244 
The proof is therefore absolute, so far as a biological demonstra- 
tion can ever be so, that the specific disease variously known as pear 
blight, fire blight and twig blight, is caused by the bacteria which 
are associated with it. For the snggestions which eventually led to 
this important and satisfactory result, I am indebted to Professor 
Prentiss of Cornell University. 
The bacteria which cause this disease—named by Professor Bur- 
rill, Micrococcus amylovorus—are not the bacteria of common 
putrefaction, or those which accompany the attack of higher fungi, 
as I have elsewhere shown *, for the latter when introduced into a 
growing pear are unable to overcome the living forces of the cells, 
and make no headway in disorganizing the plant’s tissues. It is the 
veriest nonsense, in the face of all the facts now at command, to 
pretend to account for the presence of bacteria in pear blight by 
saying that they are everywhere, in the air, in the water, the soil, 
even in our own mouths, and that when ‘‘fermentation” once 
begins one would expect to find them present as a matter of course. 
To give the reader some idea of the infinitesimal amount of truth 
in this assertion, it will be entirely safe to estimate that of the hun- 
dreds of kinds and uncountable myriads of individual bacteria 
about us constantly there is not to be found in a pear limb during 
active blight a greater ratio as compared with the specific pear blight 
bacteria,than one toa billion, and it is very doubtful if this is not far too 
liberal an estimate. There is no greater evidence, furthermore, that 
the germs of pear blight are constantly about us in the air, than that 
the germs of diphtheria or small pox are thus omnipresent. The 
ubiquitous bacteria, which are ever in mind and so constantly refer- 
red to, which are the bane of the investigator trying to maintain 
pure cultures, and of everyone wishing to keep organic matter 
unchanged, are for the most part quite innocuous, and in the pres- 
ent connection, uninteresting. 
The work of last year made it evident that the bacteria (i. e. the 
specific pear blight bacteria, Micrococcus amylovorus, as these are 
always meant unless otherwise stated) very likely gained entrance to 
the interior through the ‘‘delicate surface tissues of expanding 
buds.” The observations of this year have so fully confirmed this sug- 
gestion as to amount toa practical demonstration. The earliest 
spontaneous instances of blight were seen June 30, in a small, neg- 
lected hawthorn tree in a pasture. A large proportion of the 
branches bore dead leaves and flowers at their extremities. The fruit 
on the uninjured parts was at this time two-thirds grown, but on 
the blighted parts had not advanced beyond the flowering condition. 
As hawthorns were in flower in this section, about May 15 to 20, it 
is apparent that they could not have been attacked by the disease 
later than that date. There was, moreover, satisfactory evidence 
in the changed appearance of the tissues, that the disease had started 
in the flowers, or at points where there were no flowers in the tender, 
growing tip of the branch. The same observations were soon 
extended to a great number of other hawthorns, and to pears, apples 
and quinces. ‘The only reasonable conclusion to be drawn is, that 

*Third Annual Report, page 364; Amer. Nat., XIX, p. 1185. 

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