EFFECT UPON THE HOST. 



19 



killed by freezing, or otherwise being mechanically destroyed. It is 

 maintained that in extreme cold weather mistletoe has been largely 

 killed out over large areas; for example, in the unprecedented cold 

 wave of February, 1900. Its own bulkiness and brittleness would 

 seem to operate toward the self-destruction of the mistletoe bush. 

 In the case of infection upon the mesquite, spoken of elsewhere, the 

 parasite so injures the branch upon which it sits as to kill the branch 

 and thereby destroy itself. Usually, however, accident to the aerial 

 part of the parasite merely results in stimulating the development 

 of adventitious buds — and thereby in multiplying the number of 

 mistletoe shoots upon a gradually widening area of infection. 



EFFECT UPON THE HOST. 



Perhaps in the majority of 

 cases the original point of 

 infection is upon a small, re- 

 mote branch. The habit of 

 the mistletoe is of course to 

 draw sustenance from the 

 branch and in increasing 

 quantity as the parasite in- 

 creases in size. The imme- 

 diate result is to starve that 

 portion of the branch lying 

 beyond the point of infec- 

 tion, and while this part may 

 persist for some years alive 

 without noticeable growth, in 

 the end it dies, and the mis- 

 tletoe thus comes to occupy 

 the end of the branch. (See 

 fig. 5.) This habit is particu- 

 larly well shown in the water 

 oak. where very large clusters 2 to 3 feet in diameter swing 

 from the end of a long slender branch not more than an inch 

 in diameter next the swollen point of junction of the host and 

 parasite (PL II, fig. 1). At the point of attachment also the 

 branch is stimulated to excessive growth, which gives rise to deformi- 

 ties of varying shapes on different host species. In the water oak 

 just mentioned both branch and mistletoe are enlarged like a 

 clumsy piece of welding. Frequently, as in the Osage orange, the 

 branch is stimulated to an excessive formation of shoots, forming a 

 sort of witches' broom. (See fig. 6 and PL II, fig. 2.) Large 

 branches, and not infrequently the main trunks of trees, may be 

 greatly deformed by the mistletoe. This happens where infection 

 occurred when the tree was young and has persisted to misshape all 



166 



Fig. 7— The trunk of a mesquite tree at Waco, Tex. 

 deformed by long-standing mistletoe infection. 



