24 THE MISTLETOE PEST IN THE SOUTHWEST. 



them, and that the immunity which some trees seem to have is due 

 to the mechanical hindrance exercised by the bark or cork, or the 

 simple epidermis, to penetration by the primary sinker, or possibly 

 to the density of shade, as in the case of the China tree ( Melia 

 azedarach) . b This relative immunity of species becomes, however, 

 a matter of practical importance as indicating one of the qualities 

 (resistance to mistletoe) that will influence the choice of trees for 

 planting for shade or wind-break or ornament. 



Further, it should be noted that the mistletoe varies considerably 

 in habit on different hosts, and especially where the hosts occupy 

 notably different situations as to soil and climate, being in this 

 respect plastic, like the majority of plants capable of existing in 

 widely different soil and climatic conditions. Probably the qualities 

 acquired in any given situation are not such as to forbid a gradual 

 or even ready tolerance for other conditions. 



LOCATION OF INFECTED TBEES. 



Speaking generally, mistletoe-infected trees are^those which occupy 

 moist soils, namely, in river bottoms and along creeks and ravines 

 leading to the uplands. In the South generally, certain swamp- 

 inhabiting trees (gums and water oaks) are notably mistletoe hosts. 

 There is no doubt a reason for this in the more or less constant 

 demand which the mistletoe makes on its host for water. In the 

 central Texas mistletoe belt this preference of mistletoe for trees of 

 moist situations is obvious, but it is by no means limited to trees of 

 such locations. Quite on the contrary, the most noteworthy and 

 destructive instances of infection are on isolated trees or clumps of 

 trees in dooryards or parks and along streets and highways. There 

 is a notable preference for isolated trees. Even in timbered bottom 

 lands the heavily infected trees are those which stand out conspicu- 

 ously in or about a field or clearing. (PL I, figs. 1 and 2 ; PL II, fig. 1 .) 

 In close stands of timber the average tree seems much less likely to 

 be infected than the same species in the open. Those which are 

 infected in heavy stands are the trees which overtop their neighbors. 

 Perhaps the need of mistletoe for abundant sunlight explains in part 

 its absence in close forests and the predominance of it in exposed 

 trees. This fact is of the greatest consequence in the present con- 

 sideration, for while the loss to forest trees in a close stand is negligible, 

 the concentrated attack of mistletoe on isolated trees is a blow on a 

 vital spot, for the maintenance of adequate shade trees is a difficult 

 matter, quite apart from the role played by mistletoe. 



b One correspondent states that mistletoe has been observed to grow upon China 

 trees in southern Texas. 

 166 



