APPROPRIATION OF ORGANIC MATTERS. 337 



by a process of reduction the sulphur is set free to unite with the 

 albuminous matters already described. 



The abundant occurrence, in conducting tissues of stems and 

 petioles, of calcic oxalate resulting from the changes described has 

 been held to indicate the probable seat of albuminous synthesis. 1 



885. The general statements which have now been made re- 

 specting the appropriation of carbon, nitrogen, and sulphur hold 

 good for all ordinary land and water plants. There are a few 

 plants, however, concerning which they must be somewhat modi- 

 fied, and these are here for convenience treated of together ; 

 as humus-plants, parasites, insectivorous plants, and epiphytes. 

 It must be remembered that in all these apparently exceptional 

 cases the mechanism of nutrition is not radically different from 

 that which other plants possess at some period of their lives 

 or in some slight degree. 



APPROPRIATION OF ORGANIC MATTERS. 



886. Humus-plants, 2 or Saprophytes. Among the higher plants 

 there are some (for example, Epipogium 3 ) which derive all their 



1 Sachs : Text-book, 2d Eng. ed., 1882, p. 711. 



2 As a matter chiefly of historical interest, the "humus theory " must be 

 referred to. As stated in the words of Liebig, its author, it is briefly as 

 follows : 



" Woody fibre in a state of decay is the substance called humus. . . . 

 Humus acts in the same manner in a soil permeable to air as the air itself ; it 

 is a continued source of carbonic acid, which it emits very slowly. An atmos- 

 phere of carbonic acid, formed at the expense of the oxygen of the air, sur- 

 rounds every particle of decaying humus. The cultivation of land by tilling 

 and loosening the soil, causes a free and unobstructed access of air. An atmos- 

 phere of carbonic acid is therefore contained in every fertile soil, and is the 

 first and most important food for the young plants which grow in it. ... The 

 roots perform the functions of the leaves from the first moment of their forma- 

 tion : they extract from the soil their proper nutriment, namely, the carbonic 

 acid generated by the humus. . . . When a plant is quite matured, and when 

 the organs, by which it obtains food from the atmosphere, are formed, the car- 

 bonic acid of the soil is no further required. . . . Humus does not nourish 

 plants by being assimilated in its unaltered state, but by presenting a slow 

 and lasting source of carbonic acid, which is absorbed by the roots " (Chem- 

 istry in its Application to Agriculture, American edition, 1842, pp. 65 et seq). 



It has been shown by the investigations referred to in the text that plants 

 can be grown with vigor and carried to complete maturity without the supply 

 of carbonic acid to the roots, and hence the " humus theory " is emptied of all 

 its value ; but, as will be shown later, the decaying vegetable matter in soils 

 has important functions. 



3 Pfeffer's Pflanzenphysiologie, i., 1881, p. 226. 



22 



