632 The American Naturalist. [August, 
Wiesner’s hypothesis is somewhat different. It has been 
known for some time that various essential oils and other vege- 
table products, e. g. thymol, salicylic acid, benzoic acid, tan- 
nin, quinine, oil of cinnamon, oil of peppermint, ete., exert a 
powerful restraining influence on the growth of many bacteria, 
and it is not improbable that a great variety of bactericidal 
and protective substances occur in plants. On the other hand 
there may be and probably are bacterial parasites capable of 
thriving in the very plants which Wiesner found most resist- 
ent to continuous spray, to the saprophytic bacteria of stagnant 
water, and to those of decaying meat infusions, the exact con- 
ditions under which any given microorganism will thrive 
being determinable only by experiment. It must also be re- 
membered that the physiological requirements of bacteria often 
become profoundly modified to suit changed environments, | 
and that all parasites have undoubtedly descended from sapro- 
phytic forms. Prof. Wiesner has, however, opened up a very 
inviting field and its further investigation by some careful ex- 
perimenter, trained in bacteriological methods, might lead to 
very interesting discoveries. 
Most of the recent books on vegetable pathology devote a 
chapter to the bacterial diseases of plants, but these books have 
not been written by bacteriologists and consequently the state- 
ments given are usually very meager and unsatisfactory, 
and forcibly illustrate the fact that no one can write acceptably 
on a subject with which he is not familiar, not even if he pos- 
sesses a logical mind and has read all the “authorities.” 
Excepting Prof. W. Migula, who reviewed the subject briefly but 
somewhat carefully in 1892," and Dr. H. L. Russell, who gavea 
brief summary in tabular form the same year at the end of 
his Thesis," no one seems to have gone over the field critically 
since de Bary’s time, although there isnow a considerable body 
of literature. It is proposed, therefore, in the following pages to 
examine the literature of this subject from the standpoint of the 
10 Migula: (10) Kritische Uebersicht derjenigen Planzenkrankheiten, welche ange- 
blich durch Baktrien verursacht werden. Semarang. Midden-Java. 1892. Exp. 
Sta. 
u Russel: l. c., pp. 36-41. 
