372 The American Naturalist. [April, 
nected with the organic food supply of these latter; he thinks that the 
Algæ, furnished with nitrogen by the bacteria, assimilate the carbo- 
hydrate material, part of which goes to their own maintenance, but 
part also to that of the microérganisms. It is, therefore, in his belief, 
an instance of symbiosis in which each supplies the wants of the other. 
There are many facts, partly the result of his own observations, partly 
the result of those of others, which uphold this view. Ifthe mixed 
culture be placed in the light, there is a far more noticeable nitrogen 
increase than when in darkness. Again, if a rich supply of carbon 
dioxide gas be provided, this is marked by a decided rise in nitrogen- 
fixing powers. Both these conditions are such as are known to influ- 
ence carbohydrate assimilation in chlorophyll-containing organisms ; 
but all experience is antagonistic to the view that light should be bene- 
ficial to the vital activity of the bacteria, and there are only one or two 
exceptional instances (Nitromonas, ete.) in which carbon dioxide can 
be directly assimilated by these microérganisms. 
Moreover, in the cases where the bacteria are bought into immediate 
contact with the Alga, as in those species of Algze which are enveloped 
in a gelatinous covering wherein the microorganisms become imbedded, 
nitrogen fixation appears to be greatly aided, and the addition of sugar 
to the culture has no such marked effect as in the instances where non- 
gelatinous Algæ are employed. The explanation of this seems to be 
that the bacteria embedded in the gelatinous sheath are amply pro- 
vided with carbohydrate food without the addition of sugar, which, 
therefore, comes more or less as a superfluity. 
All this seems to justify Kosswitsch’s view of the part played by the 
Algæ in the fixation of nitrogen ; it appears to show that they have an 
indirect, but none the less important, influence upon the process. 
(Nature, Jan. 24, 1895.) 
