152 



SOILS. 



the growth of the excrescence is completed, the swollen, quies- 

 cent bacteroids gradually collapse and become depleted of 

 their nitrogenous substance; and finally the apparently empty 

 husk remains or drops off, carrying with it the minute cocci 

 which in the soil become active bacteria again. The nodules 

 are thus found mainly on the actively-growing roots, and at 

 the time when vegetation and assimilation are most active in 

 the plant. In autumn, or when the plants are in fruit, the roots 

 may be wholly destitute of nodules. 



FIG. 25. Square-pod pea. Tetragonolobus purpureus. FIG. 26. White Lupin. I.upinus albus. 



The adhesion of the nodules to the roots is mostly very 

 loose, and their falling-off when the seedlings are carelessly 

 transplanted, doubtless accounts for much of the difficulty 

 generally found in transplanting legumes when once es- 

 tablished. 



The figures annexed show the various forms assumed by the 

 nodules in different plants, and with them also the correspond- 

 ing forms of the bacteroids of each. The latter, here shown 



