FACTORS INFLUENCING REPRODUCTION 851 



with sulphuric acid followed by constant watering during the period 

 of germination.^'' 



OBSERVATIONS 



Root Systems 



Field observations on the root systems of spruce, fir, and white pine 

 showed that detailed studies of roots would probably yield interesting 

 results. Spruce roots form a dense mat in the raw humus, or "duff" — 

 a mat so dense that hardly a square centimeter under a spruce stand 

 escapes. The rootlets keep growing toward the surface as the humus 

 deepens, those in the lower layers dying back. Hence spruce is in a 

 position to get the first water that reaches the forest floor. The quan- 

 tity absorbed by these roots must be enormous and cannot fail to be an 

 important factor in reproduction. 



Fir roots are characteristically much less branched than those of 

 spruce and seem to go more into the mineral soil, though they also feed 

 largely in the "duff." This greater penetration may possibly explain in 

 part the ability of fir to grow on drier sites than spruce. White pine 

 absorbs from both the raw humus and the mineral soil. 



The roots of all three species are often affected by a fungus which 

 produces black threads of mycelium on the root tips. These threads 

 prevent absorption and kill the portion of the root attacked; yet seed- 

 lings appear thrifty even when a large proportion of their roots are 

 affected in this way. Perhaps, since the fungus attacks only the smaller 

 rootlets, the plant is able to develop new rootlets about as fast as the 

 affected ones die off. 



Interception of Precipitation by Spruce Crowns 



A factor of more importance than hitherto recognized is dryness due 

 to the interception of precipitation by the crowns of spruce. The lack 

 of vegetation under a forest of spruce has generally been attributed to 

 lack of light. While light plays an important part, there are probably 

 many cases where lack of moisture rather than lack of light is the 

 determining factor. A rather striking illustration may be cited. Under 

 the crown of a spruce growing in the open was found a patch of forest 

 floor similar in every respect to the forest floor found under dense 

 stands of spruce. Herbaceous vegetation and tree reproduction stopped 

 abruptly at the edge of this spot, yet the crown of this tree was high 

 enough to allow the ground under it to receive ample light. The only 

 vegetation under the crown was a few grasses and asters, light-demand- 



" Hartley, Carl: "The control of damping-off of coniferous seedlings." U. S. 

 Bept. Agriculture Bull. 453, 1917. 



