338 ESSENTIALS OF BOTANY 
Seaweeds are considerably employed for fertilizing pur- 
poses along some parts of the New England coast. 
It has long been known that poor soils may be bene- 
fited by plowing under certain crops grown, upon them. 
Most of the plants employed in this way belong to the 
Pea family, and clovers and alfalfa are among the species 
Fic. 231. Roots of Red Clover with Tubercles. 
7, sections of ascending branches; b, enlarged base of stem; ¢, root- 
tubercles containing bacteria. 
commonly plowed under. It has only recently been ascer- 
tained that the value of leguminous plants for this purpose 
is chiefly due to the fact that they bear upon the roots 
tubercles containing bacteria (Fig. 231) which can take 
free nitrogen from the air and deliver it to the host-plant 
(clover, etc.) in a form which can at once be utilized by the 
