and of propagating their 

 kind; also the conditions which are favorable or unfavorable 

 to their development. The simplest forms of vegetable para- 

 sites which cause specific plant diseases are called " bacteria." 

 They are exceedingly simple in structure; they consist of an 

 outer covering, which is the cell wall, and an inner soft living 

 substance called " protoplasm." That is all there is of them. 

 There are no separate pieces of the body, or anything of that 

 kind. They propagate in an exceedingly simple manner. 

 During active growth they simply divide into two parts, each 

 of which is altogether alike, and performs all the functions of 

 the original or parent cell. Under favorable conditions 

 growth is rapid. One cell soon separates into two, the two 

 soon become four, the four eight, the eight sixteen, and con- 

 tinuing in that ratio the number of individuals rapidly becomes 

 enormous. Bacteria represent among fungi what quick grass 

 stands for among weeds. As every joint or node of the root- 

 stocks of quick grass is capable of developing into a separate 

 plant, so also every bacterial cell is capable of reproducing a 

 separate independent individual. It is not strange, therefore, 

 that it has been found very difficult to treat bacterial plant dis- 

 eases successfully. Among the diseases of this class are the 

 fire blight of the pear, quince, and apple, the stump-rot of 

 cabbages, and the wilt disease of melons and cucumbers. 

 These diseases are very hard to control. They do not seem to 

 be amenable to treatment by spraying, and other methods have 

 to be adopted. 



■ More complex in structure than the bacteria are the fila- 

 mentous fungi, in which the vegetative cells remain attached 

 to each other, and by continued growth form filaments or 

 threads, in some cases simple; in others branched. Among 

 the more highly organized fungi there is a division of labor. 

 Certain parts of the plant structure are developed for the 

 special purpose of reproducing and disseminating the species, 

 while other parts perform simply vegetative functions. Thus, 

 in the case of the fungus which causes the disease commonly 

 called " potato blight " or " mildew," the vegetative part of the 

 parasite consists of the threads or filaments of the mildew. 

 It penetrates the inner tissues of the potato plant, and absorbs 



