159 
[Note. —What power causes the ascent of sap, or the action of the spongiolis, 
has not been fully ascertained. We can easily conceive how the spongiolis might 
imbibe moisture, by the simple influence of capillary attraction; but neither the 
porosity of the roots nor the chemical modifications of the sap, which take place 
in the leaves, can fully account for the rapid ascent of sap, and the immense 
force which occasions this rise; a force which Dr. Hales found to be equal to 
the pressure of nineteen pounds to the square inch—a pressure nearly a third 
greater than that of the atmosphere.] 
In the leaves the sap is peculiarly modified and concentrated, and the 
elements necessary to the growth of the plant, that exist in the gaseous 
state, are imbibed. Baron Liebig maintains that the atmosphere is the 
source of all the carbon which plants contain, and if this is true, which 
is considered doubtful, all the carbon or woody fibre must be imbibed 
through the leaves; and they certainly do take in carbonic acid from the 
atmosphere, retain the carbon, and return the oxygen pure.'* Hence, we 
see that although the means of nutrition are two, viz.: by the leaves and 
roots, only the latter falls duly within the province of this paper. 
It will be readily seen from this cursory glance at the vegetable econo¬ 
my, that the absorption of whatever is in the atmosphere may take place in 
the leaves—for example: Carbonic acid gas, oxygen, nitrogen and hydro¬ 
gen, in small quantities; and also that the absorption of inorganic elements, 
even though insoluble, is performed by the roots. Now all the elements 
necessary to the growth of plants may be found in the atmosphere and 
water; but the plant driven to this slow and imperfect assimilation will 
be stinted in its growth for lack of material, because, although there may 
be a superabundance of all the necessary elements except one, this one 
exception nullifies the others, and the development will take place no 
farther than it can be perfect; and it is the province of manures to furnish 
at once, in suitable quantities, the requisite elements for the growth of 
plants. Here we find a strong argument for the complex nature of manures, 
and also for studying the mutual adaptation between them and soils. 
The inorganic elements greatly differ. Marine plants contain iodine and 
soda. Plants from the desert, silex. Marsh plants are soft and watery— 
plants from dry soils are hard and dry. 
Soils also differ. They are silicious, alkaline, aluminous. Manures 
also in like manner differ, being animal or vegetable, and mineral; some 
* The exact reverse in the animal kingdom. 
