TOLERANCE AND INTOLERANCE. 



33 



undertake in the woods depends so largely upon it. 

 Thus tolerant trees will often grow vigorously under 

 the shade of light- crowned trees above them, while if 

 the positions were 

 reversed the latter 

 would speedily die. 

 (See PI. XVI.)' The 

 proportion of differ- 

 ent kinds of trees in 

 a forest often depends 

 on their tolerance. 

 Thus Hemlock some- 

 times replaces White 

 Pinein Pennsylvania, 

 because it can grow 

 beneath the Pine, and 

 so be ready to fill the 

 opening whenever a 

 Pine dies. But the 

 Pine can not grow 

 under the Hemlock, 

 and can only take 

 possession of the 

 ground when a fire 

 or a windfall makes 

 an opening where it 



FIG. 30. A Pitch Pine, producing seed abun- 

 dantly, as shown by the numerous cones, but 

 with no seedlings beneath it. Fire has run 

 over the ground, and the surface is very dry. 

 A strong breeze was blowing when the pic- 

 ture was taken. New Jersey. 



can have plenty of 

 light. Some trees, 

 after being over- 

 shaded, can never recover their vigor when at last they 

 are set free. Others do recover and grow vigorously 

 even after many years of starving under heavy shade. 

 The Red Spruce, in the Adirondacks, has a wonderful 



