854 The American Naturulist. [October, 
the infection is transmitted by insects in their visits to the 
flower for nectar. The germs thus carried from an infected to 
a healthy tree are deposited on the nectary as the insect gar- 
ners the honeyed store. A lodgment effected, the germ finds 
a rich nutrient medium and it multiplies profusely, penetrat- 
ing finally the exposed cells of the nectary, which are desti- 
tute of a cuticle. With an entrance once established, the 
contagion spreads from blossom to twig, from twig to branch, 
until at last the whole tree structure becomes involved, and 
falls a prey to the disease. Since we have learned that the 
blackened leaves and branches contain the virus, it becomes 
only a matter of watchfulness and care on the part of the 
husbandman to check and ultimately stamp out the malady 
by the excision and burning of the diseased tissue. 
Another problem of vast importance not only to agricultur- 
ists, but to sanitarians, is the subject of nitrification, or the 
conversion of nitrogen bearing substances into nitrates and 
nitrites. It has been an established fact, known for years, 
that under certain conditions, there was an increase in the 
nitrous and nitric salts in the soil, but just how this was 
brought about, was for a long time an unsolved problem. 
Chemists had explained the phenomenon on a purely chemi- 
cal basis, and it was believed that the nitrogen of the air was 
oxidized, either by the action of ozone or oxygen. Pasteur 
as a result of his studies on fermentation was the first to sug- 
gest that the production of nitric acid in the soil might be due 
to organized ferments. This idea was taken up by Schloesing 
and Miintz fifteen years ago, and the working out of the pres- 
ent accepted biological theory of nitrification is largely due to 
the untiring labors of these eminent French investigators, as 
they laid the basis for the more brilliant and successful results 
of Winogradsky and Warington. 
Interesting as is the story, time will not permit entering 
upon the detailed steps by which the germ theory was finally 
proven. Fora number of years after it had been shown that 
organized ferments played the chief róle, no advance was 
made in the isolation of the organism as the different investi- 
gators failed to produce any nitrification with the germs they 
