MISTLETOE INJURY TO CONIFERS. 



the nodes and the basal scales of the terminal buds, are the chief 

 factors in the penetration of the primary root. 



The occurrence of mistletoe plants on the thick-barked branches of 

 old trees or on the main trunk are the result of earlier infection, 

 when the bark was thinner. What appears to be a recent infection 

 on the older parts of trees is often merely a retarded or suppressed 

 condition of an earlier in- 

 fection which has ex- 

 pended most of its energy 

 in the production of a sub- 

 cortical stroma and later 

 breaks through the bark. 

 Periods of suppression and 

 dominance are frequently 

 noticeable in all mistle- 

 toes,^ condition noted to 

 be in several instances di- 

 rectly referable to the 

 state of vigor of the host. 

 An excessive flow of resin 

 sometimes appears in the 

 second and third year of 

 the life of a new infection 

 on larch and yellow pine, 

 which, if not fatal to the 

 young plants, may seri- 

 ously retard their growth 

 for years. Until infection 

 by actual inoculation, 

 using natural methods, is 

 attained, all statements of 

 the ability of the parasite 

 to effect an entrance in 

 old-barked branches or 

 trunks can not be accepted 

 and must be considered 

 faulty observation. The 

 writer has never succeeded in causing the infection of branches at any 

 point older than four years. The ease of infection is found to be 

 more or less in proportion to the decrease in age of the branches 

 tested. This was proved in the case of yellow pine by inserting 

 seeds at regular intervals in the axils of the leaf sheaths of young 

 branches, from the terminal bud to the tenth internode. The results 

 of this experiment are shown in Table III. 



FIG. 5. Four-year-old yellow-pine seedlings killed by 

 mistletoe. Note the hypertrophy of the stem at 

 the point of infection and the shortening of the 

 needles. The two seedlings on the right were 

 killed principally by having the wood and cambium 

 in the swelling infiltrated with pitch. The para- 

 site killed the seedling on the left by invading the 

 terminal shoot. 



