MISTLETOE INJURY TO CONIFERS. 29 
girdled area and initiates an abnormal swelling (fig. 26). The swell- 
ing continues to increase in size and weight, likewise all members of 
the crown, so that eventually the slender stem below can no longer 
support the overdeveloped crown and is broken down by the wind. 
A specimen in the laboratory shows the number of rings of the stem 
at the girdled area at the time it was cut to be eight, with a diameter 
of 1 inch. The swelling just above and within the same internode 
showed 15 rings, with a diameter of 3 inches. The same phenomenon 
is sometimes produced in yel- 
low pine by Peridermium fila- 
mentosum Pk. When it is re- 
called that the cambium and 
the outer wood of the girdled 
area are actually dead, the 
length of time the crown con- 
tinues alive is really remark- 
able. 
In point of general insect at- 
tack it has been noted that the 
beginning of an_ infestation 
may start with trees badly 
suppressed by mistletoe. The 
fact that trees heavily sup- 
pressed by mistletoe have a 
weak flow of sap causes them 
to be first selected by certain’ 
forest-tree insects. For this 
reason mistletoe areas form 
centers from which infesta- 
tions may spread. Again, nu- 
merous infestations may start 
simultaneously over a wide 
territory, owing to the weak- rie. 26—a young yellow pine, showing com- 
ening of the trees by these par- plete girdling of the stem by a combined at- 
. : tack of mistletoe and insects. The cambium 
asites instead of from a few is destroyed, but the crown remains alive and 
detached areas. as is often the continues to elaborate food materials, which 
‘ : are stored just above the girdled area, 
ease. This has been found par- 
ticularly true in the case of yellow pine and the red turpentine beetle 
mentioned above. In all regions of heavy mistletoe infection of the 
Douglas fir, Dendroctonus pseudotsuga Hopk. is usually very abun- 
dant. This was the rule in the Whitman National Forest, Oreg., and 
though the numerous dead trees of this species in the forest were 
undoubtedly the result of an immediate attack by the beetles, their 
work was hastened, it seemed, by the serious mistletoe suppression 
which was exhibited by most of the dead trees. During the season 
