The flesh-forming principles of food are, as I have already stated, almost identical with the principal nitrogenous constituents of animals. Unlike the non-plastic substances, they are convertible into each other with little, if any, loss either of matter or of force. Not many years since it was the fashion to estimate the nutritive value of a food-substance by its proportion of nitrogen; but this method—not yet quite abandoned—was based on erroneous views, and yielded results very far from the truth. No doubt all the more concentrated and valuable kinds of food are rich in nitrogenous principles; but there are other varieties, the nutritive value of which is very low, and yet their proportion of nitrogen is very high. This point requires explanation. Both the plastic and the non-plastic materials of food exist in two distinct states—in one of which they are easily digestible, and in the other either altogether unassimilable or so nearly so as to be almost useless. Thus, for example, the cellular tissue of plants, when newly formed, is to a great extent digestible, whilst the old woody fibre is nearly, if not quite, incapable of assimilation. Gelatine, which in raw bones is easily digested in the stomachs of the carnivora, loses a large proportion of its nutritive value on being subjected to the action of steam. Again, a portion of the nitrogen of young succulent plants is in a form not sufficiently organic to admit of its being assimilated to the animal body. But, independently of these strong objections to the method of estimating the nutritive value of food by its per-centage of flesh-formers, there are many other reasons which as clearly prove the fallacy of this rule. If we were, for instance, to estimate the value of albumen according to the tables of food equivalents which were constructed some years ago by Boussingault and other chemists, we would find one pound weight of it to be equivalent to four pounds weight of oil-cake, or to twelve pounds weight of hay; yet, it is a fact that a horse would speedily die if confined to a purely albuminous diet, whereas hay is capable of supporting the animal's life for an indefinite period.
It is clear, then, from what I have stated, that neither the amount of flesh-formers, nor of fat-formers, contained in a given quantity of a substance is a measure of its nutritive value; nevertheless it would be incorrect to infer from this that the numerous analyses of feeding substances which have been made are valueless. On the contrary, I am disposed to believe that the composition of these substances, when correctly stated by the chemist, enables the physiologist to determine pretty accurately their relative alimentary value. Theory is certainly against the assumption that food is valuable in proportion to its content of nitrogen; nor has practice less strongly disproved its truth. An illustration drawn from the nutrition of plants will make this matter more apparent. Every intelligent agriculturist knows that guano contains nitrogen and phosphoric acid; both substances are indispensable to the development of plants, and therefore it would be incorrect to estimate the manurial value of the guano in proportion to the quantity of nitrogen it was capable of yielding. If the value of manures were determined only by their per-centage of nitrogen—a mode by which certain chemists still estimate the nutritive value of food—then woollen rags would be worth more than bones, and bones would be more valuable than superphosphate of lime. The truth is, that the analysis of feeding stuffs and manures is sometimes of little value if the condition in which the constituents of these substances exist be undetermined. For example, the analysis of one manure may show it to contain 40 per cent. of phosphate of lime, and three per cent. of ammonia, whilst, according to analysis, another fertiliser may include 20 per cent. of phosphate of lime, and two per cent. of ammonia. Viewed by this light solely, the first manure would be considered the more valuable of the two, whereas it might, in reality, be very much inferior. If the phosphate of lime in the manure, containing 40 per cent. of that body, were derived from coprolites or apatite, and its ammonia from horns, the former would be worth little or nothing, and the latter, by reason of its exceedingly slow evolution from the horns, would possess a very low value. If, on the contrary, the phosphate of lime, in the manure comparatively poor in phosphate, were a constituent of bones, and its ammonia ready formed (say as sulphate of ammonia), then, its value, both commercial and manurial, would be far greater than the other.
In estimating the money value of an article of food, we should omit such considerations as the relative adjustment of its flesh-formers and fat-formers, and its suitability to particular kinds of animals, as well as to animals in a certain stage of development. The manure supplied to plants contains several elements indispensable to vegetable nutrition; and, although the agriculturist most commonly purchases all these elements combined in the one article, still he frequently buys each ingredient separately. Ammonia is one of these principles, and, whether it be bought per se, or as a constituent of a compound manure, the price it commands is invariable. This principle should prevail in the purchase of food: each constituent of which should have a certain value placed upon it; and the sums of all the values of the constituents would then be the value of the article of food taken as a whole. There are, no doubt, practical difficulties in the way which prevent this method of valuation from giving more than approximatively correct results; but are there not precisely similar difficulties in the way of the correct estimation of the value of a manure according to its analysis? There are several constituents of food, the money value of which is easily determinable: these are sugar, starch, and fat. No matter what substance they are found in, the nutritive value of each varies only within very narrow limits. The value of cellulose and woody fibre is not so easily ascertained, as it varies with the age and nature of the vegetable structure in which these principles occur. There is little doubt but that the cellulose and fibre of young grass, clover, and other succulent plants, are, for the most part, digestible; and we should not be far astray if we were to assume that four pounds weight of soft fibre and cellulose are equivalent to three pounds weight of starch. As to old hard fibre, we are not in a position to say whether or not it possesses any nutrimental value worth taking into account. The estimation of the value of the flesh-forming materials is far more difficult than that of sugar, starch, pectine compounds, and fat. The nitrogenous constituents of food must be in a highly elaborated state before they are capable of being assimilated. In seeds—in which vegetable substances attain their highest degree of development—they probably exist in the most digestible form, whilst much of the nitrogen found in the stems and leaves of succulent plants, is either in a purely mineral state, or in so low a degree of elaboration as to be unavailable for the purpose of nutrition. But even plastic materials, in a high degree of organisation, present many points of difference, which greatly affect their relative alimental value; for example, many of them are naturally associated with substances possessing a disagreeable flavor: and as their separation from these substances is often practically impossible, the animal that consumes both will not assimilate the plastic matters so well as if they were endowed with a pleasant flavor. In seeds and other perfectly matured vegetable structures, the flesh-formers may exist in different degrees of availability. The nitrogen of the testa, or covering of the seeds, will hardly be so assimilable as that which exists in their cotyledons. The solubility of the flesh-formers—provided they be highly elaborated—is a very good criterion of their nutritive power. In linseed the muscle-forming substances are more soluble than in linseed-cake—the heat which is generally employed in the extraction of oil from linseed rendering the plastic materials of the resultant cake less soluble, and diminishing thereby their digestibility, as practice has proved.
