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 temper

atures.

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 5° 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: Observations by Gasparin at 2 p. m. daily.

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.

Another study into the total radiation received by the plants in sunshine was made by Gasparin by placing a thermometer in the center of a globe 1 decimeter in diameter, made of thin copper and covered with a layer of lampblack. Having found by comparison that bulbs of different sizes gave different temperatures, he recommends this size to all meteorologists; but I do not know of observations made by others until Violle (1879) urged the same construction and size for his conjugate bulbs. This bulb in the full sunshine and at a standard distance above the ground seemed, to Gasparin, to give what he calls the temperature of a dry opaque body. The difference between this and the temperature of the air gave a surplus showing the effect of solar radiation on the leaves; again, the difference between this dry, black bulb and the temperature of the surface of the moist earth gave him some idea of the nature and amount of the influence of the sunshine on the surface of the soil, which he illustrates by the following table, derived from seventeen years of observations:

We see how much the difference of temperatures of the stems and the roots ought to modify the flow of the sap, and there is here an interesting subject for physiological study which should redound to the profit of agriculture. The solar heat contributes also in a remarkable manner to cause the differences in the vegtation of the mountains and the plains. On mountain tops it is the heat of the surface soil and the roots in the sunshine and the effect of sunshine on the leaves that makes possible the existence of a great variety of phænogams. The direct action of the solar heat is the explanation of the possibility of raising cereals and other southern crops in high northcrn latitudes.

Gasparin (1852, p. 100) gave the following table, compiled for western Europe, showing the mean temperatures of the day during which the respective plants leaf out, flower, or ripen. This early effort to apply meteorological data to the study of plants takes no account, as the author himself says, of other meteorological conditions than temperature such as introduce considerable variations into the phænological phenomena, but he gives it in hopes of helping thus to fix the rela

tions of natural vegetation to cultivated plants. If in addition to recording temperature, rainfall, sunshine, and other meteorological elements, we could keep a parallel record of the stages of development of cultivated and uncultivated plants we could use the latter as an index to the effect of the weather during any season and predict from that the behavior of the cultivated plants.

Temperatures at the respective phænological epochs for plants in European climates (by Gasparin).

Apple tree (Malus communis); cherry tree (Cerasus communis).

8.0

Furze or gorse (Uler europœus); box (Burus sempervirens); white poplar (Populus alba)

4. 0

Sainfoin or French grass (Hedysarum onobrychis, Leguminosa).

12.7

Currants; raspberries; strawberries; cherries..

Morella cherry tree; apricot; plum tree; barley; oats

Rye

17.8

18.0

19.0

During decreasing heat (for fruits which have, received a sufficient quan

NOTE.-It can be easily understood that the fruits which require the greatest prolongation of heat ripen last and are gathered at periods of the lowest temperatures.

Lachmann, in his Entwickelung der Vegetation, counts the sum total of all the temperatures at his station (Braunschweig, Germany) from February 21 onward.

Linsser, for north temperate countries, counts from the date when the temperature 0° C. is attained, but for warmer countries he counts from the date when the lowest temperature of the year is attained; which date would, according to his calculations, be the 8th of February at Braunschweig instead of the 21st of February; but, according to the normal values resulting from the thirty years of observation by Lachmann, this change would only make his sum totals about 10° C. larger.

Tomaschek, as quoted by Fritsch (1866, LXIII, p. 297), takes the mean of all positive temperatures as observed at 6 a. m., 2 p. m., and 10 p. m., omitting the individual negative observations instead of the negative daily averages. He counts the sums from January 1; this method gives figures that agree very closely, at least in Europe, with those given by Fritsch's method.

Kabsch, as quoted by Fritsch, attempted an improvement on the method of Boussingault. His formula is especially appropriate to the annuals, but not to the perennial plants. His method of computing the thermal constant is expressed by Fritsch in the following formula: