16 



THE MISTLETOE PEST IN THE SOUTHWEST. 



mistletoe bush its strongly buttressed attachment to the branch (fig. 

 4). The presence of these persistent cortical roots interrupts the 

 formation of host tissue exterior to them, and thus they come to lie 

 more or less exposed along the surface of the branch. At these 

 exposed places (possibly also from the younger cortical roots still 

 within the cortex zone) adventitious buds are developed which may 

 give rise to new mistletoe shoots. (See figs. 5 and 6.) They do this 

 in considerable numbers if from any cause the original mistletoe plant 

 is injured, or especially if it be broken off, as is most frequently done 



when the effort is made 

 to rid a tree of the para- 

 site. In this way the 

 cortical roots are stim- 

 ulated to further 

 growth and to giving 

 rise to repeated crops 

 of mistletoe shoots. 

 Scraping off the old 

 plants stimulates the 

 development of more 

 adventitious buds and 

 thus tends to enlarge 

 the area of infection. 



It has been observed 

 that in some trees, or 

 at least in some situa- 

 tions on the branch, 

 there is no considerable 

 spread of cortical roots 

 and no development of 

 shoots from adventi- 

 tious buds. Thus, in 

 the case of the water 

 oak one may find huge 

 bushes of mistletoe a 

 yard in diameter on branches eight or ten years old with not a single 

 indication of the spread of the infection along the branch. This may 

 be observed also in the mesquite and doubtless in any other tree where 

 the primary mistletoe plant started on a very young branch and has 

 developed unharmed. (See fig. 5.) In the course of time, however, 

 the original plant is destroyed by some agency or other, but the cortical 

 roots persist and thenceforth develop secondary shoots from adven- 

 titious buds. In this way may be explained the characteristic occur- 

 rence of mistletoe on old branches and even on trunks, as shown not 



166 



Fig. 4.— Sectional view, partly diagrammatic, of a branch infected 

 with mistletoe, showing the relation between parasite and host: 

 a, Branch of host tree; 6, mistletoe; c, primary sinker; d, sinker 

 from cortical root e; f, cortex or soft bark; g, cambium or growth 

 ring; ft, wood of branch. The starving and dwarfing of the branch 

 beyond the mistletoe is shown at i. 