From the considerations which I have now entered into, it is obvious that the chemical analysis of food substances as generally performed, though of great utility, does not afford strictly accurate information as to their commercial value, and still less reliable in relation to their nutritive power. At the same time, they as clearly establish the feasibility of analyses being made whereby the money value of feeding-stuffs may be estimated with tolerable exactitude. Let the chemist determine the presence and relative amounts of the ingredients of food-substances, and—if it be possible so to do with a degree of exactness that would render the results useful—place on each a money value. This done, let the physiologist and the feeder combine the food in such proportions as they may find best adapted to the nature, age, and condition of the animal to be fed.
It is to be regretted that the market price of feeding stuffs is not, in consequence of our defective knowledge, strictly determined by their nutritive value, for if such were the case, the feeder would merely have to adapt each to the nature and condition of his stock. Even amongst practical men there prevails, unfortunately, great diversity of opinion as to the relative nutritive value of the greater number of food substances; and I am quite certain that many of these command higher prices than others which in no respect are inferior. It would lead me too far from my immediate subject were I to enter minutely into the consideration of such questions as—whether an acre of grass yields more or less nutriment than an acre of turnips? I shall merely describe the composition and properties of grass and of turnips, and of the various other important food substances, and compare their nutritive power, so far as comparisons are admissible; but I shall say but little on the subject of the various economic and other conditions which affect the production of forage plants. When I shall have described the chemical nature and physical condition of the various articles of food, and the results of actual feeding experiments made with them, the feeder will then be in a position to determine which are the most economical to produce or to purchase.
SECTION II.
PROXIMATE CONSTITUENTS OF VEGETABLES.
The saccharine, or amylaceous substances constitute the most abundant of the proximate constituents of plants. They are composed of carbon, hydrogen, and oxygen. I shall briefly describe the more important members of this group of substances, namely, starch, sugar, inulin, gum, pectin, and cellulose.
Starch, or fecula, occurs largely in dicotyledonous seeds, peas, &c., and still more abundantly in certain monocotyledonous seeds, such as wheat and barley. It constitutes the great bulk of many tubers and roots—for example, the potato and tapioca. It consists of flattened ovate granules, which vary in size according to the plant. In the beetroot they are 1⁄3500 of an inch in diameter, whilst in tous les mois they are nearly 1⁄200 of an inch in diameter. Most of the starch granules are marked by a series of concentric rings. Starch is heavier than water, and is insoluble in that fluid when cold; neither is it dissolved by alcohol or ether. When heated in water having a temperature of at least 140° Fahrenheit, it increases greatly in volume, and acquires a gelatinous consistence. When the water is allowed to cool, a portion of the starch becomes insoluble, whilst another portion remains in solution; the latter form of starch is sometimes termed amidin, from the French word for starch, amidon. When dry starch is heated to 400° Fahr., it is converted, without any change in its composition, into a soluble gum-like substance, termed dextrin, or British gum. On being boiled in diluted sulphuric acid it is converted into a kind of sugar; and the same effect is produced by fermentation—for example, in the germination of seeds. Fresh rice contains 82, wheat 60, and potatoes 20 per cent. of starch. This substance constitutes a nutritious and easily digestible food, but alone cannot support life. Arrowroot is only a pure form of starch.
Sugar occurs less abundantly in plants than starch. There are several varieties of this substance, of which the kinds termed cane sugar (sucrose) and grape sugar (glucose), are only of importance to agriculturists. The former enters largely into the composition of the sugar-cane, the beetroot, the sugar-maple, the sorgho grass, pumpkins, carrots, and a great variety of other plants. Grape sugar is found in fruits, especially when dried—raisins and figs—in malted corn, and in honey. In the sugar-cane there is 18 per cent., and in the beetroot 10 per cent. of sugar.