18 BULLETIN 1112, U. S.. DEPARTMENT OF AGRICULTURE. 
very probable that the increased diameter growth continues to a 
certain point, beyond which there is a pronounced decrease in the 
rate of growth, until the limb eventually dies. This increased diam- 
eter growth is probably caused by the formation of an excessive 
amount of pathological parenchyma tissue, as a result of the irritation 
of the cambium by the parasite. The softer parenchyma adjacent 
to the center of infection renders the infected wood somewhat weaker 
than the more highly-lignified tissues of normal, uninfected wood. 
The presence of the mistletoe in the limb produces a stimulated 
growth of the host near the center of infection. 
TABLE 12.—Comparison of the average diameter growth or accretion of 54 limbs on 18 
standing black jacks, for a 5-year period, grouped according to degree of mistletoe in- 
fection. 
Average diameter.1 Accretion, 1910-1915. 
cial of infection. 
. meee Basis. 
As eriodic 
1910 1915 Periodic. Sal 
Inches Inches. Inch. Inch Limbs 
COPS Sl ye 5 en oe EY ee aE ee et ail ig 3. 84 3.98 0.14 0. 20 
2 taal who. od ap oy Be i sie lac ES aaa eB eee sia a ot Ty, 3.90 4. 06 .16 032 6 
2D, CAE ad ok OR UES Soe aga Ue Ag Ea eG eg SVE 4.35 4. 62 wah 054 7 
ERR Uns ree ey verk ak 2 Ops es ee vee Sa 4.08 4. 46 38 076 21 
1 Average of two sets of measurements taken at 6 and 18 inches from the bole of the tree. 
TABLE 13.—Com parison of the average diameter growth or accretion of 8 limbs on 4 felled 
western yellow pine trees, for a 25-year period, grouped according to degree of mistletoe 
infection. 
Average | tion, |; age | tion, | age 
tion, | age | tion, | age | tion, 
diameter.! | 1890- | diam-| 1895- | diam-| 1900- | di 
diam- | 1905— | diam-| 1910— 
1905. | eter.1 | 1910. | eter.1 | 1915. 
Accre-| Aver- | Accre-| Aver- | Accre-| Aver- Accre- Aver- | Accre- 
Degree of An TENE SS Ee ET EE EE ES: Ee 
infection. Basis 
Fee ear Peri- Park. Hau 
~ | odic odic » || odie odic ec 
18905) 1895 |) ons a}, 100 |e AOD, | ne | OLB E |S dO 
nual. nual nual nual nual 
In. | In: In. In. In. In. In In In. In. In. |\Limbs 
De eee e en 1.685] 2. 84.) 02032: | *2201 | 09034) 62521505040 253441 70:026 11 2246 150.0 4 
Kee FI OLA 3.06 | 322 +1142 052. | 93268 072 | 4.04 072 | 4.34] .060|] 4.56 044 4 
} | 
1 Average of two sets of measurements taken at 6 and 18 inches from the bole of the tree. 
Eccentricities, especially hyponastic growth, are considerably more 
pronounced in limbs heavily infected with mistletoe than in healthy 
limbs. The formation of a large witches’ broom produces an abnor- 
mally heavy load which must be borne by that portion of the limb 
nearest to the-bole of the tree. This abnormal load produces stresses 
sufficient to stimulate the formation of wood on the under side of the 
limbs through the deposition of food materials, which enables the 
wood to withstand the compression below. The tension above is over- 
