556 
Ward . — Symbiosis . 
food-materials by the one symbiont for the other may be an 
important factor ; e. g. an Alga supplies a Fungus with carbo- 
hydrates, or a Fungus converts starch into the fermentable 
sugars which the associated yeast needs. In other cases the 
advantage derived is one of protection from some injurious 
agent — e. g. the aerobic Bacterium prevents the access of 
oxygen to the anaerobic one. But there is evidence which 
suggests that mere nutrition or protection is not the only 
or even the principal factor involved. It is well known that 
the products of fermentative actions are frequently poisons, 
and we all know of cases where such poisonous excreta of 
living cells act as stimuli to other living cells, if supplied 
to them in minimal doses and very gradually: I need only 
instance the effects of tobacco or alcohol on man, in illustration 
of this. 
Several observers have shown that in presence of a particular 
food-substance the living cell is stimulated to produce and 
excrete a particular enzyme, while the substitution of another 
food stimulates the organism to excrete a totally different 
enzyme. 
Now let us see if there is any evidence to support the hypo- 
thesis that some such stimulative action is exerted by one 
symbiont on another. To a certain extent we find such evidence 
in the remarkable vigour and large size of the algal cells in 
a Lichen as compared with the same cells living an independent 
life, and in the persistent zone of brilliant green and often 
hypertrophied cells of leaves in which certain Fungi are living, 
the gigantic cells of the nodules on leguminous roots in which 
the bacteroids are living, and many other cases ; but since it 
is impossible to say how far these are cases of merely enhanced 
nutrition, we will pass them by and seek for other instances. 
One of the earliest I can find is Hugo Schulz’s demonstra- 
tion in 1888 ( 41 ) that minute quantities of poisons such as 
corrosive sublimate, iodine, iodide of potassium, bromine, 
arsenious acid, chromic acid, sodium salicylate, or formic acid, 
when added to yeast in 10 per cent, grape-sugar solution, 
immediately raise the fermentative activity of the organism — 
