July is, 1915 W allrothiella Arceuthobii 377 



mistletoe, provided it is susceptible to attack, owing to the usually damp 

 condition of the compact moss-covered brooms. It remains to be seen 

 under just what conditions the fungus will propagate itself. To this end 

 it is being introduced into mistletoe regions of all types of exposure. 



The ease with which the fungus seems to infect its host leads the writer 

 to believe that it may be of some economic importance in the control of 

 certain species of mistletoe, at least for small areas. For a mistletoe 

 species to propagate itself, it must produce seeds abundantly, in order 

 to insure the infection of the young growing forest. The proportion of 

 mistletoe seeds actually causing infection to the total number produced 

 is very small indeed. Some fall to the ground; some fall on plants not 

 susceptible; most of them fall on parts of the host too old to be pene- 

 trated by the young root of the seed. With the exception of a few rare 

 instances, where infections have been known to occur on wound tissue 

 of mature parts of trees, the writer has not yet found either in nature or 

 by actual inoculation a seed taking effect on any part of its host other 

 than the more tender shoots or their equivalents in tenderness of bark 

 and then only when the primary sinker found its way to a leaf scar, leaf 

 scale, or other more vulnerable irregularities of the substratum. Again, 

 the seed must fall in such a position that the protruding root may directly 

 find its way under a leaf scale or be sheltered by the thick bunch of 

 needles at each node of growth or at the base of a leaf or leaf sheath; 

 otherwise it may fail of its purpose. The seed may germinate and expend 

 its stored materials in the production of a primary root of half an inch 

 or more, but before the growing point can penetrate the stem, provided 

 it is in such a position as to be drawn toward it, the young hypocotyl is 

 exhausted. Very few seeds cause an infection when not very favorably 

 located or directly through the smooth epidermis possessing a suberized 

 layer. 



With the exception of the small forms mentioned in this paper most of 

 the members of the genus are prolific seed producers. If so few seeds find 

 a vulnerable point on their hosts even with an abundant production of 

 seed, so much less will the chances of infection be if the seed production 

 is lessened. An estimate of the number of seed that should have been 

 produced by the lodgepole-pine mistletoe on a small broom was about 

 400. Not a single mistletoe seed on this broom had reached maturity. 

 All were attacked by the fungus. The biologic control of organic 

 agents destructive to plant life is in most cases a thing very much in 

 the realm of fancy. It seems, however, that a fungus of the nature 

 of W. arceuthobii may be introduced into mistletoe regions possessing 

 certain climatic conditions with the prospect of reducing the seed pro- 

 duction of these parasites, and thus reducing the damage caused by 

 the mistletoe. 



