Chap. I] CHARACTERS OF THE TEMPERATE CLIMATE 415 



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

 observations on this question are 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 — 

 zones of light-climate \ in contrast to those denoting the distribution of 

 heat, zones of temperature-climate — 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 day 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 IN DIFFERENT LATITUDES. 



Owing to the greater length of the day the insolation of a point 

 in the temperate zones during a summer day 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 81 °/ o of the luminous rays, 75% of the heat rays, 

 40°/ o of the chemical rays to pass; with increasing distance from the 

 zenith, the absorption will naturally become proportionately greater. 



Whilst absorption of the sun's rays 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. Chemical 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 cJiemical 

 energy than one in the tropics. 



iii. A TMOSPHERIC PRECIPITA TIONS. 



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 year predominate, for instance 



