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green weight, although larger in their dry weight, after what would ordinarily be called very heavy manuring. These facts are quite in accord with the general results of work at experimental farms, which, according to the South Carolina department of agriculture, have shown that increasing the amounts of the fertilizers beyond a certain point gives no corresponding increase in the amount of grain, and but few of the applications pay for their cost. There is abundant experimental proof that for any given soil there is a limit to the amount of profitable manuring. The process of improving the soil, like the process of fattening cattle, is comparatively gradual and requires time. The margin of profit in the application of manures is narrower than is generally supposed. It is equally important to . attend to the selection of the seed, the thorough cultivation, and the natural fertilization that results from the cultivation of the Leguminosae and the rotation of crops.
PART II.-EXPERIENCE IN OPEN AIR OR NATURAL CLIMATE.
Under the general heading we shall consider, first, the wild plants and their natural habits; second, the plants cultivated at experiment stations under instructive experimental conditions, and, third, the statistics of each and the experience of farmers in general from a practical point of view. The study of the forest or natural habits of plants leads us into the phenology of plant life. Phenology is a term first applied by Ch. Morren to that branch of science which studies the periodic phenomena in the vegetable and animal world in so far as they depend upon the climate of any locality. Among the prominent students of this subject, one of the most minute observers was Karl Fritsch, of Austria, who in his Instructions (1859) gives some account of the literature of similar works up to that date. He distinguishes the following epochs in the lives of plants, and especially recommends the observation of perennial or forest trees that have remained undisturbed for at least several years. His epochs are: (1) The first flower. (2) The first ripe fruit. The next important are, for the annuals: (3) The date of sowing. (4) The date of first visible sprouting. In order to assure greater precision he adds: (5) The first formation of spikes or ears. As Fritsch considers that the development of the plant so far as its vegetative process is concerned depends principally upon temperature and moisture, but that its reproductive process depends principally upon the influence of direct sunlight, therefore he adds a sixth epoch for trees and shrubs—viz: (6) The first unfolding of the leaf or the leaf bud or frondescence. This is the epoch when by the swelling of the buds a bright zone is recognized which opens out and the green leaf issues forth. Corresponding with the formation of the leaf is its ripening and fall from the tree, which Fritsch adds to his list of epochs, viz: (7) The fall of the leaf or the time when the tree has shed fully one-half of its leaves; as the wind and heavy rains accelerate this process the date is liable to considerable uncertainty independent of the vitality of the plant. Therefore, in this, as in all other epochs, Fritsch, in endeavoring to lay the foundations of the study, rejected those cases in which any unusual phenomenon, such as wind or drought or insects, had a decided influence on the observed dates. Many plants blossom a second time in the autumn, although they may not ripen their fruits; therefore in special cases Fritsch adds an eighth epoch, viz: ~ (8) The second date of flowering. Of course it is understood that if the second flowering is brought about artificially, as by irrigation, pruning, or mowing, that fact must be mentioned. When the flowers blossom in clusters, such that the individuals are lost sight of in the general effect, then, in addition to the first flower, we note the following item: (9) The general flowering or the time when the flowers are most uniformly distributed over the plant. For 118 varieties Fritsch gives in detail the phenomena that characterize the date of the ripening of the fruit. He also gives an equally elaborate system of observations on birds, mammals, fishes, reptiles, and insects, and especially the mollusks or garden snails and slugs.
THE RELATION OF TEMPERATURE AND SUNSHINE TO THE DEVELOPMENT OF PLANTS-THERMOMETRIC AND ACTINOMETRIC CONSTANTS. Reaumur was the first to make an exact comparison of the different quantities of heat required to bring a plant up to the given stage of maturity, and since then many authors have written on this subject. I will here give a brief summary of views that have been held by prominent authorities as to the proper method of ascertaining and stating the relation between temperature and the development of plants. Reaumur (1735) adopted simply the sum of the mean daily temperatures of the air as recorded by a thermometer in the shade and counting from any given phenological epoch to any other epoch. He employed the average of the daily maximum and minimum as a sufficiently close approximation to the average daily temperatures, and evidently in the absence of hourly observations any of the recognized combinations of observations may be used for this purpose. Reaumur found from his observations that the sum of these daily temperatures was approximately constant for the period of development of any plant from year to year; hence this constant sum is called a thermal constant in phenology. For the three growing
months—April, May, and June, 1734—the sum of the daily temperatures for ninety-one days was equivalent to 1,160° C., but for 1735 it was 1,015° C., whence he concluded that the ripening of the vegetation would be retarded in 1735 as compared with the preceding year.
