CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 713 



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

 found to bear peculiar gall-like outgrowths termed tubercles, which seem to 

 be more numerous and larger tbe smaller the proportion of combined nitrogen 

 contained in the soil. The tubercles are the result of the attack of a fungus i 

 which penetrates into the root through the root-hairs. The green plant and \ 

 the fungus appear to exist in a state of symbiosis (p. 273), as in the case of the 

 mycorhiza already mentioned, with the result that tbe 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 atmosi^here 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 nitro- 

 gen : but it is a question whether this process takes place in ihe tubercle itself ; 

 or whether it is not carried on in the soil by a Schizomycete, which may either 

 be derived from the tubercles, or be an independent organism. It seems prob- 

 able that the latter suggestion is nearer the truth. It is, in fact, known that 

 a bacterioid organism exists in the soil having the property of forming nitro- 

 genous compounds from free nitrogen in the presence of non-nitrogenous organic 

 substance [e.g. glucose). It may be that the development of this organism is 

 especially favoured by the presence of the tubercular roots of the Papilioneffi 

 in the soil, and that the nitrogenous substances which it produces are absorbed 

 by tbe roots after having undergone nitrification (see p. 712). 



The tubercles are structures formed by the hypertrophy of the cortex of tbe 

 root, resulting from the attack of tbe fungus at various points : their cells are 

 rich in sugar and starch : the branches of the mycelium penetrate most of the 

 cells, and there bud ofif 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 

 substances 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 phosphates, 

 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 

 be indispensable in the case of one plant only, the Buckwheat 

 (^Polygonum Fagopyrum). 



Iron, though it is met with in very small quantities, is absolutely 

 necessary for the formation of chlorophyll. The leaves produced 

 by plants which are not supplied with iron during their growth, 

 are white so soon as their own store of iron is exhausted ; these 

 leaves, which are said to be chlorolic, become green in consequence 



