30 PLANT GEOGRAPHY 



tufted plant, when grown in shade, elongates and 

 produces smaller leaves. Shade-plants, on the other 

 hand, have their chlorophyll in the epidermal ceils of 

 the leaf, and are commonly few-flowered. These con- 

 trasting conditions may occur, according to surround- 

 ings, in the structures of the same species. 



Very intense light, especially if violet, disintegrates 

 chlorophyll, turning it yellow or brown; and is actually 

 fatal to protoplasm, thus causing the death of many 

 Bacteria and Confervae. On the other hand, the absence 

 of light produces " etiolation," a lengthening of slender 

 internodes, with small leaves, and the replacement of 

 chlorophyll by the pale yellow etiolin. 



The long-continued light of clear northern latitudes, 

 though at a low angle, favours rapidity of development. 

 Thus barley passes through its life-cycle, from sprouting 

 to ripening, more rapidly there than farther south. 



The slope of the ground and its aspect, or the point of 

 the compass towards which sloping ground faces, have 

 an important action in determining the time when, and 

 the angle at which, light falls upon plants. Thus it is 

 only on the cliffs of Spitzbergen that many Arctic species 

 receive enough radiant energy from the sun to ripen 

 fruit. Trees commonly come into leaf earlier on the 

 side facing the sun. 



B. HEAT 



Heat is far more influential than light in determining 

 plant-distribution. Its distribution over the earth's 

 surface depends mainly upon latitude, the grouping of 

 seas and land-masses, and altitude. The inclination of 

 the earth's axis towards the plane of the ecliptic produces 

 those differences in the length of day and night which 

 constitute seasons, and divides the earth's surface into 

 latitudinal zones, the Torrid Zone extending for 23^ on 

 either side of the Equator, the Frigid Zones for 23^ from 

 either Pole, while the two Temperate Zones (North and 

 South) each occupy the 43 between one of the Tropics 

 and the Arctic or Antarctic Circle. But though these 

 mathematical zones do bound or limit areas according 

 to the total proportion of solar radiation they receive, 

 they are very far from bounding regions having the 

 same climates or even temperatures. Of the sun's rays 



