712 PART IV. THE PHYSIOLOGY OP PLANTS. 



juices, or fruits, etc. ; and Saprolegnia which attacks the corpses of animals. 

 S"ine of these Fungi, notably the Yeasts and ihe various kinds of Bacteria 

 (Schizomycetes), are peculiar in that th^y not only decompose the amount of 

 organic substance which they require for their nutrition, but they give rise to 

 widespread decompositions which are known as fermentation and putrefaction. 

 Amongst the higher plants there are many saprophytes which grow in soils rich 

 in humus : they may be almost destitute of chlorophyll (e.g. Monotropa ; 

 Neottia and some other Orchids) : or they may possess it in considerable 

 quantity (e.g. some Orchids; Pyrola; Ericaceae; Diapensiacese), in which case 

 they are probably only partially saprophytic ; plants of this kind grow mostly 

 in the leaf-soil of forests, or in peat on moors. 



Hydrogen. The hydrogen of the plant is mainly absorbed in the 

 form of water (H 2 0), but it may also be absorbed in combination 

 with nitrogen as ammonia-compounds (NH 3 ), and also in com- 

 bination with carbon when complex carbon-compounds are 

 absorbed by the plant. 



Oxygen is absorbed in combination with carbon, as C0 2 , and 

 with hydrogen, as H 2 0, and in many of the inorganic salts of the 

 food, such as sulphates, phosphates and nitrates, as well as in more 

 complex carbon-compounds. Oxygen is also absorbed uncombined, 

 in connexion with the catabolic processes, in respiration. 



Nitrogen, which is an essential constituent of proteid substan- 

 ces, is only exceptionally assimilated in the free form ; although 

 it is present in large quantities in the atmosphere, a plant perishes 

 if the soil in which it grows contains no compounds of nitrogen. 

 Nitrates and compounds of ammonia are widely distributed, and 

 it is in this form that nitrogen is mainly taken up by plants ; it 

 seems probable that plants possessing chlorophyll absorb their 

 nitrogen in the form of nitrates only. 



Although it is usual to manure green crops with ammonia-compounds (e.g. 

 sulphate of ammonia), the nitrogen is, nevertheless, absorbed in the form of 

 nitrates. For there exist in the soil certain Schizomycetes, termed Nitro- 

 bncteria, which oxidise the ammonia-compounds present to nitrites, and these 

 again to nitrates. This oxidising process is termed nitrification. 



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

 phytes, and carnivorous plants, in the form of nitrogenous carbon- 

 compounds (see pp. 710, 692). 



Although it is generally true that plants cannot assimilate uncombined nitro- 

 gen, nevertheless certain plants (Papilioneas, such as Peas, 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 at- 

 tained is not yet completely determined, but the principal facts are briefly as 



