TREES MOST SUSCEPTIBLE TO INFECTION. 21 



begins in the majority of cases on the remote young branches, where 

 the protective covering is thinner and the lenticels (breathing pores) 

 still numerous. Here also the buds offer a point of attack. Cannon 

 states that in the case of a mesquite branch he has seen as many as 

 a half dozen successive buds bearing mistletoe. The hypocotyl of 

 the seedling pushes its way between bud scales to the more easily 

 penetrated tissue at their base. Cannon also holds that even on the 

 youngest branches the penetration of a sinker is conditioned upon its 

 finding a lenticel or inserting itself between bud scales; but in view 

 of the abundance and varying aspects of infection it seems necessary 

 to ascribe a more active role to the sinker than that of mechanically 

 pushing its way through openings and crevices, and that a less hap- 

 hazard mode of infection prevails. However, no actual proof is at 

 hand that the sinker can secrete an enzyme that will dissolve cutin- 

 ized or cork cell walls. 



The location of the point of infection upon a tree appears also to 

 have some relation to the perching habits of the birds which eat the 

 berries. This idea is held notably in the case of mocking birds. 

 Large areas of mesquite-covered country to the westward of San 

 Antonio show a predominance of cases where a single plant of mis- 

 tletoe occupies the most conspicuous spot upon the tree, namely," 

 near the apex of the farthest spreading top branch. This appears 

 to coincide with the choice of perching places by the bird in its flight 

 from tree to tree. The habits of cedar birds in their repeated short 

 flights and perching during their spring visits in search of berry mast 

 are also such as to afford the largest likelihood of mistletoe seed being 

 deposited on the youngest branches. 



TREES MOST SUSCEPTIBLE TO MISTLETOE INFECTION. 



It is a question whether any tree is wholly immune to attacks 

 from mistletoe. Certainly there is reason to believe that mistletoe 

 could be induced to grow upon any living woody plant. But from 

 the actual status of infection in any community where mistletoe 

 grows there are some trees which are practically immune. One of 

 the curious things about the matter is the prevalence of infection 

 upon different species in different localities. Thus, in the vicinity of 

 Austin, the hackberry and the cedar elm are the trees most frequently 

 and heavily infected. The sycamore, though common in central 

 Texas, both native and cultivated, has not been reported to have 

 mistletoe in any case; whereas in the river bottoms of Arkansas and 

 southeastern Oklahoma it is one of the most commonly infected trees. 

 Broad-leaved elms are practically immune in the Austin region, but 

 at Muskogee, Okla., and northward such elms are as thoroughly 

 beset with mistletoe as the cedar elm is at Austin. At San Antonio 



166 



