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Evidently this whole process of respiration depends largely upon the temperature of the air and is more active as the temperature increases. It goes on both in darkness and in light, but with this difference—that in darkness more carbonic-acid gas is given out than the oxygen that is absorbed, whereas, on the other hand, under the influence of light more oxygen is given out than the carbonic-acid gas that is absorbed. Both these processes are stimulated by heat. The assimilation or nutrition of the plant depends upon this mechanical influence of light in disengaging oxygen and "fixing" the carbon of the gas in the cells of the plant. Plant respiration is accompanied by two distinct but correlated phenomena, which are defined by Marie-Davy (1882) as "evaporation" and "transpiration."

Evaporation. This is a purely physical phenomenon. All bodies lose water from their external surfaces when in contact with dry air, and do so faster in proportion as the wind is stronger and the air is drier. Evaporation takes place for dead and living surfaces alike.

Transpiration. This is a physiological and not a purely physical phenomenon. It occurs only in living plants and under the influence of light; it is independent of the dryness of the air and is only indirectly dependent on temperature. It is intimately connected with assimilation, since by its means materials are furnished to complete the work of the growth of the plant.

DRYNESS, TEMPERATURE, AND VELOCITY OF THE WIND.

The evaporation from the leaves, the flow of sap, and the development of the plant depend almost as much on the wind and the dryness of the air as they do on the temperature of the air considered by itself, since all these are necessary in order to bring the supply of nutritious water up to the leaf. Therefore, the temperature of the air must not be considered as the only important climatic element controlling vegetation. At the time of the bursting of the buds in the spring, when no leaves are on the trees and when the respiration of the plant and the evaporation are at their minimum, the temperature and dryness of the air have their least influence, while the temperature and moisture of the soil may have their maximum relative importance. These latter are the elements that determine how much water shall be absorbed and pushed upward as sap. It is under the influence of this upward pressure of the sap that the sunlight manufactures the first buds and leaves. The temperature of the air flowing among the branches and buds may have any value without seriously affecting the development of the plant, provided it is above freezing and below a destructive temperature, such as 120° F., and above a destructive dryness, such as 5 or 10 per cent of relative humidity. Ordinarily a warm spring day implies a warm, moist soil and a warm,

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moist atmosphere. Man naturally observes first the latter feature, which is so important to him, and then associates it with the budding of the plant, but he recognizes his mistake when he considers that the plant is firmly established in the earth and that its nourishment and growth must depend primarily on the condition of the soil and roots.

TEMPERATURE AND MOISTURE OF THE SOIL.

The temperature of the soil a short distance below the immediate surface does not depend, by way of cause and effect, primarily on the temperature of the air. It is not warmed or cooled appreciably by conduction of atmospheric heat, but by direct absorption or loss of the radiation that falls upon it. To a slight extent (perhaps 5 per cent) this sunshine is reflected from the surface particles of the ground according to the laws of simple reflection; the remainder is absorbed by the surface and warms it. This warmed surface layer immediately radiates back a small quantity (10 per cent) as long waves into the atmosphere and through that into space, since the atmosphere does not absorb these long waves, but it gives up a larger part, perhaps 50 per cent, by conduction to the adjacent lowest layer of air, which being thus warmed quickly rises and by convection distributes this 50 per cent of heat throughout the atmosphere, whence it is eventually radiated back into space. The remaining 40 per cent of the solar heat is by conduction carried downward through the solid earth; a large portion is consumed in the evaporation of soil water and returns to the atmosphere with the aqueous vapor; the rest goes on downward, warming up the soil until it arrives at a layer 30 to 50 feet below the earth's surface, where the gradient of temperature just in front of it is the same as that just behind it. Here the heat would accumulate and push its way still deeper were it not that by this time, in most cases, the diurnal and annual changes of temperature at the earth's surface, where this heat wave started, have brought about a deficiency just below the earth's surface; consequently the heat that had reached the depth of 30 or 50 feet now finds the temperature gradient just above it beginning to reverse, wherefore this heat begins to flow back, upward, and outward. In this manner the temperature of the ground increases downward to a depth of a few yards during certain months and then upward during other months, in diurnal and annual fluctuations interspersed with irregular changes, depending on cloud and wind and rain, all of which are easily recognized by examining any system of curves representing the earth temperatures at different depths throughout the year.

The ground is warmed by the air only in case the temperature of the surface soil is lower than that of the air, and, although this happens frequently, yet the quantity of heat thereby communicated

to the ground is comparatively slight, owing to the slow conductivity of the soil and the small specific heat of the atmosphere. This point has been carefully developed by Maurer, of Zurich (1885). But when rain and snow fall, then the latent heat formerly contained in the atmospheric vapor is quickly given to the surface soil and directly conducted deeper into the ground, and the latter is warmed or cooled according as the rain or snow is warmer or cooler than it. In general, the warming of the soil by warm rain is less important than the cooling by cold rains, melting snows, and evaporating winds.

CLOUDINESS.

When clouds intervene the soil receives a smaller proportion of direct solar heat, and the proportion diminishes as the thickness of the cloud layer increases or as the proportion of cloudy sky to clear sky increases. We may adopt the approximate rule that the warming effect of the sunshine is inversely as the cloudiness of the sky within 45° of the zenith; thus for a sky covered by 10 cumulus or 10 stratus the direct solar heat at the ground is 0; for 10 cirrus or cirro-cumulus or cirro-stratus the solar heat is about 5, while for 0 cloudiness the radiation that the observer receives is 10.

