32 THE MISTLETOE PEST IX THE SOUTHWEST. 



conditions than in the closer stands and denser foliage of hninid 

 climate forests. 



(3) The harml'nlness of mistletoe is due in part to its mechanical 

 injury to trees (deformity of branches and trunk, wounds followed 

 by decay), but more especially to its drain upon the trees' vitality 

 by withdrawing water and nutriment substances from them. The 

 sinkers which connect the parasite with the water-transporting vessels 

 of the wood and the cortical roots which ramify in the soft bark are 

 the means by which the parasite withdraws substances from its host. 



(4) The first infection of a tree by mistletoe takes place only 

 through the agency of a germinating seed placed upon the body or 

 branch of the tree by birds (mostly mocking birds, wax wings or 

 cedar birds, and robins), except in the rare case where berries fall 

 upon a branch from a bunch of mistletoe in an overtopping adjacent 

 tree. The subsequent spread of infection upon a tree may take place 

 by the falling or washing of berries upon other parts of the tree from 

 the previously established mistletoe shrubs, or by the spread of 

 cortical roots from which new mistletoe shoots arise. Spreading by 

 cortical roots occurs more readily upon some species of trees than 

 upon others, and is especially stimulated by the injury or removal 

 of the original shoot. 



(5) The mistletoe seed and seedling exhibit unusual powers of 

 resistance to drying out, and are thereby enabled to survive in con- 

 siderable numbers the critical period from the time the berry is 

 placed upon a branch until the parasitic plantlet becomes established. 

 This period may extend beyond the first growing season. 



(6) A tree may become infected at any point where living tissue 

 is exposed or covered only by a thin layer of cork with breathing 

 pores, but the most vulnerable points are the young branches and, 

 sometimes, buds. The sinker of the mistletoe seedling is able to 

 penetrate certain tissues by dissolving the walls of cells lying in its 

 path. It is uncertain whether cutinized or corky cell walls can be so 

 dissolved, but the writer believes that they can. 



(7) The trees most liable to infection are those which occur singly 

 or in clumps or rows along streets and highways, in vacant lots and 

 parks, along the border of fields, and narrow strips of timber along 

 streams. The damage to trees in forest stands is negligible. Shade 

 and ornamental trees suffer most. 



(8) While it is not certain that any broad-leaved tree is wholly 

 immune to attack from the American mistletoe, some are practically 

 so, although freedom from infection seems to vary with locality. In 

 the choice of trees for planting the question of the ability of a tree to 

 resist infection might profitably be considered. It is believed that 

 any tree subject to infection may be infected by seed from mistletoe 

 growing upon any other species; e. g., the hackberry may be infected 



1GG 



