PRIMER OF PRIMERS. 189 



ing power of the sun's rays concentrated by len- 

 ticular, resinous, or gummy nodules. 



"When its velocity is great, the wind sometimes 

 sweeps away the trees from extended areas. This 

 action of the wind is limited mainly to the edges 

 of the forest or to openings made in them by any 

 cause. 



By overflowing their banks, rivers sometimes 

 undermine and carry away thousands of acres of 

 forest trees. The trees accumulate in the bed 

 of rivers and form masses called rafts. 



An avalanche sweeping down the slope of a 

 mountain often completely removes the forest. A 

 plot of forest land, properly placed, will often 

 check the movements of avalanches. 



The animate enemies of the forest often produce 

 their greatest destruction by the aid of the inani- 

 mate enemies. Thus, man destroys forests by fire ; 

 the beaver, by floods. 



Various parasitic plants may grow on, and 

 thus cause the death of, even the most vigorous 

 trees. 



Some forms of fungus-growth cause considera- 

 ble damage to the trees on which they grow. 



The animal enemies of the forest vary in size 

 from minute insects to animals of large size. 



