Many of the plants observed by Hoffmann show such discordant sums from year to year as to prove that his method has no meaning for them, but for others the agreement is such that he recommends them to be observed in connection with the observations of the sunshine thermometer, as follows:

For the following plants observe the temperature sums from the first swelling of the buds to the first flower blossom: Castanea vesca, Bupleurum falcatum, Corydalis fabacea, Dianthus carthusianorum, Lonicera alpigena, Salix daphanoides, Syringa vulgaris, Amygdalus nana, Alnus incana, Alnus viridis, Atropa belladonna, Betula alba, Crataegus oxyacantha, Larix europaea (up to the date when the pollen first falls from the anthers), Ligustrum vulgare, Lonicera tatarica, Prenanthes purpurea, Prunus padus, Prunus spinosa, Rhamnus frangula, Ribes aureum, Rosa arvensis, Rosa alpina, Salix caprea, male (for the catkin, or the flowers of the willow, the beginning of pollination, as ascertained by a light stroke on the flower, is to be considered as the date of the first blossom).

Hoffmann also applies his summation of sunshine maxima temperatures to the interval from January 1 to the ripening of the fruits and shows an excellent agreement between the numbers for 1880 and those for 1881 at Giessen.

In the Zeitschrift for 1884 Hoffmann gives his results for 1882, 1883, and 1884 as collected in the preceding table and says that the vexed question of the thermal constant for vegetation is still far from being settled; either temperature and vegetation are independent of each other, which no one can easily believe, or they stand to each other in a relation for which the correct expression is still unknown. Pfeffer in his Pflanzen Physiologie (Vol. II, p. 114) has stated that the approximate uniformity of the sums of temperature, from year to year, can only mean that, in general, for each year the heat received from the sun amounts to about the same sum total for the same date annually; but this is not in strict accordance with facts, for if it were true a small change in the date should make a small change in the sums, which is not always the case. Thus, if for Linosyris vulgaris the dates of blossoming are August 15, 18, or 20, the sums from January 1 for different years will be as follows:

From these figures we see that, the sums vary from year to year quite independently of the change of date.

The thermometer B1, similar to B2, having been sent to Upsala for observations at that place, it gave from January 1 to the first blossom

2667-05 M16

sums that agree so well with those found at Giessen that Hoffmann thinks no better can be expected.

In the Zeitschrift for 1885 Hoffmann continues to give the comparative observations at Giessen and Upsala, and remarks that the question is not as to whether his method is correct and the others are wrong, but as to which of all methods is even a little better than the others. Of these others only one can, he thinks, be compared with his own, viz, that of Karl Fritsch, who takes the sum of all positive mean daily shade temperatures. Hoffmann applies Fritsch's method to the observations at Giessen and Upsala and finds the argument not in its favor. He also tries another form of thermometer, viz, the so-called black bulb in vacuo, but finds it too sensitive, which he thinks is because its bulb is too small.

In the Zeitschrift for 1886 (p. 546) Hoffmann gives a summary of observations at Giessen and Upsala during 1886. In general the sums are smaller at Upsala and so also for high Alpine stations. He is thus led to the laws established by Karl Linsser, as published in St. Petersburg in 1867 and 1869, which laws he expresses as follows: "Every wild plant has in the course of time so adapted itself to the surrounding local climate that it utilizes this climate to the best advantage. For any given phase of vegetation it uses a certain proportional part of the available annual sum total of heat. Thus, if the annual sum at Venice is 4,000 and if the corresponding sum at St. Petersburg is 2,000 and if the plant utilizes one-fourth in order to bring it to the flowering stage, then it will require 1,000 at Venice and 500 at St. Petersburg." From Linsser's law he concludes; (1) plants that have been raised in the north and are transplanted to the south reach their phenological epochs earlier than plants already living there, while southerly plants carried to the north are retarded as compared with those already acclimatized; (2) plants raised on colder highlands when transplanted to the warmer lowlands have their epochs accelerated as compared with those already domesticated; plants raised in the lowlands and transplanted to the colder highlands develop more slowly than the acclimatized plants.

In the Zeitschrift for 1886 (p. 113) Hoffmann determines the relative retardation of vegetation as determined by the dates of the first blossom of several plants at different altitudes. The result is for the Pyrus communis (pear tree) and allied varieties a retardation of 3.7 days per 100 meters, and corresponding to this a retardation of 2.8 days per 1° of latitude. The analogous data for Pyrus malus (apples) are 2 days per 100 meters and 4.4 days per 1° of latitude. Charts are given showing by means of isophenological lines the gradual progress northward of the development of vegetation as spring advances.

In Petermann's Geog. Mitth. for 1881 Hoffmann gives a general phenological chart for central Europe showing the acceleration or retardation of the phases of vegetation with respect to Giessen.

