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 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 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 Cupulifera are conspicuous, exhibit another symbiosis 

 which is of the greatest interest and importance. The 



