78 PLANT PHYSIOLOGY 



pounds of nitrogen. These plants fall into one of three 

 classes, the humus plants, the carnivorous plants, and the 

 parasites. 



HUMUS PLANTS 



The humus plants live in soil containing a large amount 

 of organic matter, mainly of vegetable origin, in a more or 

 less decomposed condition. Besides these organic remains, 

 living saprophytic fungi form an important constituent of 

 humus. Owing to the very diverse composition of humus 

 soils (loam, leaf-mould, etc.), it is very difficult to deter- 

 mine what substances are absorbed from them by plants, 

 but since the majority of the humus plants contain either 

 no chlorophyll or only a little, they must absorb elaborated 

 non-nitrogenous, as well as nitrogenous, carbon compounds. 

 These soils consist largely of substances insoluble or only 

 slightly soluble in water. It is very probable, therefore, 

 that there is a solvent action exerted by the underground 

 parts of humus plants. The fact that even the immediately 

 soluble constituents of humus soils diffuse only slowly, 

 strengthens the supposition that some if not all of the 

 humus plants dissolve the nutritious substances upon which 

 their life depends. Those growing in humus soils containing 

 little soluble food-material, but incapable of secreting acids 

 or other solvents, depend upon other organisms which can 

 exert a solvent action and with these they live in more or 

 less intimate association. 



The plants most active in converting the insoluble nitro- 

 genous substances in lifeless remains of higher organisms 

 into soluble and hence more generally available compounds, 

 are, as we have seen, the bacteria. Almost equally impor- 

 tant are the saprophytic fungi toadstools and similar 

 plants common in all sufficiently moist humus. These fungi 

 and the bacteria, in nourishing themselves, prepare the 

 materials needed and used by other humus plants growing 

 with them. One step farther some of the higher humus 

 plants go. Instead of living merely close to the lower 

 humus organisms, upon the activities of which they are de- 

 pendent, they come into actual contact with these, their 





