418 ZONES AND REGIONS [Ft. Ill, Sect. II 



although its constituents, as regards their demands on heat, exhibit great 

 differences, which are by no means expressed by the quite temperate mean 

 temperature, nor by the sum total of the degrees of heat. The oecological 

 optimum temperature sometimes exhibits a curve that is nearly as flat as 

 those of tropical plants, but sometimes a steep ascent from low to high 

 degrees of temperature and as steep a descent. Moreover, the absolute 

 minimum of plant-life is proved to be very uneven, whilst data are not 

 yet available regarding the maximum, which is probably just as unequal. 



Whilst unequal demands on atmospheric precipitations appear to be 

 limited chiefly to groups of a low rank, from the genera downwards, the 

 relation in regard to temperature is frequently characteristic of groups of 

 a higher rank, so that apart from historical causes the differences in the 

 flora within either of the two temperate zones are greater than within the 

 tropics, where only atmospheric precipitations play an essential part. As 

 regards their floras, in the first place, belts with mild winters are marked 

 off from those with cold winters ; in the second place, districts with a 

 maritime climate are distinguished from those with a continental climate. 



General considerations on the effects of heat on growth, transpiration, 

 and other functions in the temperate zones must be omitted here, since the 

 great differences in temperature of the temperate zones determine corre- 

 sponding differences in the vegetative functions. 



ii. EFFECTS OF LIGHT. 



The difference in relation to plant-life between the amount and intensity 

 of light is most strikingly apparent when we compare tropical with tem- 

 perate zones. Under an equally dense crown of leaves the sum total of 

 luminous energy increases towards the poles, but the ability of plants to 

 live under it diminishes. Shade-vegetation is accordingly much more 

 strongly developed in the tropics than in the temperate zones *. The 

 unequal intensity of light in the temperate and tropical zones also induces 

 a different fixed lie of the leaves in relation to the light. In the tropics 

 direct insolation is the controlling factor ; leaves arrange themselves obliquely 

 or parallel to the rays, but in the temperate zones they expose their 

 surfaces at right angles to the direction of the brightest diffuse light, regard- 

 less of the direct insolation. In spite of the exposed position, the 

 destructive effects of light on chlorophyll are far less in temperate than in 

 tropical zones. The foliage of Scandinavian vegetation is considered to 

 be even of a more intense and pure green than that of Central Europe, 

 although it is almost continuously illuminated during summer. 



Many effects of light associated with less high intensities are naturally 

 more strikingly displayed as duration of daylight increases. Thus, the 



1 See p. 224. 



