Page images
PDF
EPUB

purposes of digestion. They have, as it were, merely to divide, macerate, and dissolve the nitrogenous and carbonaceous tissues which they ingest, and to rearrange them in their own economy, by the mysterious function of assimilation and of organic nutrition.

NUTRITION IN PLANTS.

Nutrition in plants differs from nutrition in animals in the leading fact, that whereas the food of animals is principally derived from the organic world, that of plants is solely derived from the inorganic world.

All vegetable substances live and increase on food extracted from the soil or from the atmosphere, which their vital force enables them to decompose, if necessary, and to assimilate.

Plants, like animals, are formed of the elementary gases, hydrogen, oxygen, and nitrogen, of carbon, of sulphur, of phosphorus, and of other inorganic elements, and salts.

By far the greater part of their bulk, however, is formed of carbon, which constitutes one-half of their weight in the dry state. It is entirely extracted from the atmosphere, the carbonic acid of which is absorbed by the leaves, the carbon retained, and the oxygen in great part emitted. It was formerly supposed that a great part of the carbon of plants was extracted from the soil by the roots, but it is now generally admitted that in green-leaved plants

nearly the whole of the carbon is taken from the atmosphere.

For carbon to be thus extracted from the atmosphere and used in the formation of the plant, two conditions are all but essential-viz., green (chlorophyll holding) tissue in its leaves or branches, and light. The stronger the light, the more active is the carbon-extracting nutritive function.

The proportion of the other principal elementary components of vegetable substances, according to Professor Johnston, "Agricultural Chemistry," are— Oxygen rather more than one-third.

Hydrogen little more than 5 per cent.
Nitrogen from 1 to 4 per cent.
Sulphur 1 to 5 per cent.

Phosphorus about a thousandth part. The following table, from the same source, gives a very clear idea of the proportions in which Nitrogen, Hydrogen, Oxygen, Carbon, and residual ash enter into the composition of the principal vegetable substances used as food. The figures apply to 1,000 lbs. of such substances, perfectly dry:

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

The oxygen is partly derived directly from the atmosphere, and partly from the decomposition of water, taken up by the roots, which also furnishes hydrogen. The nitrogen which plants contain, as in animals, is not supposed to be extracted directly from the atmosphere, but from nitrates and ammonia salts, or from other soluble substances containing nitric acid, and taken up from the soil.

The sulphur, phosphorus, alkalies, lime, magnesia, chlorine, silicon, and iron which plants contain, and which remain in the ash after combustion, are taken up directly from the soil by the roots.

The small proportionate amount of the ashes which remain after combustion is no criterion as to their importance. They are absolutely necessary to the health, nay existence, of plants, and vary much in amount in different plants, as shown by the table.

In some of the lowest forms of animal life the distinction between the animal and the plant becomes difficult, for both plants and animals contain ternary hydrocarbons, and quaternary azotized albuminoids. When structural conditions are scarcely sufficient to guide us, the mode of nutrition becomes one of the chief distinctive characteristics between them.

In plants, the ternary principles, the hydrocarbons (Oxygen, Hydrogen, Carbon) constitute the tissues themselves, are their intrinsical and fundamental components; whereas the quaternary or azotized principles may be considered deposits, incrustations. In animals it is just the contrary; their tissues

are essentially composed of quaternary or albuminoid principles (Oxygen, Hydrogen, Carbon, and Nitrogen), and the ternary principles form fatty deposits, pigments," when they are not in the circulation.

Plants perform a double part on the surface of the globe. First, by absorbing carbonic acid from the atmosphere, and fixing its carbon, whilst emitting its oxygen, they purify the atmosphere, contaminated by the respiration of animals and by combustion. Were it not for this agency of vegetable life, the atmosphere which surrounds the globe would probably, in the course of time, cease to be respirable both by men and by animals.

Secondly, they elaborate and prepare, in their own organizations, the elementary substances of the inorganic world, so as to render them fit to become the food of the animal creation. Such an intermediary is necessary, because animals are not organized to extract the elements of nutrition, or food, with the exception of oxygen, directly from inorganic matter.

It is also to the vegetable world that the earth owes its beauty, its surpassing loveliness. Were it not for the plants which clothe it in verdure, the earth would be a barren rock, a mere cinder, a mass of scoriæ, such as our cold satellite, the atmosphereless moon, is supposed to be at the present day.

[blocks in formation]

NUTRITION may be defective, from :-
I. Deficient vital nutritive power.

II. From the existence of acute disease.

III. From imperfect digestion and assimilation that is, from the imperfect performance of the processes which are concerned in the transformation of food.

I. DEFECTIVE NUTRITION FROM DEFICIENT VITAL

POWER.

The transformation of food into chyle and tissue, and the creation of animal heat and force by organic combustion, are vital functions which take place under the influence of the vital power transmitted by the parent to the offspring. If the parents are young and healthy, their nutritive power is vigorous, and in their offspring it will also, under favourable circumstances, be equally vigorous. Nutrition in the latter, therefore, will be perfectly performed, and a sound and healthy organization will be

« PreviousContinue »