This table of average intervals shows how variable is the acceleration of one place over another during the different seasons of the year. This acceleration even often changes into retardation, consequently the isanthesic lines are far from remaining parallel. We therefore conclude that latitudes and longitudes are not the only and principal causes which regulate the phenomena that are engaging our attention, because these unchangeable causes could not produce different effects; it is the same with regard to altitudes, we must only consider them as intermediary agents, and we should do wrong to take them as the basis of calculations for determining the epochs of natural phenomena." Let us see whether temperatures will give more satisfactory results. In order to facilitate the comparison I have gathered in the table (which unfortunately has not been completed for all the localities) the average temperatures for years, seasons, and months.c I must limit myself to consulting these elements, as I have not the necessary data to compute the base of daily temperatures and particularly to take the action of the sun into consideration. This first work will perhaps make us feel the incompleteness of the system of meteorological observations adopted at present (1849) in Europe. I have also been obliged to exclude the influence of the temperature of the earth, although it is absolutely necessary to consider it, in order to treat the phenomena of vegetation in a complete manner.

The mean temperature in winter at Brussels is 2° C. The most favored localities in comparison with it are Naples, Alais, and Polperro (near Lands End, England). I have not been able to deter

a It will be understood that I wish here to speak only of the action of geographical circumstances considered outside of the influence of temperature. This action has been but little studied up to the present time, but it is well worthy of our consideration. The following is what one of the most distinguished living botanists of the present time has written to me on this subject: "The distribution and extension of each species of plant over the earth shows us that the plants in general and each species as a unit are subject to organic changes dependent upon longitude and latitude. Each has a limited range; between these boundaries it has its paradise, where it thrives best. The organic changes which take place in individual plants, if one compares those that are native in different places, are such that we might presume that even their periodic phenomena must be affected. For example, all plants are stunted in height and in the number of their leaves toward their northern limit (or rather polar limit). They change their general appearance in going from east to west on the same parallel; they alter as to the extent of inflorescence and the size of flowers in going north or south on the same meridian. Now, as it is only by means of these organs that the plant vegetates in the presence of the world outside of it, it is necessary in our observations to begin with the relation of those organs, or rather the consideration of the developed organs ought to enter into our notation of their vital action. It further follows from this that we ought to study plants whose natural boundaries are known to us; these are the true barometers for vegetable life" [i. e., as the barometer is the measure of the activity of the atmospheric forces, so the natural geographic boundaries are the measures of the vital activity of plant life]. (Letter of M. de Martin's Observation of periodic phenomena, "Mem. Acad. Royal," Brussels, Vol. XVI, p. 11.)

Further, it has sometimes been necessary to give the temperature of a neighboring locality instead of that of the place itself; thus for the temperature of Polperro I have taken that of Penzance, and the temperature of Makerstoun has been replaced by that of Edinburgh, etc.

c I have omitted these figures in my copy of Quetelet's table.-C. A.

I should have liked to supplement this work with maps showing the principal epochs in vegetation, but the collected observations are not yet sufficiently

mine the epoch of the awakening of the plants in the first two places, but in the last mentioned the acceleration is forty-one days. This acceleration is also very great at the other stations of England, as well as at Valogne, which has also probably a sea temperature.

It has also been impossible for me to fix the time of awakening for places where the winter is the most rigorous, such as Lapland, Sweden, and the United States. We have seen, however, that there is twenty days retardation in places where the mean temperature is very little below zero. Jever seems to be an exception to this rule; but the results obtained in this place were only deduced from three observations.

The epoch of leafing corresponds, as we have said, with the end of March and the month of April, and that of the flowering with the months of May and June. The first includes the commencement of spring, the other the end of it. Thus the temperature of Brussels in spring is 10° C. The greatest variations besides are at Naples and at Alais. It is also in these places that the leafing takes place first. Venice, Parma, and Guastala are very little in advance, but the month of March and the beginning of April are scarcely any warmer than at Brussels. The difference of temperature is only felt in a marked manner in the following months. The flowering also takes place about eighteen days sooner.

Polperro, in regard to leafing, is about ten days in advance. The temperature in March is much higher than that of Brussels, while in April it is about the same. The advantage is lost in the following months, when, as regards flowering, Brussels is in advance of Polperro, as well as of the localities in England.

