206 CLIMBERS, PARASITES, SAPROPHYTES. 



distinguished from parasitism, where one organism lives at 

 the expense of another. 



There is no dividing line between total and partial sapro- 

 phytism ; even Neottia has some chlorophyll. 



239. Partial Saprophytes also obtain food by means of 

 a mycorhiza, but since they have green leaves and therefore 

 carry on photosynthesis, they are not so completely dependent 

 on the fungus-servant. 



The roots of most forest-trees, and of very many other 

 plants, especially those which grow in the rich humus of 

 woods and plantations, have an external mycorhiza like that 

 of Monotropa, while in Ericaceae (Ling, Heaths, Bilberry, 

 etc.) the mycorhiza is usually internal as in Neottia. 



In the case of partial saprophytes with an external 

 mycorhiza, it is probable that the fungus-threads act as 

 root-hairs, and absorb water and inorganic salts in addition 

 to organic compounds. The latter are probably chiefly 

 absorbed for the sake of the carbon they contain, since the 

 green plants can absorb carbon dioxide from the air. Plants 

 provided with a mycorhiza produce few or no root-hairs, since 

 the projecting fungus-threads form a far more effective means 

 of absorbing food-materials than do root-hairs. 



240. Assimilation of Nitrogen by Leguminous 

 Plants. The free nitrogen of the atmosphere, although 

 abundant, is not made use of by the green plant. There is, 

 however, one important order of flowering plants, the Legu- 

 minosae, in which the nitrogen of the atmosphere is indirectly 

 used. It was for a long time recognised that leguminous 

 plants would readily grow in a soil containing little or no 

 combined nitrogen, and that as a matter of fact the soil was 

 oftener richer in nitrogen after a leguminous crop had been 

 grown. These facts, which were at first extremely puzzling, 

 have now been explained. 



Numerous small nodules or tubercles are found on the 

 roots of these leguminous plants. When the tubercles are 

 carefully examined they are seen to be filled with small oval 

 unicellular bodies called bacteroids. By some these are 

 regarded as bacteria ; by others as the " spores " of a fungus. 

 They are always present in the soil and infect the roots of 



