156 BOTANICAL GAZETTE [AUGUST 
affected by a fungus which produces black threads of mycelium on 
the root tips. These threads prevent absorption and kill the por- 
tion of the root attacked. Yet seedlings 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. 
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 illustra- 
tion 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 végetation under 
_ the crown was a few grasses and asters, light demanding but com- 
_ paratively drought resistant plants. The bareness of this piece of 
_ forest floor was due to lack of moisture, not to lack of light. This 
was confirmed by moisture tests, which showed that the soil 
: _ beyond thé crown, soil which had been giving up moisture to a _ 
thick herbaceous cover all summer and should consequently be 
| rer than a spt which had given up nothing to vegetation and oe 
