20 THE MISTLETOE PEST IN THE SOUTHWEST. 



its subsequent growth. (See fig. 7.) On the other hand, very many 

 trees are infected for years without showing any noteworthy deform- 

 ity, and, as a matter of fact, this item is not one of serious economic 

 importance. 



Mistletoe not only causes mechanical injury, but it saps the vital- 

 ity of the branch and when sufficiently abundant often the whole 

 tree; and in the case of the hackberry, especially, often results in the 

 death of the entire tree. About Austin considerable numbers of 

 hackberry trees have been and are being destroyed in this way. At 

 Bryan, Tex., this is true of water oaks. It has been supposed that 

 perhaps the mistletoe merely supplants the end of the branch which 

 was starved by its presence, and that while it receives what the 

 branch would otherwise receive, it in turn contributes to the tree by 

 its assimilating activity what the displaced branch would have done. 

 This seems never to be the case at any time, and it is certain that 

 when the majority of branches become so supplanted by the mistle- 

 toe, the tree dies, showing the parasite to be always a drain upon its 

 host. One of the curious results of infection has been often observed, 

 especially in the Uvalde region, where bunches of mistletoe on remote 

 branches of the mesquite, becoming large, so injure the branch that 

 it decays beneath the mistletoe, which thus itself is starved and 

 killed and presently falls away, leaving the branch free from infec- 

 tion. This is probably due to decay induced by the starving of the 

 branch beyond the original point of infection. 



THE POINT OF ATTACK. 



In the process of establishing itself upon the host, the critical point 

 for the mistletoe seedling is to penetrate the covering of the branch 

 so as to reach the food-supplying cells of the cortex and wood. The 

 sinker can exert pressure and is doubtless able to force an entrance 

 through fissures or through natural openings, such as lenticels, and 

 between bud scales, as shown by Cannon. a 



The growing tip of the sinker has been shown to secrete an enzyme 

 capable of dissolving the walls of certain cells lying in its path. 

 Whether heavily cutinized walls or the walls of dead cork cells can 

 be dissolved by this secretion has not been fully determined. Cork 

 and bark certainly offer much more resistance to the forward growth 

 of the sinker than cellulose walls do, and it is quite unlikely that a 

 heavy layer of these could ever be penetrated by a mistletoe sinker. 

 When infection begins on the old parts of a tree it must be at some 

 fissure or thin place in the bark. Naturally the younger branches, 

 and especially twigs of last season's growth, present the most vulner- 

 able points of attack, and as therefore would be expected, infection 



a Cannon, W. A. Observations on the Germination of Phoradendron villosum and 

 P. calif ornicum. Bulletin, Torrey Botanical Club, vol. 31, pp. 435-443. 1904. 

 16G 



