18 



BULLETIN 1112, U. S. DEPARTMENT OF AGRICULTURE. 



very probable that the mcreased 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 parenchyraa 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 jachs, for a 5-year period, grouped according to deg-^ee of mistletoe in- 

 fection. 





Degree of infection. 



Average diameter.i 



Accretion, 



1910-1915. 







1910 



1915 



Periodic. 



Periodic 

 annual. 



Basis. 







Inches. 

 3.84 

 3.90 

 4.35 

 4.08 



Inches. 

 3.98 

 4.06 

 4.62 

 4.46 



Inch. 



0.14 



.16 



.27 



.38 



Inch. 



0.028 



.032 



.054 



.076 



Limbs. 



20 



X 



6 



XX 



7 



XXX 



21 







1 Average of two sets of measurements taken at and 18 inclies from the bole of the tree. 



Table 13. — Comparison 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. 



Degree of 



Average 

 diameter.i 



Accre- 

 tion, 

 1890- 

 1895. 



Aver- 

 age 

 diam- 

 eter.i 



Accre- 

 tion, 

 1895- 

 1900. 



Aver- 

 age 

 diam- 

 eter.i 



Accre- 

 tion, 

 1900- 

 1905. 



Aver- 

 age 

 diam- 

 eter.i 



Accre- 

 tion, 

 1905- 

 1910. 



Aver- 

 age 

 diam- 

 eter.! 



Accre- 

 tion, 

 1910- 

 1915. 



Basis- 



r 



Infection. 



1890 



1895 



Peri- 

 odic 

 an- 

 nual. 



1900 



Peri- 

 odic 

 an- 

 nual. 



1905 



Peri- 

 odic 

 an- 

 nual. 



1910 



Peri- 

 odic 

 an- 

 nual. 



1915 



Peri- 

 odic 

 an- 

 nual. 







In. 

 1.68 

 3.06 



In. 

 1.84 

 3.32 



In. 



0.032 



.052 



In. 

 2.01 



3.08 



In. 



0.034 



.072 



In. 



2.21 



4.04 



In. 



0.040 



.072 



In. 



2.34 



4.34 



In. 



0.026 



.060 



In. 



2.46 

 4.56 



In. 



0.024 



.044 



Limbs . 

 4 



XXX 



4 







I 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- 



