OTHEE 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 Iho 

 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 Leguminosce 

 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 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 Algae (Andbcena) and Bacteria (Pseudomonas 



