16 



DISEASES OF DECIDUOUS FOREST TREES. 



There are also a considerable number of smaller mistletoes belong- 

 ing to the genus Arceuthobium which are widely distributed through- 

 out the country. Of these there are two which may be especially 

 mentioned: Arceuthobium cryptopoda Engelmann and A. pusillum 

 Peck. The former is known to occur in various sections of the 

 Rocky Mountains and is injurious to a number of different conifer- 

 ous hosts; the latter seems to be an eastern form, limited more or 

 less closely to the Appalachian Mountains, and is definitely known 



to occur from the 

 Canadian border to 

 southeastern Penn- 

 sylvania. The differ- 

 ent species of Arceu- 

 thobium resemble 

 each other very much 

 in their habits of 

 growth, their manner 

 of seed dispersal, and 

 their effect upon 

 their hosts, so that 

 we may take the 

 best -known form, 

 Arceuthobium pusil- 

 lum, as a type of this 

 group (77). 



The seeds are 

 coated with a muci- 

 laginous substance. 

 Upon ripening they 

 are projected for sev- 

 eral feet from the 

 seed capsules, and 

 alighting upon an 

 adjacent branch or 

 twig they stick 

 tightly to the bark 

 and there germinate. 

 They may also be sometimes carried by birds. 



The young plant pierces with its holdfast the bark of the twig upon 

 which it is seated and establishes communication with the living por- 

 tions of the twig. It is thus enabled to feed upon the sap of its host, 

 and in this way a considerable amount of food material is diverted 

 from the outer end of the affected branch to the parasite. About the 

 point of infection a number of branchlets develop, and in the course 

 of a few years there is formed a compact, bushy mass of twigs which 



149 



Fig. 2. 



A black spruce tree with a large witches' broom caused by 

 dwarf mistletoe. 



