255 
on which they grow. For this reason the members of this family 
are able to thrive on soils which are unsuitable for other plants. 
Not only do these bacteria supply the nitrogen compounds neces- 
sary for the host on which they grow but the soil is enriched by 
the growth of such plants. 
Fic. 37. Insect-galls on a branch of swamp-oak. 
The value of clover as a fertilizer has long been known but 
it is only comparatively recently that the reasons for this have 
been worked out, and the value of such plants as rotation crops 
is fully recognized at the present time. 
