THE WESTERN YELLOW PINE MISTLETOE. 19 
come by the development of a special tissue on the upper side of the 
limb. 
The increased size of the limbs infected with mistletoe renders 
limbing and brush disposal in logging operations more difficult and 
consequently more expensive. The excessive secretion of resin which 
occurs in mistletoe-infected limbs has somewhat the same effect on 
brush disposal. 
HYPERTROPHY AND RESIN FLOW. 
Although mistletoe infection causes swellings on both branches and 
trunks, the stem swellings frequently become less noticeable as the 
tree grows. In many cases there are no pronounced swellings on the 
larger boles even when very old mistletoe infections are present, about 
the only means of detecting the infection being the presence of the 
mistletoe or the unusual roughness of the bark. (Pl. IV, Fig. 1.) 
When, however, infection occurs during the early life of the tree, 
it frequently results in the formation of a burl which may attain 
_ very conspicuous proportions. The fusiform swellings on both the 
stems and limbs resulting from mistletoe infections are centers where 
abnormal amounts of food materials are stored, which condition is 
evidently stimulated by the decided pathogenic tendencies of the 
mistletoe. The cortex is frequently gnawed from the lesions by 
rodents, especially porcupines and squirrels, attracted by the soft 
spongy nature and the greater thickness of the inner bark of mistletoe- 
infected branches and stems. This fact establishes a casual relation- 
ship between mistletoe infection and rodent injury. 
As previously stated, there is more or less hypertrophy at the point 
of mistletoe infection on both limbs and stems. This hypertrophy 
is frequently accompanied by a copious flow of resin, which appears 
as small drops, usually on the lower side of limbs, which are heavily 
infected with mistletoe. A cross section through such a limb shows 
large areas of the sapwood thoroughly infiltrated with resin. The 
process of infiltration continues on many of the limbs until the fibro- 
vascular system is thoroughly clogged with resin; the limb is girdled 
as far as receiving any food supply is concerned, and is finally killed. 
This process is usually very gradual, but none the less sure. 
The resin flow from such mistletoe-infected limbs is not caused by 
insect punctures. A careful examination of the resin-infiltrated areas 
fails to show any signs of such injury. On many of the trees which 
are heavily infected with mistletoe, resin cankers (Pl. IV, Fig. 2) of 
varying sizes are found on the bole. These cankers are often directly 
associated with local mistletoe infection. The resin flows on the bole 
are usually indicative of the decline of the tree; in fact, trees with 
- marked resin-flow cankers usually die in a relatively short time. 
