But at temperatures of 13° and 14° C. nothing was seen even at the end of seven hours. At a temperature below 6° the leaves remained uncolored for fifteen days in the diffuse light of the room.

Again, the pale shoots of cabbage placed in the window, and therefore in full sunshine and at temperatures of 13° or 14° C., became green at the end of twenty-four hours; but under temperatures of 3° to 5° C. only traces of green color were seen at the end of three days, and the coloration was not complete until at the end of seven days.

Herve Mangon, by employing the electric light in place of sunlight, has arrived at similar results for rye. Marić-Davy, by the use of a single gaslight, has obtained similar results for the strawberry plant. Similarly De Candolle caused mustard and other plants to become green by the light of four argand lamps.

Evidently a very feeble light suffices to produce the greening, for the feeble individual effects accumulate and add together; but when a bright light is used secondary reactions set in, transforming and utilizing the chlorophyll itself. The light that determines the production of the chlorophyll and its green color also proceeds to destroy the chlorophyll. Thus the direct light of the sun rapidly decolors the alcoholic extract of chlorophyll, while diffuse light acts more slowly; but in a living plant the action of light is different, since it may become so intense for a special plant that the destruction of the chlorophyll may go on faster than its formation. If a green plant is carried into a dark room the chlorophyll ceases to form and a gradual process of destruction, or rather of transformation and assimilation, goes on until the plant becomes pale yellow. This mutability of chlorophyll makes it the essential medium through which the plant is nourished.

Draper, Desains, and others have shown that the chlorophyll absorbs certain rays of the spectrum; that is to say, that the work of forming and transforming chlorophyll is accomplished by means of radiations that have a certain velocity of vibration or a certain wave length, and that they are mostly those that form the red, orange, yellow, green, and blue portions of the spectrum. Awaiting a more detailed study of this phenomenon, we must at present adopt the general rule that the variation in efficiency of each of these agents is approximately proportional to the variation in the total energy of the solar radiation, although our present knowledge points to the conclusion that a radiant beam generally contains specific active wave lengths in proportions and intensities that have no necessary relation to each other.

INFLUENCE OF ABSORBENT MEDIA ON CHLOROPHYLL.

The action of sunlight on the chlorophyll within the cell is not materially modified if the light passes first through layers of cells that do not contain chlorophyll, such as those of the red colored cabbage leaf, since in those cells, as in yellow cells and others, the radiation that is absorbed is not to any extent that special radiation which the chlorophyll absorbs. The absorption of light by the yellow cells of the yellow leaves of an alder bush was examined by T. W. Engelmann (Agr. Sci., Vol. II, p. 139), who found that these absorbed most from the middle of the spectrum and least at either end, whereas the chlorophyll absorption is complementary to this. He also found that the green leaves of the alder bush, when exposed to the light side by side with the yellow leaves, set free far more oxygen than these, so that it seems probable that if the yellow cells contain only pure xanthopyll there assimilating power would be zero.

INFLUENCE ON THE SUPPLY OF SAP.

The action of sunshine in producing or altering the colors of fruits, especially the black Hamburg grape, has been experimentally studied by Laurent. (Agr. Sci., Vol. IV, p. 147.) Bunches of iramature grapes quite shielded from the sunlight ripened, colored, and flavored as usual, but bunches whose food supply had been cut off by ringing the base of the stock supporting the bunch, and then also kept in the dark, remained green, small, and sour. Bunches that had been subjected to the ringing process, but which were exposed to the sunlight, produced berries of normal size, some reddish and others green and of an acid flavor. He concludes that the coloring matter of grapes may be formed in the absence of sunshine, provided a sufficient supply of nourishment be at hand, but if this supply be arrested then the color remains imperfect.

CLIMATE AND THE LOCATION OF CHLOROPHYLL CELLS.

Guntz (1886) has studied the anatomical structure of the leaves of cereals and grasses in their relations to locality and climate. This connection is infinitely complex. Among other items brought out by him we note that the green assimilating organism consists of many cells of various shapes and in most cases fills the spaces between the nerves of the leaves; in tropical grasses the green cells occur most in the inclosing sheath, but in the grasses of the steppes it lies on either side of the grooves or ridges. The intercellular gaps, according as they are larger or smaller, indicate a moist or a dry soil and, equally so, a moist or dry atmosphere. The bast in the leaves of the grasses serves primarily to strengthen the whole structure, but the bast increases with the dryness of the locality, and its proportional distribution is an appropriate, indirect indication of the climate.

THE INFLUENCE OF CLOUD AND FOG.

There are some parasitic plants, says Marié-Davy (1881 and 1882), that require only moisture and warmth in order to vegetate. They mature and propagate while entirely cut off from sunlight, but they derive this power from organic matter or cells that have been previously formed by the action of sunshine upon the plant on which the parasite itself feeds.

Similarly certain bulbous plants will flower and mature in darkness, but in doing so the bulb itself is wholly consumed and dies; the plant lives on organic matter that was elaborated and stored up by its parent and predecessor in preceding years when it had sunshine to do the work for it. If a new bulb is to be formed as a basis for the flowering of the next year then the present bulb and plant must be allowed the necessary sunlight.

