76 NATURE STUDY AND AGRICULTURE 
of nitrates in the soil is easily ex- 
hausted, so that it must be replen- 
ished from time to time. 
There are several thousand spe- 
cies of legumes, the most familiar 
examples being peas, beans, the 
clovers, alfalfa, sweet clover, prairie 
clover, the wild vetches, and the 
milk vetches. The family also in- 
cludes several trees, as the Siberian 
pea tree and the locust. 
The Family of the Composites. 
—This is the largest family of 
Rep Cover Roors our common plants and includes 
With nodules. 
many of our most familiar flowers. 
The term composite refers to the fact that what we regard 
as a single flower is really composed of a large number 
of flowers, thus making a sort of compound 
flower. The cultivated sunflower, on ac- 
count of its large size, is perhaps the best 
example for studying the structure of com- 
posites, but the common thistle, the blaz- 
ing star, or the wild sunflower will do 
very well. 
Let us take the sunflower as our ex- 
ample. Upon careful examination we find 
that the ‘‘flower’’ consists of scores of tiny 
florets massed together ina compact cluster 
which botanists call a ead. On the out- 
side of this there are several overlapping 
Pops or LEGUMES 
a, bean; 6, alfalfa; 
c, clover. 
rows of small, leaf-like scales that resemble a calyx and 
are called the involucre. Next within this green involucre 
