XXXIV INTRODUCTION. 



be in a great measure suspended under the stimulus of the 

 strong light emitted from the sky at that season.* 



* It seems probable that amongst the complicated elements determining 

 the geographical distribulion of plants may be the duration of their exposure to 

 direct or diffuse solar light, and the modified and diminished barometric pres- 

 sure resulting from altitude, with perhaps other scarcely subordinate conditions, 

 dependent on electricity and similar meteorological agencies, not hitherto taken 

 into account. It is alleged that many tropical plants in our stoves suffer from 

 the too long-conlinued action of light upon their leaves in summer, depriving 

 them of that sleep or repose from functional activity which the invariable alter- 

 nation of equal day and night ensures to them, in their native climate, for the 

 renewal of their exhausted irritability. Such plants, possessing in their peren- 

 nial foliage organs permanently excitable, would probably, irrespective of tem- 

 perature, he as unfit to exist for any length of time in regions exposed to long- 

 protracted daylight at one period of the year as to tolerate the deprivation of 

 its absence for the same space of time at another season. In northern latitudes 

 the cold of winter compensates for the continued stimulus of light through the 

 long days of summer by the torpor and suspension of the circulation it induces 

 in plants, whose respiratory functions have been previously arrested by the natu- 

 ral decay of the leaves, from constant excitement during the season of vegetable 

 activity. 



Even between the tropics, certain trees, having leaves analogous in their 

 thin transpirable texture to those of species inhabiting the temperate zone, 

 pass a short period of the year in repose, and are as truly deciduous as any of 

 colder countries ; but, since the stimulus of light is withdrawn, in the former 

 case, for many hours out of the twenty-four, at all seasons alike, a shorter ces- 

 sation of the vegetable functions suffices to restore impaired irritability than in 

 the latter, where the excitement is kept up, with but partial or imperfect remis- 

 sions, for weeks or months together. In countries verging upon or within the 

 arctic circle this continuous action of light augments in an increasing ratio of 

 intensity and duration with the latitude attained, and so makes up, by its stimu- 

 lating power on vegetation, for the low mean temperature which (contrary to 

 popular belief), marks the short-lived summer of such high latitudes. For it is 

 certainly a great though very prevalent error to suppose that the summers of high 

 northern latitudes, though short, are hot : all meteorological tables prove the 

 contrary, it being a fact that the mean temperature of the year, and of each 

 month in it, diminishes from the equator to the pole (local causes of deviation 

 from the general law excepted) by a decrement capable of exact calculation, 

 but varying betwixt certain parallels, and greatest about the middle lati- 

 tudes. 



It is true, as Humboldt long ago observed, that the mean temperature 

 of summer diminishes less rapidly than that of winter in receding from the 

 equator; but so far is the temperature of the former season from increasing 

 towards the arctic parallels, that the prolonged presence of the sun above the 

 horizon cannot compensate for his lower altitude and consequent obliquity of 

 his rays sufficiently to maintain a mean summer heat equal to, much less sur- 

 passing, that of any latitude farther from the pole than the one assumed to 



