28 BULLETIN 360, U. S. DEPARTMENT OF AGRICULTURE. 



It is a well-known fact that wounds heal quickly in young or in 

 strongly growing trees, principally due to the protection afforded by 

 an abundant flow of resin. It may be assumed that trees having their 

 life functions brought to a low ebb by excessive mistletoe infections, 

 with resulting decrease in annual increment, will not be able to heal 

 or protect their wounds as quickly as normal trees ; hence, are more 

 liable to infection. This may be one of the reasons why so many 

 open burls are formed on infected larches. These open burls are 

 seldom, if ever, healed, although the parasite in its tissues has long 

 since died. There is a slight increase in the number of resin passages 

 in early burl formations, but this*s entirely offset by the early dying 

 out of the bark of the burl exposing the wood. It is an observed fact, 

 experimentally proved by the writer, that strongly suppressed yellow 

 pine, larch, and Douglas fir do not as readily form traumatic wood 

 or exude the normal quantity of resin on being wounded on any part 

 as do normal, healthy trees. Such a tardy reaction to injury does not 

 afford a ready antisepsis against the entrance of fungi which may 

 attack these trees. Since turpentine orcharding is becoming more 

 extensively practiced in the West it would be an interesting experi- 

 ment to determine the relative flow of pitch from trees strongly sup- 

 pressed by mistletoe and from those in a high state of health. 



RELATION OF MISTLETOE INJURY TO INSECTS. 



In the same manner that burls and other types of mistletoe injury 

 on some conifers are open doors to fungi, they are found to afford 

 a ready means of entrance for some species of forest-tree insects 

 which do not in this region habitually attack vigorous unwounded 

 trees. Old mistletoe burls on larches are almost invariably attacked 

 by borers (figs. 23 and 24), and burls on yeliow r pine are, in the ex- 

 perience of the writer, quite as frequently infested by bark and wood 

 boring beetles. In this connection a very curious and interesting phe- 

 nomenon often occurs on young yellow pines from 10 to 20 years 

 of age. An infection by mistletoe will have occurred, completely 

 enveloping the trunk some 2 or 3 feet from the ground. The parasite 

 having advanced somewhat each way from the point of original 

 infection, the intervening space is attacked by Dendroctonus valens 

 Lee. The combined influence of the beetle and mistletoe results in the 

 complete infiltration with resin of the space between the two edges 

 of the advancing mistletoe, so that the cambium dries out and dies. 

 Strange to state, this does not always kill the tree. The crown goes 

 on manufacturing food materials, being supplied with water through 

 the inner wood of the girdled area. The elaborated food not being 

 able to travel downward, since the cambial tissues of the entire cir- 

 cumference of the stem have been destroyed, is stored just above the 



