186 



flowering season, particularly as regards Prague, where the tempera- 

 ture 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 tempera- 

 ture tables, and also in regard to the epoch of ripening (fructifi- 

 cation). 



I have already had occasion to call attention elsewhere to the fact 

 that the falling of the leaves (effeuillaison) depends less upon the tem- 

 perature 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 w*ould be superfluous to consider the influence of the other me- 

 teorological agents when we still possess so little information as to 

 the mode of action of the princii^al 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 tempera- 

 ture, 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 ther- 

 mometer? By folloAving 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. Ge- 

 neva and Lausanne are 4° 30' farther south than Brussels, while their 

 elevation averages about 420 meters greater, which shoAvs that a de- 

 gree of latitude farther north is about equal to an increase in height 

 of 120 meters. At Munich and Groningen 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 knoAv the annual 

 temperature of Groningen. Berlin and Stettin seem to approach 

 that locality very nearly in the natural epochs of their plants. In- 

 deed 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, there- 

 fore, 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 con- 

 firmed 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, 