Brussels is about eighteen to twenty days ahead of the towns of Holland and Germany in the epoch of leafing, and is behind in the

complete to allow of undertaking such a task. The first chart would have shown by a series of lines drawn over Europe the awakening of plants for each ten days, that is to say, a first line would indicate the localities where the awakening first takes place immediately after the coldest day of the year, which with us is about the 20th of January; a second line would pass through places where the awakening is on an average ten days later, and so on. Another system of similar lines traced upon a second chart would have indicated in the same way the beginning of budding, always proceeding by intervals of ten days. We should also have made similar charts for flowering and ripening and the fall of the leaves. By comparing these charts we should be able to see at a glance the principal changes which take place in these various systems of lines. In order to complete this study we should imagine other systems of lines relating to temperatures. Thus one system would show the localities in Europe where frosts first cease, always advancing at intervals of ten days; then another system for places which, at successive intervals of ten days, and beginning from the awakening of the plants, have reached a sum total of temperatures amounting to 183° C., corresponding to the epoch of leafing; further, a third system of lines which should pass through places that, counting from the time of awakening, have successively attained the total number of degrees of temperature necessary for the flowering of plants; and so on for further systems.

The charts relating to vegetation and those relative to temperatures would, by comparing them, give much curious information. Unfortunately the observations we possess of daily temperatures are still as rare as those of the flowering. I have therefore been compelled to renounce that portion of my work.

flowering season, particularly as regards Prague, where the temperature in April, May, and June is a little higher than that of Brussels.

The retardation for stations in Sweden, the United States, and Lapland is sufficiently explained by an examination of the temperature tables, and also in regard to the epoch of ripening (fructification).

I have already had occasion to call attention elsewhere to the fact that the falling of the leaves (effeuillaison) depends less upon the temperature of the year than upon the effects of the first cold. Thus the leaves fall sooner in the north than in the south, unless they fall sooner here on account of a season of great dryness or excessive heat. It would be superfluous to consider the influence of the other meteorological agents when we still possess so little information as to the mode of action of the principal cause, which, in our climate, dominates in some degree all the phenomena of vegetation.

The temperature month by month at Geneva and Lausanne vary little from that observed at Brussels. The winter months there are a little colder and the vegetation is a little behind. Toward the time of ripening this retardation changes into an advance. The temperature, however, in spring and winter is no higher than that of Brussels.

Is not this advantage to be attributed to the fact that Geneva and Lausanne, having a higher elevation, enjoy purer air and a more efficient solar radiation, elements which are not indicated by the thermometer? By following the mode of calculation generally adopted one would say that the difference of latitude between Brussels and the two Swiss cities is compensated by their different altitudes. Geneva and Lausanne are 4° 30′ farther south than Brussels, while their elevation averages about 420 meters greater, which shows that a degree of latitude farther north is about equal to an increase in height of 120 meters. At Munich and Gröningen the same plants flower almost simultaneously, yet their latitudes and elevations are very different. Munich is 5° 4′ farther south, but is 524 meters higher. Here again a degree of south latitude nearly compensates 100 meters of elevation. It is to be regretted that we do not know the annual temperature of Gröningen. Berlin and Stettin seem to approach that locality very nearly in the natural epochs of their plants. Indeed there is very little difference in their latitudes, their elevations, and probably, also, in their temperatures.

Carlsruhe and Brussels have about the same annual temperature. The winter and early spring are a little colder in the first than in the second of these cities, consequently the vegetation is a little later; on the other hand the months of April and May are warmer, therefore, we see the vegetation changes its retardation into an advance.

Carlsruhe is about 2 degeees south of Brussels. For this reason alone vegetation should be about eight days in advance as at Paris; but on the other hand its altitude is about 300 meters greater than that of Brussels, and its vegetation should for this reason be about twelve days later. Combining the effects of these two causes, Carlsruhe would still have a retardation of more than four days, which is confirmed by experience for the first portion of the year; but in the second part we see this retardation change to an advance of fifteen days. Should we not here again remark, as was done before, that,

other things being equal, vegetation is much more active on high plateaus, where the radiation is greater, as well as in localities where the annual variations are very marked? This activity is further reenforced if the locality is near the polar regions, where the light acts almost uninterruptedly when once the awakening of the plants has taken place. In this respect Russia and Lapland present us with notable examples of this reënforcement.

