THE ART OF THE SECOND GROWTH 



B. Natural laws govern the organization of the species 

 and regulate the communal life (symbiosis) and messmateship 

 (commensalism) of individuals with their own kin, with relatives 

 and with other plants belongings to the same household and feeding 

 at the same table. 



C. The most important ecologic factors are: 



I. Air. Oxygen, nitrogen and carbonic acid, the main com- 

 ponents of air, are essential for plant life. The relative proportion 

 of the two integral parts, 79% N., 21% 0., varies but little with 

 altitude, latitude and elevation. Salt particles in the air near 

 oceans and sulphuric acid in the air near smelter works are in- 

 jurious to plant life. 



II. Light. Intensity depends on: 



Season. 



Latitude. 



Altitude. 



Direct insolation is said to be, on the whole, of less importance 

 than diffused light (excepting polar regions). 



Light is not required for germination of seeds. Without light, 

 however, there is no assimilation, and hence no possibility of tree 

 life. Assimilation increases with increasing intensity of insolation; 

 excessive insolation is, however, destructive. For each species, and 

 for each stage of its growth, there exists a certain optimum, mini- 

 mum and maximum of insolation with reference to the possibilities 

 of its success. The damaging influence of excessive insolation is 

 prevented by the inner organization of the plant. 



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

 the 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 a greater sensitiveness of chloro- 

 phyl and thanks to a more favorable inclination, form and position 

 of the leaves, some s-pecies exceed others in assimilation and vitality 

 under the same influx of light. Shade bearing are such leaves as 

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

 the fact that but 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 interstices of light 

 13 



