f) THEORY AND PRACTICE OF PRUNING. 307 t^' 
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somewhat better -when exposed to the alternations of day and night, since they exhale a small quantity 
of oxygen by day from their own substance, and by night form carbonic acid with this, which they 
again decompose by day. A plant will live in this way, though with very great difficulty and without 
exhibiting any growth; but if the small quantity of oxygen formed by day be removed by sulphur 
and iron filings, or the carbonic acid by lime-water, it can form no more of these gases, and dies. 
It is evident from these facts that the process, accompanied by evolution of oxygen gas, under the 
influence of light, is related to the nutritive processes, since the latter become abnormal when the 
former is interfered with ; yet, the plant may survive a continuance of this interruption. But the 
other process, in which oxygen is absorbed and carbonic acid exhaled, occurring, moreover, in all parts 
of plants, is intimately connected with the life of the plant, and when the chemical changes which are 
produced by the action of oxygen upon the vegetable structures, and which are continually going on 
in all the organs, are interfered with, the plant is, like an animal, thrown into a kind of asphyxia and 
rapidly dies. 
Thus, this latter operation, in which oxygen is consumed by plants, is far more reasonably called 
the " respiratory," than the former action in the green parts, in which oxygen is evolved, for this 
belongs clearly to the nutrient functions ; and in this point of view the respiration of plants is exactly 
analogous to the respiration of animals, in its immediate relation to life ; and oxygen gas is a true 
vital air to plants. The relation of the plant to the atmosphere, however, becomes somewhat more 
complex than that of the animal, from the fact that the former absorbs oxygen, notonly from without, 
but in part from the supply prepared by its own green organs. 
Water and carbonic acid being so universally distributed, plants find the principal elements of their 
substance (oxygen, hydrogen, and carbon,) everywhere in a condition fit to be appropriated by them; 
but they have not everywhere the opportunity to absorb the amount of nitrogen necessary for vigorous 
development; hence the important influence of nitrogenous substances in manuring. The nitrogen gas 
of the atmosphere seems to be an indifferent substance to plants, and it has long been assumed that 
they take up nitrogen in the form of solutions of organic substances or of ammonia. The latter view is 
supported by Liebig, who demonstrated the presence of ammoniacal vapours in the atmosphere and all 
rain and snow water, as well as of ammoniacal salts in the ascending sap of many trees. It has been 
stated by him, that the ammonia of the atmosphere suffices to supply nitrogen to wild plants, and 
that cultivated vegetables require a greater supply onty because they are intended to produce a 
greater quantity of inflorescence. But it has never been shown that plants can obtain nourishment 
from the ammoniacal vapours of the atmosphere, and it is doubtful whether this is the case even with 
the ammoniacal salts in the soil, for Bouchardat's experiments go to prove that ammoniacal salts are 
poisonous in very minute proportions when absorbed in watery solutions by plants. Yet numerous 
experiments testify that the admixture of ammoniacal salts with the soil greatly favours the growth of 
plants. Hence it seems most probable that the ammoniacal salts form compounds with substances 
contained in the soil, and that those exert a different influence on the plant than that of the pure 
ammoniacal salts. Mulder's experiments on the humous substances are exceedingly valuable in 
reference to this question, and they tend to show that ammonia is always taken up by plants in combi- 
nation with the organic acids of the soil (so abundant in black vegetable mould), and this would 
explain the difference between the poisonous action of their pure ammoniacal salts, and their favourable 
effect when they are mixed with the soil. 
Taking all circumstances into consideration, it seems to be the legitimate conclusion that plants 
absorb substances furnishing nitrogen through the roots and not the leaves ; while, on the other hand, 
the latter play the particularly active part in the absorption of the carbonic acid, which yields the 
principal element of all their solid structures. 
r 
i 
THEORY AND PEACTICE OE PRUNING. 
By Mr. II. BAILEY, Ga.ude.neu to G. V. Haucouut, Esq.., M.P., Kuneham Pair. 
the PLUM. 
N almost every garden we find the Plum-tree trained in the fan-shaped form, and nurserymen 
invariably prepare their trees for this mode of training. But, notwithstanding its general adop- 
<\ tion, there are great objections to it for the Plum, — the more vertical and central shoots growing with x> 
't* a degree of vigour inconsistent with fruitfulness ; and, when laid in at length, being liable to produce 
, {m but few irregularly placed spurs. Something may be done to avert this result by carefully bending 
