182 PHARMACEUTICAL BOTANY 
omously after attaining a certain length. The branches fre- 
quently rebranch at their tips which grow out into long 
thread-like structures from 1-3 cm. in length that may also 
branch and become entwined about the roots of other plants. 
The color of the youngest tubercles is a pinkish-gray brown. As 
the tubercles become older their color changes to brown, dark- 
brown and even black. 
Tue Nirrocen Cycie 
Living plants make use of nitrogen in the form of nitrates 
found in the soil and water. The nitrates are absorbed by the 
plant and split up within its cells into the 
component elements, the nitrogen of which 
is linked with some other simple elements 
there present including carbon, hydrogen, 
oxygen, sulfur and sometimes phosphorus 
to form proteins. Animals eat the plants 
and part of the proteins are digested by 
them and excreted as urea, uric acid and 
hippuric acid. These products are later 
Fic. 115. Endo- decomposed by bacteria (micrococcus ure@, 
trophic mycorrhiza in etc.) to form ammonia. The remainder of 
epidermal cells of the : . ; 
root of a member of the the proteins are assimilated and stored in 
heath family, Andromeda the growing animal cells. When the ani- 
polfolia, the root shown mals die their bodies decay and the pro- 
in cross section. (Palla- |. : ‘ 
din.) tein or nitrogenous compounds are split 
into decomposition products (peptones, 
amino-acids, ammonia) of which ammonia is the final end 
product. 
The soil contains Nirriryinc Bacteria. Of these the Nitro- 
somonas and WNitrosococcus secrete enzymes which oxidize ammonia 
to nitrous acid. Some of the nitrous acid is neutralized by 
alkaline substances found in the soil to form nitrites. Another 
nitrifying organism, the Nitrobacter secretes enzymes which oxi- 
dize nitrous acid and nitrites forming nitric acid; free nitric acid 
then combines with other substances (bases) in the soil forming 
nitrates, thus completing the cycle. 
