178 PHARMACEUTICAL BOTANY 
shaped organism which appears widely distributed in soils. It is 
apparently attracted to the root-hairs of leguminous plants by a 
chemotaxic influence probably due to the secretions poured out 
by these structures. A number of these organisms penetrate the 
walls of the root-hairs by enzymic action. Upon entering the 
hairs they form bacterial strands which branch and rebranch and 
extend into the middle cortex 
% ® ( cells. Within the cortex cells 
yK >. the organisms multiply rapidly 
a a ® SI . producing nest-like aggrega- 
4 4 1 tions. Their presence here 
stimulates the rapid multipli- 
with the consequent forma- 
en ve a # cation of the parenchyma cells 
y 
me ° tion of nodules or tubercles. 
Ww o a ; 
te R 5 Under oil-immersion magni- 
S 3 ° eo fication these bacteria are often 
Fic. 110.—Pseudomonas radicicola. 1, fouaid 0 aes : VariGnsly 
From Melilotus alba; 2 and 3, from Shaped forms called involution 
Medicago sativa; 4 and 5, from Vicia villosa. forms. They remain within 
(Marshall, after Harrison and Barlow from the cells of the middle cortex 
Lipman.) 3 S 
region breaking down the 
starch and sugar present into simpler compounds. In doing 
this the Pseudomonas obtains the energy required to ‘‘fix’’ the 
free atmospheric nitrogen. This process is called “fixation of 
atmospheric nitrogen” and consists of uniting the free nitrogen 
of the air with some other elements to form a nitrogenous com- . 
pound. This is assimilated into its own body. ; 
The bacteria gradually swell up into zodglwa masses, until 
finally their bodies break down into soluble nitrogenous sub- 
stances which are partly absorbed and assimilated and partly 
stored as reserve nitrogenous food by the green leguminous 
plant. 
Rotation or Crops.—Upon the decay of the leguminous 
roots and their bacteria in the soil, the nitrogenous compounds 
are set free and are ready to be decomposed by various soil 
organisms into the various decomposition products, including 
amino-acids and ammonia, 
