OTHER METHODS OF OBTAINING FOOD 203 
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 protem food-substances. The power 
was shown to be connected with the formation of certain 
tubercular structures upon the roots of the leeuminous 
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 
hodies. The exact nature of the fungus hay 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 filaments 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. 
A somewhat similar symbiosis is met with in the roots of 
Cycas. A regular system of spaces in the cortex, extending 
round it in an almost regular cylinder, is occupied by an 
assemblage of Algw (Anabeena) and Bacteria (Pseudomonas 
