A FEW IMPORTANT PLANT FAMILIES 75 
them being grown together except at the upper end. In some 
species all ten stamens are thus grown together. The pistil, 
when ripe, forms a dry pod containing a 
single row of seeds, as we see in the peas 
and beans. The leaves of this family of 
plants are alternate and are usually com- 
pound. 
The great value of legumes is due to the 
nodules or tubercles that grow on the roots 
of the plants. These are tiny, whitish 
swellings not generally much larger than 
the head of a pin. They are the home of 
swarms of minute beings called bacteria 
that have the power of causing the ordinary 
+ nitrogen of the air to 
unite with certain sub- SWEET PEA Biossom 
stances of the soil and form what is 
called nitrates. These nitrates are then 
taken up into all parts of the plant, 
including the seeds, giving them a supe- 
rior food value as seen in clover and 
alfalfa, and in peas and beans. The 
soil in which legumes grow is also 
greatly enriched by the nitrates in the 
roots and nodules. To appreciate the 
importance of all this the student must 
~ understand that nitrates are our most 
APhewemia: thie slainene valuable food constituent. They form 
and pistil; the ripened the basis of the proteids that are so 
pod. essential for the support of man and 
beast. They are indispensable to the growth of plants, 
and plants must obtain them from the soil. But the supply 
SWEET PEA 