Kupffer, in his "Note relating to the temperature of the soil and of the air at the limits of the region of cultivation of cereals," gives the following temperatures for the three principal boundary points of this region:

"A comparison of the curves for Nertchinsk, Irkutsk, and Archangel demonstrates in a striking manner," says Kupffer, "under what climatic conditions the cultivation of cereals can be carried on notwithstanding the lowness of the average annual temperature. All the curves agree together in spring and autumn, whence it results that it is especially the temperature of spring and autumn which influences the cultivation of cereals; it is in these seasons, in fact, that occur the two most important periods of the year for agriculture the time of sowing and the time of reaping. In the cultivation of rye autumn plays a still more important part, because rye is sowed also in autumn." Kupffer calls attention in another part of his note to the fact that some kinds of farming are carried on where the soil below the surface is frozen. "Experiments in farming," he says, "have been made at Irkutsk, on a very small scale it is true, but which in many respects have been a success. This is due to the fact that the soil becomes soft on the surface and is thus capable of developing the germs received by it; its mean temperature is above zero four months in the year, which is sufficient to ripen the cereals in a country where continuity of the sunshine makes up for the weakness of solar action. Snow often falls upon the sheaves, but still they harvest them." These examples confirm what we have said in regard to annual changes of temperature. In no locality in the world are these variations greater than here; at Yakutsk the difference of temperature between the warmest and the coldest month of the year is 50.9° C.; at Irkutsk, it is 24°.1; at Nertchinsk, 39.°1; at Archangel, 28.2° C.

It might be said, it is true, that the average temperature of the year should not be considered here, not even that of the free air, so long as the plants are covered by snow to shield them, for in this case the temperature of the air does not at all represent that of the plants. In this respect the conditions of vegetation would be the same at each

locality about the time of the winter awakening, and we should particularly consider the temperature that follows after the thermometer has passed the freezing point, as well as the quantity of light radiated by the sun.

It must therefore be admitted that cold, as long as it does not destroy the life of the plant, may be more or less severe or more or less prolonged, and thus lower the average yearly temperature, without causing any marked difference in the epochs of vegetation. This reflection explains, independent of all hypothesis, that for any equable mean annual temperature the acceleration in vegetation should be in favor of localities where the annual variation is the greatest, particularly in northern countries, where the frost prevails during many months of the year and where duing many of the following months the sunlight never ceases to fill the sky. Admitting the hypothesis that the action is proportional to the sum of the squares of the temperatures, the results are still more positive; for, other things being equal, the greater the annual variation the greater will be the sum of the square of the positive ordinates in the curves of temperatures.

I will now present some conclusions that one can deduce from all that precedes. I must first of all warn my readers that this work must be considered only as an attempt to solve a problem as difficult as it is interesting, the principal elements for the solution of which are still wanting.

1. A great number of factors combine to produce variations in the periodic phenomena of vegetation, the most important of which in our climate is temperature.

2. It may be estimated that the progress of vegetation is in proportion to the sum of the temperatures, or, better, to the sum of the squares of temperatures, calculated above the freezing point, starting with the epoch of the awakening of vegetation after the winter sleep. 3. The cold of winter, if it does not injure the vitality of the plant, does not cause any perceptible retardation in its future development, particularly if the ground has been covered with snow.

The effects that can be produced by the cold of winter must, however, be considered, and especially the condition of the plant when it entered upon its winter sleep, a condition which should correspond to a certain sum of acquired temperatures (or heat stored up). As to the ripening of the harvest and because plants develop under the influence of the sun, we must consult a thermometer exposed to its direct action, and not a thermometer exposed to its direct action, and not a thermometer placed in the shade, as is commonly done.

4. The temperatures at night are not comparable with those of the day as to their effects on vegetation. The quantity of light received by the plants must also be taken into consideration.

5. An increase of 1° in latitude produces about the same retardation in vegetation as an increase in elevation of 100 meters; that is to say, in our climate, a retardation of about four days.

This result should be looked upon as only a kind of average of quantities that vary during the year, the differences of latitude and elevation having scarcely any real influence further than as they produce differences of temperature.

6. The variations of temperature, other things being equal, are