In the Zeitschrift, 1882, Vol. XVII, page 457, Hoffmann gives the results of his study of observations collected by Karl Fritsch, showing the dates of blossoming and ripening of fruits in Europe, as reduced to the latitude and altitude of Giessen; and, second, the thermal constant by Hoffmann's method from observations at Giessen for the years 1881 and 1882, as collated in the preceding table. He also shows that the advance of vegetation in the early and very warm spring of 1882 did not materially diminish the sums total of maximum temperatures, the figures for which I have reproduced in the preceding table (p. 240).

MARIE-DAVY.

The extensive researches conducted at the observatory of Montsouris (Paris) are scattered through many annual volumes, from which I have culled sufficient to show the views held by Marié-Davy and his coworkers, who distinguish very clearly between thermometry and actinometry, and attempt to determine separately the constant amounts of air temperature and of sunshine which constitute the total molecular energy needed to develop the plant.

In his Annuaire for 1877 Marié-Davy quotes from Tisserand (1875) and Schuebeler (1862) the results of a series of observations on the culture of grain in Europe. Special praise is given to the records from Norway and to the high state of education among the Norwegian farmers. The durations of the periods from sowing to ripening are as follows:

For other plants-oats, peas, beans, vetches, etc.—the duration of the vegetating period diminishes in a similar manner as the latitude increases or as the temperature diminishes; therefore we can not assume at once that warmth hastens the ripening, for in this case cold appears to hasten it. I say "appears," because with the cold comes in another influence, viz, the amount of sunshine. Thus as we go

northward we have a greater amount of possible sunshine during the growing period, although the actual sunshine is very materially diminished by the quantity of cloud and fog. Tisserand calls attention to the maximum possible duration of sunshine as given in the following table for the season of spring wheat from sowing to ripening:

These numbers of possible hours of sunshine should be diminished to actual hours of sunshine on account of cloudiness. Moreover, actual actinometric observations would have shown that owing to the atmospheric absorption the efficiency of the sunshine is less at low altitudes and, therefore, at high latitudes. But in the absence of fundumental climatic data Tisserand is probably correct in concluding that the temperature of the air has apparently little to do, in and of itself, with the duration of the time from sowing to ripening, but that this depends principally on the sunshine, so that at northern latitudes the wheat ripens best in localities that have the least cloudiness or the sunniest exposure. On the other hand, the temperature of the air does appear to materially affect the chemical constitution of the grain, since the northern crops are richer in hydrocarbons, and the proportion and quality of the starchy principle increases and the nitrogenous compounds diminish as the locality approaches the equator.

The acclimatization of plants is accompanied by notable changes in their nature; frequently the leaves increase in size relatively to the rest of the plant, and their colors are more pronounced, as if the plant sought to supplement the low temperature by a more complete absorption of the solar rays. A similar change as to the leaves and colors takes place in the flora of high mountains as compared with that of the plains below. The aromatic principles of plants are also developed in a remarkable manner in high latitudes. Thus the beans have a more decided flavor in Norway in proportion as we go northward, and at Alten (lat. 70° N.) the most aromatic cumin (Cuminum cyminum) of all Europe is cultivated.

The incident sunshine seems to be the productive climatic element in effecting the growth of plants; it furnishes the total vis viva, or

the mechanical or molecular energy, that is at the disposition of the plant, but it is also the last consideration to be studied and understood.

The temperature is the next important climatic element and that which has been most studied; the heat involved in temperature is the mechanical, molecular energy that is utilized by the vital powers of the plant. Each plant utilizes a fraction of the molecular energy that is at its disposition, according as its sunshine, temperature, and sap are favorable to the formation of the chemical substances that it can elaborate within its cells. The remaining elements important to the production of crops are:

(a) The water that enters the root, which may be natural rain or artificial irrigation.

(b) The chemicals dissolved in the water.

(c) The soil that furnishes these chemicals.

(d) The atmosphere that furnishes nitrogen, oxygen, and carbonic

acid gas.

(e) The evaporation of moisture from the plant and soil, mostly through the influence of the wind and heat.

Of these, only the rain water, the gases in the atmosphere, and the evaporation are, properly speaking, meteorological or climatic elements not under the control of man; whereas the irrigation of the soil and its chemical constitutents are largely under his control.

The quantity of water actually consumed by the plant or evaporated from its leaves and that which is daily evaporated from the soil or which drains away to other localities, and thus becomes useless to the plant, have been the subject of many experiments, some of whose results may be summarized as follows:

Thus, for example, Lawes and Gilbert, at Rothamsted, England, from experiments in vases entirely under their control, derived the following numbers, showing the weight of water evaporated relative to the weight of grain produced per unit area of ground:

In these experiments, therefore, the ground during the wheat season consumed water equivalent to a rainfall of from 184 to 212 millimeters in order to produce a harvest of 30 hectoliters, or 80 kilograms in weight per hectare.

a Is it not in fact the vital power of the plant?-C. A.