feature of the year's returns of cattle was the augmentation of the number of young stock under one year old, noticeable in the English and Welsh breeding counties. The cow stock of the country is again at a level higher than in any year since 1892, although both in the aggregate, and still more in relation to the population, even after the present recovery, the numbers of cows and heifers in milk or in calf fall short of those returned in 1892 and 1891. The addition in the past year to the cows and heifers in Great Britain amounted to 2.2 per cent., and that of other cattle over two years to 4.4 per cent.

Looking backward, however, over a longer series of years, and summarising, as in Table III., the cattle returns of Great Britain since 1871, the increase in the number, whether of cows or of other cattle, has not, it will be observed, been commensurate with the increase of population.

TABLE III.-Numbers of Cattle in Great Britain.

Even the recovery recorded in 1898 fails, it will be seen, to replace the numbers per 1,000 of the population at the level shown. in the earlier years a fact which is to be borne in mind as helping to explain the growing imports both of dairy produce and of

beef.

Sheep. The sheep of Great Britain more than recovered in 1898 the decline shown in 1897. About one-third of the increase of 403,000 was due to an augmentation of the breeding ewes, and about two-thirds to an addition to the number of lambs living on June 4. The total sheep stock is, however, still under that of the year 1893, and less by 2,000,000 than the number returned in the two immediately preceding years, though the latest figures, it may be recalled, represent flocks larger by more than 1,000,000 than the average shown for Great Britain for the decade 1881-90.

Since 1895 there has been, as shown in Table IV., p. 414, a yearly increase in the number of ewes kept for breeding, in spite of variations in the returns of other sheep.

TABLE IV.-Numbers of Sheep in Great Britain.

So far as the number of lambs returned in June may be taken as an index to the number born, it would appear that the lambing season of 1898 was a successful one, although, relatively to the number of ewes, the fall of lambs was not so great as in 1896. Thus for every 1,000 ewes returned there were, in June 1896, 1,013, and in June 1898, 1,026 lambs.

Considerable local variations are shown in the returns for sheep, Cumberland exhibiting an increase of 5 per cent. on the previous season, while Surrey showed a decline of 9 per cent. In Scotland the proportionate rate of increase in the number of sheep on the year was about double that of England.

Pigs.-The great reduction in the number of pigs referred to in last year's report was only partially recovered by an increase of 109,000 in those recorded in 1898. Table V. shows the fluctuations which have taken place since 1893, when the number returned was unusually small.

TABLE V.-Numbers of Pigs in Great Britain.

ORIGIN AND FORMATION OF ORGANIC MATTER IN PLANTS.'

By the ordinary method of sand culture, in which the plant is grown in sand free from organic matter, it may be demonstrated that the plant accumulates considerable quantities of carbon and

From a paper by Professor P. P. Dehérain, in Experiment Station Record, vol. ix., No. 10. (U. S. Department of Agriculture, Washington.)

nitrogen during its growth. This carbon and nitrogen with the elements of water form the organic constituents of the plants, which with a small quantity of mineral ingredients make up the roots, stem, and leaves, and give the seed its valuable nutritive qualities. Since the soil did not contain either carbon or nitrogen, the plant must have drawn these two elements from the air. It is the purpose of this article to explain the nature of this fixation of the carbon and nitrogen of the air.

ORIGIN OF THE CARBON OF PLANTS.

The classic experiments of Priestley, in 1771, established the fact that plants exhale oxygen. Later researches made by Ingenhous and by Tennebier explained the decomposition of the carbon dioxide of the air and the evolution of oxygen by the leaves under the influence of light.

PENETRATION OF THE CARBON DIOXIDE INTO THE LEAVES.

The earth's atmosphere contains only 3 parts of carbon dioxide in 10,000 of air. It is evident, therefore, that in order that plants may obtain the carbon which they require from a medium so poorly supplied with it, rapidity of absorption by the tissues must compensate for the scarcity of the element in the air.

In the first place the absorption of carbon dioxide is favoured by the form of the leaves, which is such that they offer, as compared with their weight, an enormous absorbing surface. In a tree the leaves are at the extremities of infinitely ramified, flexible branches, which are agitated by the slightest breeze, thus facilitating contact of the leaves with the constantly renewed layers of air about them. That the absorption of carbon dioxide is very rapid may be shown by placing a leaf from which the air has been exhausted by means of an air-pump in an atmosphere of carbon dioxide in an apparatus1 which measures the change of the volume. It will be observed that absorption begins instantly, but that it is largely dependent upon the quantity of the water present in the leaf. Thus, the coefficient of absorption of the carbon dioxide in old leaves of Japanese Euonymus, containing 66.3 per cent. of water, was found to be 0.70 at 15°, while in young leaves of the same tree containing 754 per cent. of water the coefficient was 0.83. A comparison, at different temperatures, of the coefficient of absorption of carbon dioxide in the leaves with that in pure water shows the absorption in the leaves to be somewhat greater than in pure water. This indicates that the carbon dioxide is not simply dissolved in the water in leaves, but that it combines with the water to form a hydrate. It will be shown later that this fact is of great importance.

