THE WESTERN YELLOW PINE MISTLETOE. 17 
The data presented in the preceding pages show that, although 
minor fluctuations are exhibited, the rate of growth of the host 
decreases with the amount of mistletoe infection; or, in other words, 
is dependent on the degree of parasitism. Possible exceptions may 
be cases of very light infection which appear to act as stimulants for 
the host tree. These results are really to be expected when the 
effects of mistletoe parasitism are fully understood. 
TABLE 11.—The effect of mistletoe infection on the rate of growth of western yellow pine 
on two intensive sample plots aggregating 24 acres on the Tusayan National Forest. 
Basis 
EAE ee oa Periodic Total (number 
F . iameter annu. annu: of trees 
Degree of infection. growth, | increment, | increment, i aeageh 4 inches 
1914-1919, | 1914-1919. | 1914-1919.1 me ‘| and over 
d. b. h.) 
Inches. Cubic feet. | Per cent. Cubioet 
i ieee ee 0. 70 4. 56 2. 54 , 636.2 359 
oe a ee 3 ge Pe ee ee - 88 5. 65 2.79 553.9 98 
oe es SA ae ae ee - 49 | 2. 23 1.83 254. 5 114 
1 On cubic-foot basis. 
NoTE.—Since there were only 13 .rees classed as moderately infected, they were omitted because of 
the small basis. 
: EFFECT ON LIMBS. 
The first visible effect of the infection of a limb by mistletoe is the 
formation of a fusiform swelling (PI. II) about the center of infection 
similar to those occasionally found on stems. This hypertrophy is 
the first stage of a later abnormal growth of branches from the en- 
larged portion, which results in the formation of witches’ brooms. 
(Pl. If, Fig. 1.) When the stimulus of the parasite has once become 
manifest through the formation of a broom, the tendency toward 
abnormal branching continues, although the aerial parts of the 
mistletoe have may died. 
The comparative diameter growth or accretion of 54 limbs on 18 
standing black jacks is averaged by infection classes in Table 12. 
_ Measurements were taken at 6 inches from the bole of the tree and 
at 6-inch intervals beyond this point. The measurements taken at 
6 to 18 inches from the bole of the tree were averaged and are pre- 
sented in the table to facilitate a clearer and more comprehensive in- 
terpretation of the significant data. Limb measurements were also 
taken on the felled trees to check the comparative growth of the 
healthy and mistletoe-infected limbs of the standing trees. From 
these data, which are summarized in Table 13, it is apparent that the 
limbs had been infected at least 25 years prior to the time at which 
they were analyzed. ‘The data presented in both tables show that, 
for the hmbs examined, the rate of growth in diameter of limbs 
infected with mistletoe is greater than that of uninfected limbs. It is 
3600°—22—Bull. 1112——3 
