SYLVICULTURE. 



The duration (number of days) of insolation is as important as 

 tlie intensity of insolation. Within the individual tree the lower 

 branches are killed gradually, being overshadowed by new upper 

 branches. Without light no bud; without bud no leafing branch; 

 without new leaves annually formed no limb can live. 



Within one and the same species a tree once acquiring superi- 

 ority over its neighbors is apt to retain superiority until death. 

 Since it enjoys more light, it assimilates better. 



Within rival species, owing to greater sensitiveness of chloro- 

 phyll and thanks to more favorable inclination, form and position of 

 leaves, some species exceed others in assimilation and vitality under 

 the same influx of iight. Shade bearing are such leaves as assimi- 

 late sufficiently (so as to bear buds at the axils) in spite of the 

 fact that only little diffused light chances to strike them. 



Many dicotyledonous trees form a so-called "leaf mosaic," the 

 .lower tiers of leaves fitting themselves into the inierstices of light 

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

 the sun according to the hourly degree of insolation (plioto-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 ^isrne 

 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 de- 

 termined 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, assimilation, 

 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 Faht. As 

 a rule, however, plant activity begins to be observable at 50 degrees 

 Faht. 



The maxima of heat compatible with plant life generally lie bo- 

 low 115 degrees Faht. Excess of temperature over' maximum is 

 more disastrous than deficiency of heat below minimum. Plants, 

 however, temporarily fortify themselves against periodical extremes : 



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