1 Dehérain and Maquenne, Ann. Agron., vol. xii., 1886, p. 525.

DECOMPOSITION OF CARBON DIOXIDE IN LEAVES.

The carbon dioxide which is absorbed by the leaves is decomposed, and the products of this decomposition are utilised in the formation of the simplest primary organic compounds, from which the more complex constituents of plants are derived. To accomplish this the principal condition is that the leaf be perfectly healthy. If it does not contain its normal proportion of water, i.e. if the roots do not draw from the soil as much water as is given off through the leaves, the decomposition of carbon dioxide is checked. Assimilation has ceased when, as at the end of a summer day, the leaves of the tobacco plant, for instance, are hanging down the stem, or those of the beet lie flat on the soil. In fact it has been found that the decomposition of carbon dioxide begins to decline even before the leaves have lost their turgescence.

Light is absolutely essential to the assimilation of carbon by the leaves of plants. The principal source of this energy is, of course, the sun, but attempts have been made to utilise artificial light, especially electric light, for forcing plants. Siemens in England, Bailey in America, and the author in France have made experiments of this character. Since there is no doubt that, with the increasing use of water power for the production of electricity, a large supply of electric light can be economically obtained, it is highly interesting to learn what its action is on plants. All observers have found that rays from an arc lamp without a globe exert an injurious influence, blackening the epidermis of the leaves. During the author's experiments in 1881 the epidermis exposed to the direct rays became black, while the parts protected by the upper leaves preserved their beautiful green colour. The line of demarcation was as sharp as in a photographic plate. The injurious influence ceased as soon as the lamp was surrounded by a white glass globe through which the ultra-violet rays passed with difficulty. To understand the influence which the heat rays situated at the other extremity of the spectrum exert on vegetation, we must recall to mind that in respiration leaves, like all other plant organs, absorb oxygen and exhale carbon dioxide, a process which is precisely the opposite of that which occurs in assimilation.

It must also be remembered that the activity of respiration increases with elevation of temperature, while rise in temperature has only a very slight effect on assimilation. Maquenne and the author1 some years ago made a careful study of the action of both light and heat rays on leaves. In this research two sources of light were used, the Drummond light, which is obtained by rendering a piece of quicklime incandescent by means of the oxhydrogen blowpipe, and the Bourbouze lamp, which is composed of a cylinder of platinum wire gauze, which becomes incandescent when heated with illuminating gas, the combustion of which is promoted by a strong current of air. The leaves were introduced into tubes containing an atmo

1 Ann. Agron., vol. v., 1879, p. 401.

sphere of known composition, and were placed very near the lights, but were protected by screens containing transparent liquids of varying diathermanous properties. In some cases water was used, which allowed the light rays to pass but retained the heat rays. In other cases the screens were filled with benzene or with chloroform, which are also transparent but much more diathermanous than water. Exposing the leaves to the action of the Drummond light, which is poor in heat rays, and surrounding them with a screen filled with water, promoted reduction, the proportion of carbon dioxide in the tube diminishing, while the oxygen increased. When the screens were filled with chloroform, however, and the Bourbouze lamp was used, which is rich in heat rays, the opposite effect was obtained, i.e. the carbon dioxide increased and the oxygen diminished. In this case the phenomena of respiration took the place of those of

assimilation.

Passing from the study of the chemical and heat rays to that of the light rays in the central part of the spectrum, we find that the latter produce very different effects from the former. Draper demonstrated long ago that the orange rays are the most active in decomposing carbonic acid in the leaves. This conclusion was fully confirmed by the researches of Sachs, Cailletet, and the author, made nearly 30 years ago. The reasons for this special action of the rays of this part of the spectrum were not investigated until the Russian physiologist, Timiria zeff, took up the subject. He found that the rays which are most active in decomposing carbon dioxide are the orange and yellow, which are absorbed by chlorophyll when the latter is examined with the spectroscope. The same fact has been beautifully demonstrated by Engelmann. He received a ray of light upon a prism so placed under the objective of a microscope that on looking through the instrument the different rays of the spectrum could be seen. He then put a drop of water on a slide and added a filament of green alga and some putrefactive bacteria, which were aerobic. It was observed that the bacteria congregated in great numbers on that part of the alga lighted by the yellow and orange rays. In the green region only a few were observed, and these finally collected in the blue portion. In other words, the bacteria collected in the different rays in numbers approximately proportionate to their activity in assisting the decomposition of the carbon dioxide by chlorophyll.

Evidently the rays which pass freely through the chlorophyll exert no action. So it happens, as shown above, that the extreme red or the green rays are without effect on the decomposition of carbon dioxide. On the other hand, the orange and blue rays are retained and absorbed by the chlorophyll, and thus made available for the work of decomposing carbon dioxide. The fact that orange rays are much more effective than the blue is easily explained. The decomposition of the carbon dioxide, with the evolution of oxygen, requires an expenditure of energy equal to that involved in the burning of carbon in oxygen. In order, therefore that the rays may be effective for reducing carbon dioxide, they must be not only