558 PHYSIOLOGY 



truly, that green plants have the power of feeding upon inorganic 

 substances, and fixing them in definite organic compounds : 

 secondly, but with less justice, that this is the universal law of 

 vegetable nutrition that plants live exclusively on inorganic sub- 

 stances, which they convert into organic matters unfit for their 

 own use, aud only assimilable after a new decomposition. In 

 regard to certain plants this last assertion is altogether inadmis- 

 sible, namely the Fungi, the so-called insectivorous plants such as 

 Drosera, &c. (in which animal substances are dissolved, absorbed, 

 and appropriated by the action of a ferment), and, above all, the 

 colourless parasites ; and not only is it contradicted by the phe- 

 nomena of their life, but it is opposed to the universal experience 

 derived from observation of the cultivation of plants. Lastly, we 

 know of no cause why plants should refuse to absorb organic 

 substances presented to them in a state of solution favourable to 

 endosmotic action in the roots. 



It is very true that many even of the higher plants will grow upon soil 

 almost destitute of organic matters, as we see on sandy heaths &c. ; but 

 the kind of vegetation which characterizes such soils is very different 

 from that which clothes land covered with vegetable mould. And the 

 influence of manures in agriculture must be attributed in a great measure 

 to the extensive aid afforded to the plant in the shape of additional sup- 

 plies of organic matters, which bear a kind of compound interest, since 

 the increased growth they produce gives increased power of independent 

 assimilative action. 



Spontaneous vegetation is nourished principally by carbonic 

 dioxide and ammonia always existing in sufficient proportions in 

 the atmosphere. The former substance is taken up by the leaves, 

 and the latter is also absorbed by the aerial organs of plants : but 

 the principal supply to the higher plants seems to be furnished 

 through the soil, which receives ammonia dissolved in rain and 

 dew, and, where porous, absorbs it greedily. Soil, and especially 

 the carbonaceous portions, has also the power of absorbing 

 ammonia from the atmosphere. 



Plants growing upon soil abounding in decaying vegetable and 

 animal matters are doubtless supplied with part of their food from 

 these sources. Ammonia is a constant product of decomposition 

 of animal substance, carbonic dioxide of this and vegetable matter. 

 But from the researches of Mulder it would appear probable 

 that the old vegetable matters may pass into the living plants 

 without undergoing decomposition into carbonic dioxide and water. 

 The black decaying matter of vegetable origin, called humus, is 

 decomposed in the soil into a series of organic acids, of which 

 the last members possess much affinity for ammonia, and form both 

 with it and the alkalies soluble salts, which may be absorbed as 



