180 PHARMACEUTICAL BOTANY 
The tubercles occur either singly, as is frequently the case on 
subterranean branches, in small groups the size of a pea, or in 
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Fic. 112,—Nitrogen cycle in the life of alfalfa, a leguminous plant. (From Brownell?s 
General Science.) 
larger coralloid loose or compact clusters which frequently attain 
the size of a black walnut. Each tubercle is a short, cylindrical, 
blunt-ended, root-like structure which branches di- or trichot- 
