THE FUNGI WHICH CAUSE PLANT DISEASE 17 



Constancy of Species. Bacteria in nature and under artificial 

 conditions remain true to species. There may be variation from 

 generation to generation as among all other plants or animals of 

 the world, and by the slow process of evolution, a species may in 

 many generations become modified, leading eventually to new 

 races, varieties, and possibly species. That one species can change 

 directly and suddenly to another, much less a species of one genus 

 into a species of another genus, is not to be credited. Marked 

 variation is brought about in many species by change in tempera- 

 ture, food, oxygen supply, etc., changes in size, form, sporulation, 

 flagellation, virulence, chromogenesis, fermentative power, group- 

 ing, etc. These changes belong to the life cycle of the species and 

 occur as reactions to the environment. 



Bacteria were discovered by Loewenhoek in 1683. That they 

 do not originate spontaneously was shown by Pasteur in 1860^. 

 The first disease producing bacteria were recognized in anthrax by 

 Pollander & Davaine in 1849; and the first definite proof that 

 bacteria actually cause animal disease was made by Koch with 

 anthrax in 1875-1878. The first plant disease to be definitely as- 

 cribed to bacteria was the pear blight by Burrill in 1879. The 

 invention of the cotton plug, Schroeder & Dusch, 1853, the gela- 

 tine method of plating for the isolation of species, Koch, 1881, 

 and the use of stains, Weigert, 1875, were practically necessary 

 prerequisites to any considerable advance in bacteriology. For 

 long it was contended, especially by European bacteriologists, 

 that bacteria do not cause plant diseases but most convincing 

 proof to the contrary was adduced by E. F. Smith. 



Entrance to the host plant is made in various ways, very often 

 through wounds, particularly wounds caused by insects, through 

 roots, stomata, water pores, through delicate tissues as blossoms, 

 etc. Once in the tissue, bacteria may migrate rapidly by means 

 of the vessels, intercellular spaces or more slowly through cavities 

 dissolved by the aid of enzymes. 



Classification. In all there are some thirty-six well recognized 

 genera embracing twelve hundred or thirteen hundred purported 

 species of bacteria. This number will doubtless be greatly de- 

 creased when the organisms have been well studied, by finding 

 that many so-called species are not really distinct. The number 



