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SUNSHINE AND TRANSPIRATION (DEHERAIN AND MARIÉ

DAVY).

Studies in the transpiration of plants were made in England as early as 1691 by S. H. Woodward, who experimented on aquatic plants. He showed that the consumption of water by the plant, or the weight of water evaporated from it, varied within narrow limits, while the growth of the plant under the same temperature and sunshine, varied according to the amount of nourishment in the water; thus of pure spring water 170 grains had to be evaporated in order to make an increase of 1 grain in the weight of the plant, but only 96 grains of the rich water of the Thames was required to make the same increase in the weight of the plant.

In 1848 Guettard, experimenting upon a creeping nightshade, showed that a plant kept in a warm place without sunshine would transpire less than one in a colder place with sunshine.

Deherain, as quoted by Marie Davy (1880, p. 231) introduced the leaves or stems of a living plant into a tube suitably closed; under these circumstances, by reason of the small, calm space of air surrounding the leaves, the evaporation in the ordinary sense would be inappreciable, but the transpired water was found to increase the weight of the tube, as shown in the accompanying table.

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The effect of sunshine in stimulating transpiration is very clearly seen by a study of these figures. The small transpiration from the leaf when kept in darkness is supposed to be, at least in part, due to a persistency of the stimulus given to the plant by the light; so that, as is well known, the growth of the plant goes on at its maximum rate in the late afternoons, sometimes even after sunset, and does not attain its minimum until early morning.

Deherain also arranged the following experiments showing the effect of temperature. Some living leaves of wheat were kept within a glass tube which lay in a water bath at a uniform temperature of 15° C. and the following measurements taken:

In full sunshine the transpiration was 0.939 gram of water per hour per gram weight of leaf.

In darkness the transpiration was 0.016 gram of water per hour per gram weight of leaf.

The water bath was then reduced to a temperature of 0° C., and the temperature of the leaf within the tube must therefore have been at the freezing point. In this condition the transpiration in full sunshine was 1.088 grams of water per hour per gram weight of leaf.

Thus leaves in sunshine in free air at 28° C. and leaves in the air at 15° C., and again in the water bath at 0° C., give us the transpiration under these conditions 0.882, 0.939, 1.088, respectively. It is evident that this transpiration is not due to evaporation alone, else it would be independent of sunshine and depend wholly on heat; the decided differences here shown must be attributed to the special excitement of the cell by the solar radiation.

Marie Davy gives for July 24 and 25, 1877, the following record from a self-registering apparatus showing the diurnal periodicity of the transpiration from the leaves of four plants of haricot beans which were watered daily at 7 p. m.:

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These same four plants showed the transpiration day by day, as given in the first column of the following table (Marie Davy, 1880, p. 239). The third and fourth columns, respectively, show the relation of this transpiration to the daily mean temperature and the daily mean radiation, as shown by the conjugate thermometers.

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The figures in the above table are influenced by the quantity of moisture in the soil; therefore Marie Davy occasionally omitted the evening watering, and the transpiration for the day after such omission was smaller. In general, Marie Davy concludes that the relation between transpiration and temperature is very variable from day to day, while that between transpiration and radiation is very regular, a regularity that would very probably be heightened if the cloudiness and the evaporating power of the wind, as depending on its dryness and velocity, had been considered. The belief is that sunshine excites the contraction of the stomata of the leaves and thus stimulates transpiration; but the stomata can not exude water to a greater extent than as supplied by the roots; therefore the transpiration is limited by the humidity of the soil adjacent to the roots. Thus on the 30th the radiation averaged 45.5 actinometric degrees, and the plant transpired 2.167 grams of water; on the 31st the radiation was 64.1 and the transpiration correspondingly increased to 2.710 grams; but on this day the reserve moisture in the soil was drawn upon very heavily, and in the evening the leaves of the plant were flabby and drooping and evidently wilting for the want of moisture.

The results by Deherain at temperatures of 15° C. and 0° C. and those by Marie Davy seem to demonstrate satisfactorily the slight influence of the temperature of the air as such upon transpiration.

Daubeny (1836), Deherain, and Wiesner have studied the effect of radiation in different parts of the spectrum, and their work shows that the radiations that are absorbed by chlorophyl, the so-called chlorophyl-absorption bands, are those that are efficient in stimulating transpiration; also that xanthophyl acts similarly, but weaker than chlorophyl; that the violet and ultraviolet have no appreciable influence; that the ultrared rays have an appreciable action, but feebler than the visible rays between the red and blue, notwithstand

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.

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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 wherenotwithstanding 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 difference 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.) singault's results are given in the accompanying table:

Day degrees required at different latitudes.

Plant and place.

Latitudes
north.

Duration of the cul

ture.

Mean air temperature during culture.

Bous

Product of the days by the temperature.

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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

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