This idea had been familiar to Reaumur for some time previously, and in 1735, as cited by Gasparin, Met. Agric., Vol. II, 1st ed., Paris, 1844, he says:
It would be interesting to continue such comparisons between the temperature and the epoch of ripening and to push the study even further, comparing the sum of the degrees of heat for one year with the similar sums of temperatures for many other years; it would be interesting to make comparisons of the sums that are effective during any given year in warm countries with the effective sums in cold and temperate climates, or to compare among themselves the sums for the same months in different countries.
Again, Reaumur says:
The same grain is harvested in very different climates. It would be interesting to make a comparison of the sum of the temperatures for the months during which the cereals accomplish the greater part of their growth and arrive at a perfect maturity both in warm countries like Spain and Africa, in temperate countries like France, and in cold countries like those of the extreme north.
This passage, says Gasparin, is the germ of all the works which have been executed since that time in order to determine the total quantity of heat necessary to the ripening of the different plants that have been cultivated by man.
Adanson (1750) disregarded all temperatures below 0° C., and took only the sums of the positive temperatures. He expressed the law as follows: The development of the bud is determined by the sum of the daily mean temperatures since the beginning of the year.
Humboldt early insisted upon the necessity of taking the sunlight itself as such into consideration in studying the laws of plant life.
Boussingault (1837), in his Rural Economy, introduces the idea of time by adopting the principle that the duration of any vegetating period multiplied by the mean temperature of the air during that period gives a constant product. He takes the sum of the temperatures from the time when vegetation begins and finds the length of the period of vegetation from germination up to any phase, to vary from year to year, inversely as the total sums of the daily temperatures.
Thus, for winter wheat to ripen, he found that there was necessary a sum total of from 1,900° to 2,000° C. of mean daily air temperatures in the shade, which constant sum is equivalent to saying that the average temperature of the growing period is found by dividing this number by the number of days. This method of computation takes
no account of any temperature at which the growth of wheat ceases. A lower limit for such temperature has been adopted by several investigators, such as the 0° C., already mentioned as adopted by Adanson. An upper limit has not yet been ascertained. Edwards and Colin put it at 22° C.; but in Venezuela Codazzi found wheat to mature under a constant temperature of 23° or 24° C. throughout the whole period of vegetation, and, as we shall see hereafter, the upper limit undoubtedly depends upon the humidity of the air, the moisture of the soil, and the total radiation from the sun quite as much as upon temperature. Similarly Marié-Davy calls attention to the fact that maize grows poorly at Paris, where it is cloudy and warm, but well in Alsace, where it is dry and clear, the temperature of the air averaging about the same in both, the difference being in the quantity of sunshine and rain.
Gasparin (1844) adopted the mean temperature of the day as derived from observations made at any convenient hours and took the sum of such temperatures from and after the date at which the plants, especially the cereals, begin to actively develop, or to vegetate, or when the sap flows readily throughout the day. For this “ effective temperature" he adopts 5o C.
Subsequently Gasparin adopted a thermometer placed in full sunshine on the sod as giving a temperature more appropriate to plant studies, but still retaining the lower limit of 5° C. for the mean daily temperature of the initial date. Thus he obtained for wheat a sum total of 2,450° C. as the sum of the effective daily temperatures from sowing to maturity.
Gasparin also observed the temperature of a blackened metallic disk in the sunshine and the temperature of the sunny side of a vertical wall, and again the temperature of a thermometer at the surface of a sandy, horizontal soil, all in full sunshine. He recognized that the loss of heat by evaporation must keep the temperature of the soil slightly lower than that of the surface of the wall; but, in default of better methods, he kept a record of the temperature of the wall for many years. From his average results I give the following abstract:
44.1 30.2 22.0
The warmth in the sunshine is to the warmth of the air in the shade as though one had been transported in latitude from 3 to 6 degrees farther south.