45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 191 



probable that plants possessing chlorophyll absorb their nitrogen 

 in the form of nitrates only. 



Nitrogen may be also absorbed, at any rate, by parasites, sapro- 

 phytes, and insectivorous plants, in the form of "nitrogenous carbon- 

 compounds. 



Although it is generally true that plants cannot assimilate un- 

 combined nitrogen, nevertheless certain plants (Papilionese, such as 

 Peis, Beans, etc.) will grow and flourish in a soil from which all 

 traces of nitrogen-compounds have been carefully removed. The 

 nature of the means by which this result is attained is not yet 

 completely determined, but the principal facts are briefly as follows. 

 In the first place, the roots of these papilionaceous plants have been 

 found to bear peculiar gall-like outgrowths termed tubercles. The 

 tubercles are the result of the attack of a Fungus which penetrates 

 into the root through the root-hairs. The green plant and the Fun- 

 gus appear to exist in a state of symbiosis, with the result that the 

 green plant is adequately supplied with combined nitrogen although 

 growing in a soil from which such compounds are originally absent. 

 In explanation of these facts there can, first, be no doubt that 

 the supply of combined nitrogen obtained by the green plant is 

 ultimately derived from the free nitrogen of the atmosphere ; and, 

 secondly, that the supply is not obtained from the atmosphere 

 directly by the leaves, but indirectly by the roots through the soil. 

 Nor can there be much doubt that the tubercles are associated with 

 the process of the assimilation of the free nitrogen, and that it is 

 effected by the Fungus. 



The tubercles are structures formed by the hypertrophy of the 

 cortex of the root : their cells are rich in sugar and starch : the 

 branches of the fungus-mycelium penetrate most of the cells, and 

 there bud off innumerable gemmules (sometimes called bacterioids). 

 The tubercle eventually becomes disorganised ; the gemmules are 

 then set free into the soil, and are doubtless the means by which 

 other roots become attacked by the Fungus. 



Sulphur, which is a constituent of proteids and a few other sub- 

 stances occurring in plants, such as oil of Mustard, is derived from 

 the sulphates of the soil. 



Phosphorus is absorbed from the soil in the form of phos- 

 phates, and enters into the composition of some of the proteid 

 substances ; phosphates constitute a large proportion of the ash of 

 seeds. 



As regards Chlorine, it has been experimentally proved so far to 



