SYLVICULTURE. 



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 rapid 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 flowers in year preceding 

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

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



IV. Moisture of air and precipitations. 



Water is at hand 



a. to increase the toughness of wood (imbibition water of 

 cell walls) ; 



b. to allow of solution of cell contents (cell sap) ; 



c. to serve as plant food, through assimilation; 



d. to allow of osmotic movement of sap; 



e. to assist in photometric movement of leaves (through swell- 

 ing ana irritation) ; 



f. to reduce rapidity of change of. temperature by evaporation. 

 Only some lichens survive a process of absolute drying. Lack 



of moisture causes crippled growth, and frequently subterranean 

 forests ( mesquit ) . 



After Henry Mayr, the minimum of moisture compatible with 

 tree growth is two inches of rainfall and fifty per cent, of relative 

 humidity during period of vegetation. 



Phanerogamous plants are unable to absorb water directly 

 through the epidermis, obtaining it instead through the spongiolae 

 of the roots and, in gaseous form, through the stomata of the leaves. 

 Mosses and lichens, however, absorb water directly through the epi- 

 dermis. The hygroscopic power of a dead cover of mosses on the 

 ground equals that of a live cover. 



Wet climate creates evergreen woods (Pacific coast and Ant- 

 arctic forests of South America). 



A dry climate gives rise to annual species, to a distinct period 

 of rest, to rapid flowering and fruiting. 



Precipitations equally distributed over the twelve months of an 

 entire year and precipitations falling during a few weeks result in 

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