Chap. I] CHARACTERS OF TIIIC TEMPERATE CLIMATI': 41.-, 



apparently have a considerable bearing on the processes of vegetation ; but 

 observations on this question arc not available. 



ii. LIGHT. 



As the non-calorific rays of the sun's energy can act on vegetation 

 exclusively by radiation, the zones denoting the distribution of light — 

 :oiits of /ig/it-cliiitatt\ in contrast to those denoting the distribution of 

 heat, ::oiics of tcvipcraturc-cliiiiate — are parallel with the equator : under 

 the heading of light may be included the ultra-violet rays, so far as they 

 act upon the plant. The total intensity of light diminishes uniformly 

 towards the poles, but the length of the da)- during the greatest part 

 of the vegetative season increases in the same direction — the illumination 

 during the winter sleep is without significance. 



DURATION OF DAYLIGHT IX DIFFERENT LATITUDES. 



Owing to the greater length of the da>- the insolation of a point 

 in the temperate zones during a summer da\- would be much stronger 

 than that of a point at the equator if absorption by the atmosphere did 

 not likewise increase towards the poles. With the sun in the zenith, 

 the atmosphere allows Si % of the luminous rays, 75% of the heat raj-s, 

 40°/^ of the chemical raj-s to pass; with increasing distance from the 

 zenith, the absorption will naturally become proportionately greater. 



Whilst absorption of the sun's raj's increases with the distance from 

 the equator, this is compensated to a certain extent by the increase of 

 diffuse light that pari passu takes place, so that the sum total of luminous 

 energy falling on a point in the temperate zones in summer remains 

 greater than that at the same time at the equator. Chcmual radiation, 

 on the other hand, in consequence of its stronger absorption becomes 

 considerably weakened. Hence with equal surfaces a plant of the temperate 

 zone during the vegetative season receives more luminous but less chemical 

 energy than one in the tropics. 



iii. ATMOSPHERIC PRECIPITATIONS. 



The alternation of dry and wet seasons that is almost general, if not 



everywhere equally pronounced, within the tropics, is continued beyond 



them up to nearly 40° N. and S. latitudes. In higher latitudes, districts 



with precipitation at all seasons of the }-ear predominate, for instance 



