AMERICAN 
JOURNAL OF BOTANY 
Vol. VIII January, 1921 No. i 
THE FIXATION OF FREE NITROGEN BY GREEN PLANTS 
Frank B. Wann 
(Received for publication July 3, 1920) 
The ability of chlorophyll-bearing plants to utilize the uncombined 
nitrogen of the atmosphere has been repeatedly investigated during the 
last three or four decades, and the results of numerous observations and 
experiments, covering a wide range of species, have been quite conflicting. 
In some of the earlier experiments with higher plants in pot cultures the 
beneficial effect of a surface layer of algae was often observed, and the ability 
to increase the nitrogen content of the soil by free nitrogen fixation was 
ascribed to members of both the blue-green (Cyanophyceae) and grass- 
green (Chlorophyceae) algae. Similar increases in soil-nitrogen content 
were observed when higher plants were excluded from the cultures, and, 
though bacteria were known to be present, the fixation was generally 
ascribed to the chlorophyll-containing forms. More recently, pure cultures 
of members of the Chlorophyceae have been used but the results in these 
cases have been almost uniformly negative. This fact, together with the 
discovery of widely distributed soil bacteria of the Azotobacter and 
Clostridium types, the ability of which to fix free nitrogen can not be ques- 
tioned, has led to the belief th^at in impure cultures fixation is due not to the 
activities of the green plants but to the bacteria present in the soil. Thus 
it has come to be very generally accepted that members of the Chloro- 
phyceae, as well as the higher plants, are not able to use free nitrogen. 
However, the number of species which have been investigated in pure 
culture is small, and the culture methods employed have not always been 
those which are most favorable for the best growth of these organisms. 
Accordingly the experiments reported here were undertaken for the purpose 
of extending the observations over a larger number of species, grown on a 
variety of mineral nutrient solutions under culture conditions which would 
insure a rapid and vigorous growth. 
Literature 
A complete, and in some cases detailed, review of the literature bearing 
on the relation of the grass-green algae (Chlorophyceae) to free nitrogen is 
available in a paper by Schramm (1914 a), so that a repetition of the account 
is unnecessary here. Nothing of importance relating to this subject has 
[The Journal for December (7: 409-468) was issued January 12, 192 1.] 
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