THE WESTERN YELLOW PUsTE 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 (PI. 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. 



