AMERICAN SYLVICULTURE 



left in the upper tiers. Many leaves alter their inclination toward 

 the sun according to the hourly degree of insolation (photo-metric 

 movement). The epidermis of light demanding and sun-exposed 

 leaves is heavy, leathery. The leaves of shade bearers are thin and 

 wither quickly when picked. Light demanding leaves are often shin- 

 ing, reflecting and whitish, so especially in tropical countries, and 

 the leaf stomata are deeply sunk into the surface. On the same 

 tree, leaves growing in the shade are darker than those growing in 

 the light; old leaves darker than young ones. 



The formation of spines and thorns indicates a sun plant; hair 

 or down are usually found in light demanders more than in shade 

 plants. 



III. Heat. 



For each plant and for each step of its development can 

 be determined a minimum, optimum and maximum of heat required 

 or allowed. Without heat, growth is impossible, since cell division 

 is impossible. The formation of chlorophyll, breathing, assimila- 

 tion, germination, flowering, fruiting and transpiration depend on 

 heat. The distribution of the genera is governed, pre-eminently, by 

 heat. 



For some polar plants, life is possible below 32 degrees air 

 temperature Faht. As a rule, however, plant activity begins to be 

 observable at 50 degrees Faht. 



The maximum of heat compatible with plant life generally lies 

 below 115 degrees Faht. Excess of temperature over maximum is 

 more disastrous than deficiency of heat beloAV minimum. Plants, 

 however, temporarily fortify themselves against periodical extremes: 



1. By non-freezing cell contents. 



2. By reduced water contents (seed, rosin). 



3. By lignification. 



4. By dropping leaves during winter or during period of exces- 

 sive drought. 



5. By adequate covers (bark, hairs, bud scales, layers rich in 

 air cells, reddish color, wrappings formed by last year's leaves). 

 These covers, at least, allow the plant to escape the dangers of ra'tlid 

 changes of temperature. 



Short periods of vegetation and long periods of rest result 

 from deficient heat. Hence no annual plants in polar regions. 

 Short shoots, evergreen leaves, preparation of flow^ers in year pre- 

 ceding fruit are characteristic of a polar flora. In tropical countries 

 there are no periods of rest unless determined by periods of drought. 

 14 