SOIL THERMOMETERS.

The motions of the clouds do not affect the sum total of the intensity of the sunshine, but the variations of cloudiness are so important that it is best to make use of some form of sunshine recorder or, better still, some form of integrating actinometer as a means of determining the relative effectiveness of the sunshine for any hour or day. If any such instrument shows that during any given hour, with the sun at a known altitude, the duration of the effectiveness of the sunshine was the nth part of the maximum value for clear sky, then we may assume that the heating effect of the sun on the surface of the soil was the nth part of its maximum value and may thus ascertain and, if need be, approximately compute the irregularities of the diurnal waves of heat that penetrate the soil. But these irregularities are directly shown by thermometers buried in the soil at different depths, and the observation of such soil thermometers is an essential item in the study of climate and vegetation. The absence of these observations has necessitated much labor in unsatisfactory efforts to obtain the approximate soil temperatures from the ordinary observations of air temperature, radiation thermometers, clouds and sunshine.

Fortunately the agricultural experiment stations of the United States have begun the observation of soil temperatures as distin

guished from the deep-earth temperatures that have for a century past interested the student of terrestrial physics but do not affect agriculture. I shall hereafter give a synopsis of such records so far as they are available to me; but so much agricultural data has been collected, both in Europe and America, without corresponding soil temperatures that we also need the data and methods that may be used for estimating soil temperatures from ordinary meteorological observations.

SUNSHINE.

Climatology usually considers the temperature of the air as given by thermometers that are shaded from the effect of sunshine; this is the temperature of the air very nearly as given by the whirled or ventilated or sling thermometers and is that which is needed in dynamic meteorology. But the sunshine produces important chemical effects besides its thermal effects, and these have no simple relation to each other. It is therefore very important that we have some method of recording the duration, intensity, and quality of the total or general radiation that the plant receives from the sun and from the sun and the sky combined. Up to the early part of the nineteenth century the optical and thermal effects of sunshine were spoken of as due to certain imponderable forces called light and heat that were supposed to be combined in the complex solar rays, but which can be separated from each other. But we now believe it to be correct to speak of the sunshine as a complex influence, a radiation of energy, whose exact nature is problematical, but whose mechanical effects when it acts upon terrestrial matter we know, measure, and study as the phenomena of light, heat, electricity, gravitation, chemism, and vitality.

DISTRIBUTION OF CLIMATIC ELEMENTS RELATIVE TO THE LIFE OF THE

PLANT.

The

As before stated, plants respire during both day and night. pores of the leaves are always absorbing and emitting gases, but when the sun shines on the leaves, and more especially with the help of the yellow part of the solar spectrum, the chlorophyl in the leaf cells is able to decompose the carbonic acid absorbed by the plant, retaining carbon and rejecting the oxygen.

So long as the plant absorbs more carbon from the air and more nitrogen from the soil than it loses by any process it is continually increasing its leaf surface and the nutrition in its sap, laying up a store of nutriment for future use. This process ceases in the case of annual plants when the seed or grain or fruit begins to ripen; from this time forward the seed makes a steady draft upon the nutriment already stored up in the plant which goes to perfect the seed. In

this season of its growth the plant really needs less water than before, but still its roots have the same power of absorbing water, and if the sap is thus diluted there results a seed or fruit that is heavy with an excess of water. Of course this water will dry out, if it has an opportunity, after the harvest, but if it has no opportunity, on account of damp weather, it will remain in the seed and render the latter more subject to injury from fungi, whose spores are always floating in the air seeking a moist nidus or resting place favorable to their growth. Such moist seeds give a heavy, green harvest, but a light dried crop.

Thus it happens that the distribution of atmospheric heat, and moisture, as to time, is quite important in its effect on the local harvest.

Apparently the time of ripening of the harvest depends wholly upon the chronological distribution of water and sunshine, but the quantity and quality of the harvest, which are the important practical results to the farmer, depend upon the nutrition carried into the plant by the water that is absorbed by the roots.

IRRIGATION.

The determination of the right time for irrigation and of the proper quantity of water, in order to produce the best crop in soil of a given richness is the special problem of those planters who depend mostly upon irrigation for successful agriculture. In general it may be said that our ordinary seeds have long since been selected and acclimatized with a view to success in a climate where abundance

of moisture is available at the proper season. Hence our crops are not so likely to be injured by excess of rain as by deficiency or drought. Therefore in almost every section, from the Rocky Mountains to the Atlantic, the highest success can only be attained by making provision for artificial irrigation in times of drought. The exact times and quantities of irrigating water depend upon the seed, the soil, and the evaporation, which latter is due to dryness of the air, the velocity of the wind, and the character of the soil; but when artificial watering or irrigation is needed to supplement natural rain one must seek to approximate as closely as practicable to the conditions presented in the countries where the seed originated, and especially the conditions presented during the seasons in which the given seed produced the best crops.

IMPORTANCE OF CLIMATIC LABORATORIES.

The studies that we are entering upon are greatly facilitated by experiments on a moderate scale under conditions that are under the control of the investigator, and free from the irregularities of openair agriculture. The laws of nature can only be found out by questioning nature, as it were, by means of test experiments. Our present

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