OTHER METHODS OF OBTAINING FOOD § 197 
two constituents are a yeast and a bacterium, the former of 
which is closely surrounded by chains of the latter, making 
a fleshy mass of irregular shape, and sometimes of compara- 
tively conspicuous dimensions. The parts played by the two 
organisms are not very well understood, but there seems to 
be no doubt that the association is mutually beneficial. 
In a former chapter mention was made of a property 
which is possessed under certain conditions by various 
plants, particularly by some members of the Leguminose— 
that of being able to utilise the free nitrogen of the air in 
the construction of protein food-substances. The power 
was shown to be connected with the formation of certain 
tubercular structures upon the roots of the leguminous 
plant. These tubercles are swellings of the cortex of the 
root, the cells of which are inhabited by a particular 
fungus, which breaks up in their interior into curious 
bacterioid bodies. The exact nature of the fungus has 
not been accurately determined. The soil contains many 
of these bacterium-like bodies, which make their way into 
the interior of the leguminous plants by penetrating their 
root-hairs, and growing down them into the cortex of 
the root. In the cells of the latter the penetrating fila- 
ments bud off the bacterioid bodies in great numbers. The 
stimulus resulting from the invasion causes a considerable 
hypertrophy of the cortex of the roots at the points attacked, 
and tubercles are frequently the result. The fungus 
appears to have the power of fixing atmospheric nitrogen, 
bringing it into some combination, the exact nature of 
which is unknown, but which serves as the starting point 
of protein synthesis, either by the green plant or by the 
intruder. The relationship is clearly of great advantage 
to both organisms, the fungus obtaining its carbohydrate 
supplies from the green plant, much as is the case in the 
lichens already described. 
Many of our forest trees, among which the members of 
the Cupulifere are conspicuous, exhibit another symbiosis 
which is of the greatest interest and importance. The 
