GARDENING AS A SCIENCE. 
83 
The theory of nutrition supposes the faculty of converting the materials of 
aliment into certain products that shall add to, or enlarge the solid parts, and deposit 
specific juices in the cells of plants. Carbonic acid is always the product of the 
combustion of charcoal ; it is soluble in water, and may be considered the base of 
woody fibre. It unites greedily with a solution of lime, and deposits chalk : thus, 
if a person force his breath through a straw into a glass of bright lime-water, the 
fluid will begin, even from the first bubble, to become turbid or milky, and then 
gradually deposit ohdi^k^ {carbonate of lime). Carbonic acid is therefore emitted 
from the lungs of breathing animals ; and it is proved that, under certain conditions, 
plants produce it also. 
But have we any reason to suppose that the roots absorb it from the soil ? 
Philosophers assert the fact, but we question the possibility of arriving at the proof. 
Liebig, among others, states that the leaves inhale the gas, and subsequently 
elaborate it to the substance of woody fibre. In the absence of demonstrative 
evidence we advance no theory, nor feel inclined to add credence to any. 
As to the development of parts, and their subsequent increase, it appears 
rational to suppose that the embryo of every plant contains from the first all the 
rudiments of its future organs ; that aliment supplies the principle of growth, 
not that of creation. We may stagger at the infinite minuteness of parts, but 
minuteness ought to be no stumbling-block, while we see, and hesitate not to 
admit, that the smallest seed may protrude a tiny plant, which in a few years will 
have increased to a million times its original dimensions. 
The living principle — that vitality which prevents decay, which repels the 
effort of chemical agency, is the great mystery upon which every phenomenon 
depends. We are on this point utterly ignorant ; all our researches and most 
refined investigations lead only to conjecture ; and so it is throughout. Natural 
philosophy is an interesting and beautiful study, — it displays wonder everywhere ; 
but we in vain endeavour to search into causes. 
It were wise, therefore, to cease from appealing to Chemistry, as a clue to the 
processes of assimilation and organic action : Chemistry can only investigate the 
products of dead or inert matter ; and in vegetable structure it teaches that its 
ultimate elements, as shown by analysis, are four gases, — oxygen, hydrogen, carbonic 
acid, and nitrogen, and some saline and earthy substances in the form of ashes. 
Manures, earths, and soils contain the same elements, and therefore we are instructed 
to apply these substances in order to furnish the rough materials of sustenance ; 
and herein we observe analogy between the vegetable and animal kingdoms. Food 
must be taken into the systems of both,— or rather, those elements must be supplied 
which the vital principle can elaborate and assimilate so long as the several functions 
retain their vigour of health. 
The principle of vitality remains a profound secret, and we are placed in the 
dilemma between two opinions-— 1st, Whether the living plant be to a certain 
extent endowed with sensation and a power of volition, so as to imbibe at pleasure, 
