LARCH MISTLETOE. 9 



The infected wood of the branch, either at its base or other por- 

 tions, where not too greatly enlarged by the stimulating effects of 

 the parasite, requires a much smaller force to break it at the point of 

 infection than is the case in normal branches of like age and thick- 

 ness. The penetration and embedding of the vertical root system of 

 the parasite in the wood of the host add nothing to the strength 

 of the infected tissue, but diminish its normal strength when the 

 force, as in the case of the wind, is applied at right angles to the 

 grain of the infected branch. Since numerous infections occur at the 

 bases of branches, the point of greatest stress, much injury to the tree 

 results. The meristematic tissue in the cambium layer at the point 



Fig. 5.- — Cross section of the trunk of a larch tree, showing a typical basal branch burl. 

 Note that the dead wood is attacked by borers which do not encroach upon the living 

 sapwood. 



where the branch breaks usually produces secondary branches (see 

 fig. 2). These in turn may become infected and are lopped off, so 

 that eventually great burls are produced at this point on the trunk 

 (fig. 5), seriously reducing the merchantable material. The dead 

 wood thus exposed is a place of entrance for insects and fungi. 

 Since it requires years for the secondary branches to attain a size 

 and assimilatory surface capable of supporting the present bulk of 

 the trunk, the vigor of the tree is gradualty reduced. The younger 

 portion of the crown above, being continually encroached upon by 

 the parasite, is not able to supply the deficiency in food materials, and 

 the tree, merely existing for a time, finally becomes a prey to various 

 8521°— Bull. 317—16 2 



