ing that their heating effect is usually greater than those of the visible spectrum.

The laws of growth or vitality are the laws of physics and mechanics and chemistry as applied to living cells. The changes that go on slowly in the plant are not the same as would go on rapidly in large masses of the same chemicals when treated as in the ordinary chemical laboratory. In the plant small masses are confined within the transparent walls of the cells until that subtile influence which we call radiation can do its work in bringing about new combinations of the atoms. It matters not whether we consider the radiation as an orthogonal vibration, as in light, or a promiscuous interpenetration of the molecules, as in heat, or a radial vibration, as in the waves of sound; whatever view we take of it, or whatever the details may be, even if it be a rythmic breaking up and re-formation of the molecules, the general characteristic of radiation is an extremely rapid motion along the molecules and atoms of matter. Therefore, by radiation we understand energy or momentum in the minute atoms that go to make up the molecules and the masses that we deal with; this implies that work is done by one atom upon its neighbor, which work, according to its style, we call light, heat, evaporation, etc. Assimilation and transpiration are among the forms of work in the growth of the plant that are due to the molecular energy contained in sunshine, and it is essential to progress in agriculture that there be kept a continuous register of the intensity and nature of the solar radiations that reach the plant. But this is a difficult problem, whose satisfactory solution has not yet been attained, although the work of Violle, Bunsen and Roscoe, Marie Davy, Marchand, Langley, Rowland, Hutchins, and many others have marked out the methods which seem most promising.

ANNUAL DISTRIBUTION OF SUNSHINE.

Humboldt (1845), in his chapter on "Climate," after comparing the climates and fruits of Europe, says:

These comparisons demonstrate how important is the diversity of the distribution of heat throughout the different seasons of the year for the same mean annual temperature, as far as concerns vegetation and the culture of the fields and orchards, and as well as regards our own well-being as a consequence of these conditions.

The lines which I call isochimenal and isotheral (lines of equal temperature for winter and summer) are not parallel to the isothermal lines (lines of equal annual temperature) in those countries where— notwithstanding the myrtle grows wild in its natural state, and where no snow falls during the winter-the temperature of summer and fall scarcely suffices to bring apples to full maturity. If to give a potable wine the vine shuns the islands and nearly all sea coasts, even those of the west, the cause is not only in the moderate heat of summer upon the seashore, a circumstance which is shown by thermometers exposed

in the open air and in the shade, but it consists still more in the dif ference between direct and diffused light, between a clear sky and one veiled with clouds, a difference which is still unappreciated, although its efficaciousness may be proved by other phenomena, as, for example, the union of a mixture of chlorine and hydrogen.

Humboldt adds:

I have endeavored for a long time to call the attention of scientists and physiologists to this difference; in other words, to the yet unmeasured heat which direct light develops locally in the cell of the living plant. (Cosmos, t. I, pp. 347–349.)

TOTAL QUANTITY OF HEAT REQUIRED TO RIPEN GRAIN.

Boussingault (1834), in his Rural Economy, computes the total quantity of heat required to ripen grain by multiplying the mean daily temperature of the air in the shade in centigrade degrees by the duration, in days, of the process of vegetation. This product is known as the number of "day degrees" that the plant has experienced or has required for the development from sowing to maturity. (See Annual Report Chief Signal Officer for 1881, p. 1208.) Boussingault's results are given in the accompanying table:

Day degrees required at different latitudes.

The above table shows that the total quantity of heat required increases as the latitude diminishes.

THE SUNSHINE AND HEAT REQUIRED TO RIPEN GRAIN.

Tisserand (1875) modifies Boussingault's hypothesis that growth varies with heat and time, but adopts the rule that the work done by a plant can be represented by the product of the mean temperature

by the number of hours of sunshine, only rejecting the useless nighttime, just as one would reject the useless low temperature. In the absence of sunshine records he uses the number of hours between sunrise and sunset, or the duration of diffuse sunshine, and obtains for spring wheat and barley the data given in the accompanying table, where the last column may be said to give "sunshine hour degrees."

We see that the sunshine hour degrees diminish as the latitude increases. This diminution ought to be rather more rapid in proportion as the actual state of the cloudy atmosphere approaches the theoretical state of absolute clear sky.

Thus Halsno and Bodo, localities which have very nearly the same soil, the same altitude, the same orientation, the same distance from the sea, but which are more or less under the influence of the aqueous vapor coming from the Gulf Stream, have a cloudiness during the evolution of wheat of 5.6 and 7; during that of oats, 5.4 and 7; where 0 represents perfect freedom from clouds and 10 completely covered. If records of cloudiness could have been used, the numbers in the last column would have been computed like those in the following table:

THE SUNSHINE AND HEAT REQUIRED TO FORM CHLOROPHYLL.

After considering the preceding data Marié-Davy (1880, p. 221) presents the following as his views:

It is the chlorophyll or green coloring matter in the cells of the green leaves that alone has the property of decomposing the carbonic acid of the air. It utilizes the sunlight, but also requires a certain temperature, which may be given to it either from the air or from the sunshine itself, so that we may say that ordinarily in nature the sunshine both warms the chlorophyll by means of the red rays and enables it to decompose carbonic acid by means of the yellow rays. The decomposing action of the chlorophyll only becomes appreciable at a certain minimum temperature, which is about 15° C. when the temperature is rising. It attains its maximum activity at about 30° C., and as the temperature cools it retains an appreciable activity at about 10° C. These figures are obtained by experiments of Cloëz and Gratiolet on water plants in the full sunshine. On the other hand, Boussingault obtains 1.5° and 3.5° C. as the lower limits of temperature for the ordinary Gramineæ, but these plants were in the sunshine, and if his temperature observations had been made in the shade they would have given lower figures than these, so that undoubtedly the Gramineæ can assimilate and grow when the temperature of the air in the shade is below freezing. On the other hand, Sachs find that when the illumination is below a certain minimum, which varies with the plant and with the temperature, the color of the chlorophyll is a clearer yellow tint, and for temperatures below a certain minimum which varies with the plant it remains colorless, notwithstanding the most brilliant sunshine. Thus in 1862 the exceptionally low temperature of the month of June was sufficient to prevent the development of new leaves on the stems of maize, cucumbers, and beans, so that all these remained yellow and only became green subsequently with warmer weather and better sunshine.

The pale leaves of a sprouting bean became green in a few hours under a temperature of 30° to 33° C., but this happened only in the sunlight, for at the same temperature in the darkness they remained yellow. At a temperature of from 17° to 20° C. the greening of the leaf went on much more slowly; at 8° and 10° C. there was only a trace at the end of seven hours; below 6° C. the leaves remained fifteen days without greening.

Similarly the pale shoots of maize, even at a temperature of 24° to 35° C., did not become colored in the darkness, but in the feeble light of the interior of a room a green effect was visible at the end of an hour and a half, and at the end of seven hours the leaves were all green and of normal appearance. At a temperature between 16° and 17° C. the first traces of color were visible at the end of five hours.

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. Marié-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.