LECTURE IV. 



Source of the organic elements of plants— Source of the carbon— Fcrm in which it entcra 

 into the circulation of plants — Source of the hydrogen — Source of the oxygen— Source of 

 the nitrogen — Form in wJiich nitrogen enters into the circulation of plants — Absorption of 

 ammonia and nitric acid by plants. 



The first of the series of questions stated at the close of the preceding 

 lecture, regards the source from which plants derive the organic ele- 

 ments of which they are composed. They are supported, it is ohvious, 

 at the conjoined expense of the earth and the air — how much do they 

 owe to each, and for which elements are they chiefly and immediately 

 indebted to the soil, and for which to the atmosphere ? We must first 

 consider the source of each element separately. 



§ 1. Source of the carbon of plants. 



We have already seen reason to believe that carbon is incapable of 

 entering directly, in its solid state, into the circulation of plants. It is 

 generally considered, indeed, that solid substances of every kind are un- 

 fit for being taken up by the organs of plants, and that only such as are 

 in the liquid or gaseous state, can be absorbed by the minute vessels of 

 which the cellular substances of the roots and leaves of plants are com- 

 posed. Carbon, therefore, must enter either in the gaseous or liquid 

 form, but from what source must it be derived ? There are but two 

 sources from which it can be obtained, — the soil in which the plant 

 grows — and the air by which its stems and leaves are surrounded. 



In the soil much vegetable matter is often present, and the farmer 

 adds vegetable manure in large quantities with the view of providing 

 food for his intended crop. Are plants really fed by the vegetable mat- 

 ter which exists in the soil, or by the vegetable manure that is added to 

 it? 



This question has an important practical bearing. Let us, therefore, 

 submit it to a thorough exainination. 



1°. We know, from sacred history, what reason and science concur 

 in confirming, that there was a time wlien no vegetable matter existed 

 in tlie soil which overspread the earth's surface. The first plants must 

 have grown without the aid of either animal or vegetable matter — that 

 is, they must have been nourished from the air. 



2°. It is known that certain marly soils, raised from a great depth 

 beneath the surface, and containing apparently no vegetable matter, 

 will yet, without manure, yield luxuriant crops. The carbon in such 

 cases must also have been derived from the air. 



3°. You know that some plants grow and increase in size when sus- 

 pended in the air, and without being in contact with the soil. 



You know, also that tnany plants — bulbous flower roots for example 

 — will grow and flourish in pure water only, provided they are open to 

 the access of the atmospheric air. Seeds also will germinate, and, 

 when duly watered, w^ill rise into plants, though sown iu substances 

 that contain no trace of vegetable n alter. 



