THE ART OF THE SECOND GROWTH 



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 and irritation) ; 



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

 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 the period of vegetation. 



Phanerogamous plants are unable to absorb water directly 

 through the epidermis, obtaining it instead through the spongiolse 

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

 leaves. Mosses and lichens, however, absorb water directly through 

 the epidermis. The hygroscopic power of a dead cover of mosses 

 on the ground equals that of a live cover. 



Wet climate creates evergi-een 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 

 entirely diff"erent floras. Rain in summer stimulates growth much 

 more than rain in winter. De Candolle divides our globe according 

 to moisture and heat and on the basis of the resulting floral differ- 

 ences, into five regions. 



1. Hydromegathermal region (water great heat). Mean annual 

 temperature over 68 degrees Faht. (Amazon river region, wet tropi- 

 cal zone). 



2. Xerophilous (dry loving) region. The region and borders of 

 arid deserts, prairies, svmny slopes, etc., exhibiting a flora very 

 modest in moisture requirements. 



3. Mesothermal (medium heat) region, having mean tempera- 

 ture of 59 to 68 degrees Faht. (northern Florida, etc.). 



4. Microthermal (little heat) region of 32 to 59 degrees Faht. 



5. Hecistothermal (least heat) region of less than 32 degrees 

 Faht. 



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