518 The American Naturalist. [June, 
_ These tubercles are fine examples of symbiosis, and it has 
recently appeared probable that they exhibit mutualism of an 
unexpected kind, analogous to that claimed by Frank for his 
Mycorhiza. I can only go into this briefly. It is known that 
the plant cannot directly assimilate free nitrogen. Yet, as 
Marshall Ward puts it, “ For a long time it has been generally 
known that the Leguminosex, especially, have what we may 
term a special aptitude for seizing large quantities of nitrogen- 
ous substances from the soil, and this problem has become a 
classical puzzle in vegetable physiology.” In 1886, Hellriegel 
and Wilfarth published some investigations of this matter. 
Subsequent experiments founded on theirs have been very 
numerous, and are yet in progress, but their researches remain 
our principal authority on the subject. Without detailing 
them, I may say that these researches seem to demonstrate 
that this power of taking up large quantities of nitrogen 
depends entirely upon the presence or absence of the tubercles 
—that without them it does not exist, and that it exists in 
greater or less degree according to their abundance. Conced- 
ing this, two theories are. possible as to the cause. 
In 1888, Marshall Ward appeared to think that the parasite 
stimulated the cells to extraordinary metabolic activity, and 
that was probably all it did. This view has had no followers 
so far as I can find. 
The other possible theory is that the parasite does this work 
and the host takes advantage of it. Frank, as might be 
expected, takes this view. The most recent observations seem 
to have settled pretty thoroughly that the tubercles do assist 
the plant in some way in assimilating free nitrogen, and that 
here is a case of mutualism analogous to that of the lichens. 
The bacteria (as I assume that they are) are parasites. They 
are there for their own purposes, and are incidentally benefi- 
cial to the plant. The plant, it is generally admitted, can exist 
and thrive without them. In some cases it appears, and the 
analogy of the lichens makes this probable, that the bacteria 
“are purely parasitic, and that there is symbiosis without 
mutualism. But, in most cases of the Leguminose, it seems to 
be shown that the plants affected do better than those unaf 
