Ward ’ — Symbiosis. 551 
Several cases of symbiosis among Bacteria are now known. 
Apart from numerous instances of temporary association 
between pathogenic micro-organisms and animals such as 
earth-worms, rats, flies, ticks, and mosquitoes, which dis- 
seminate their germs and infect cattle, sheep, horses, and 
men (21) (reminding us of the transference of the spores of 
Botrytis by bees, which carry this parasite with the pollen and 
infect the stigmas of bilberries with the parasite (22)) or 
which act the part of intermediate hosts to the disease germs, 
much as certain pond snails do to the liver-fluke of sheep (23), 
we now know several cases of symbiosis between two species 
of Bacteria or of Fungi, or between a Bacterium and a Fungus, 
where each symbiont is incapable of carrying on alone the work 
which the symbiotic association is able to perform — a point 
which is essential to the definition of symbiosis in the narrower 
sense, i. e. the co-operation of two associated organisms, to their 
mutual advantage. 
A striking example is afforded by certain Bacteria concerned 
in the destruction of cellulose in ponds, bogs, rivers, &c. (24). 
Van Senus found that a certain anaerobic Bacterium, re- 
sembling, if not identical with, Van Tieghem’s B. Amylo- 
bacter (25), though incapable of dissolving cellulose by itself, 
can do so if associated with another Bacterium, also incapable 
of itself attacking cellulose. B. Amylobacter can ferment 
pectose compounds, and is thus capable of isolating cells one 
from another, but cellulose is not attacked by it. 
Van Senus believed that the one Bacillus destroys certain 
products of fermentation excreted by B. Amylobacter , which 
inhibit its cellulose-fermenting powers. 
I may remark here, that if a sound potato, rhizome, or other 
underground organ is placed in water and the air exhausted 
as completely as possible, I almost invariably find its cellulose 
walls destroyed in a few days by a mixture of Bacteria, and 
with the symptoms found in many kinds of ‘ wet rot.’ There 
is no reason to believe that these organs would rot if merely 
wet and not deprived of air, since they lie in ordinary soil — 
even moist soil — for weeks or months, with plenty of water in 