Similarly the seeds of the annuals sprout and nourish their little plants out of their own substance while still beneath the surface of the earth, but when the shoots reach up to the sunshine this furnishes the energy needed for the work of assimilation and the plant begins to live on the soil and the air. The roots can only send up to the leaf an inorganic sap with possibly here and there an organic cell scattered through it which has penetrated into the roots, as it were, by accident; it is the sunshine that sets these organic cells into activity, causing them to grow and to multiply.

If a plant in vigorous growth is removed from sunshine to darkness it draws upon its own reserves and lives upon itself as long as possible. In darkness the plant transforms the organic products that are at its disposition, but it can not manufacture any new ones. On the contrary, it consumes itself and its dry weight steadily diminishes. The experiments of Boussingault on seeds, those of Sachs on plants and seeds, those of Pagnoul on the beet, and of Macagno on the grapevine all confirm this general principle. The observations of the latter show that as between two sets of vines, one exposed to the sun and the other covered with a dark cloth, the growth of the latter, as measured by the amount of solid and gaseous material, was not 10 per cent of the growth of the vine in the sunshine. Other vines under a white cloth showed a growth of 80 per cent, thus apparently proving that the differences were not due to anything else except sunshine.

Pagnoul experimented upon sugar beets, some of which were covered by glass that had been blackened on the inside; this coating of lampblack is ordinarily said to absorb heat, but it would be more proper to say that it transforms all the short waves of the sunshine into long waves so that the plants beneath it receive neither ultraviolet nor visual rays, but only the ultra-red, or long, heat weves. Therefore beneath the black glass the temperature was somewhat warmer than beneath the transparent glass and the latter warmer

than the free air. The results of analysis at the end of the experiments showed that under the transparent glass the weight of the roots was the same as in the free air, but the weight of the leaf was much more, the weight of the sugar much less, and the weight of the nitrous salts much greater. Under the black glass the weight of the roots was 4 per cent of that in the free air, and the weight of the leaves was about 25 per cent, the weight of sugar 2 per cent, and the weight of the salts 8 per cent, thus demonstrating an almost complete stoppage of the vital processes.

Evidently the action of these artificial coverings on the experimental plants is perfectly analogous to the action of cloud and fog in

nature.

It is commonly said that on the seacoast the action of the salt brine blown by the wind up over the land is to stunt or prevent vegetable growth, but the same effect must be produced by the absence of sunlight in those regions where fog and cloud prevail.

INFLUENCE OF SHADE ON DEVELOPMENT.

According to Marchand (1875, p. 130), the influence of a diminution of sunlight on the development of the plant is apparent in the relative growth of plants on sunny and cloudy days or in sunny and shady places, but the matter was brought to exact measurement by Hellriegel. His experiments on barley gave him these results:

We see here that plants living in the greenhouse, receiving sunlight that has traversed the glass, have experienced a considerable diminution in their development as compared with those in the free air which experienced the full chemical force of the sunshine. The plants living under glass and in the diffuse light developed only a small quantity of stalk and did not perfect the seed at all.

INFLUENCE OF LONG AND SHORT WAVES OF LIGHT.

Vöchting (1887) investigated the formation of tubers as influenced especially by sunlight. Sachs had maintained that the germination was entirely prevented, or at least went on very slowly, if sunlight,

i. e., short waves, had access to the tubers. Vochting finds that, although the light does delay the growth and diminishes the distance between the tubers, still the supply of water is the important factor. (Wollny, X, p. 230.)

Sachs (1887), as the result of experiments on the effect of ultraviolet radiation upon the formation of buds, states that these rays exert on the green leaves (in addition to the assimilation produced by the yellow and neighboring rays) still another effect that consists in the development of particles that contribute to the formation of blossoms. These bud-forming particles move from the leaves into those parts of the plant where they are to bring about their own development into buds. We therefore now know of three different portions of the solar spectrum having very different physiological influences: The yellow and neighboring rays, which bring about the transformation of carbonic acid or the formation of starch; the blue and visible violet, that act as stimulants to motion; the ultraviolet rays, that produce in the green leaves the material for the formation. of buds. (Wollny, X, p. 230.)

INFLUENCE OF DRYNESS AND SUNLIGHT ON DEVELOPMENT OF TUBERS.

In the climate of Germany the flowering of different varieties of potatoes is very much restricted. Only a small number of varieties. flower regularly and bear fruit, whereas in Chile the plant flowers abundantly, but the tubers are small; in other words, in the Temperate Zone the formation of tubers is favored at the expense of fertilization; the energy of the one process increases while the other diminishes.

Knight and Langenthal have found that by detaching the young tubers they increase the blooming, and on the other hand, by cutting off the flowers they increase the development of the tubers, thereby largely increasing the harvest. Wollny, in 1886, experimented on four plats, each for many varieties of potatoes. He found that cutting off the flowers increased the crop of tubers as to number, size, and weight, but that something depended upon the time of cropping the flowers, which is best done a considerable time before they arrive at maturity. It seems probable that dryness and sunlight stimulate the formation of flowers, but humidity and cloudiness, at least up to a certain limit, stimulate the formation of tubers. This harmonizes with some recent results obtained by Sachs, who has shown that the ultraviolet rays stimulate the flowering. (Agr. Sci., Vol. II, p. 273.)