THE WESTERN YELLOW PUsTE MISTLETOE. 



27 



felled trees reported in Table 18 are meager because of the difficulty- 

 experienced in securing seed from mistletoe-infected trees, owing to 

 the sterility of the cones and their failure to develop. In fact, a 

 diligent search was made for heavily mistletoed trees which were 

 bearing seed, but very few were found. 



Table 18. — Data on the seed production of 64 felled western yelloiv pine trees classified 

 according to degree of mistletoe infection. 



Degree of infection. 



Final 



gennina- 



tion. 



Clean 



seeds 



per pound. 



Yield per tree. 



Reproduc- 

 tive value 

 per tree. 



Number of 

 viable seeds. 

 12, 942 

 13, 292 

 4,895 

 3,455 



Basis. 



Cones. 



Clean seed. 



1913 collection: 







Per cent. 

 82.8 

 84.1 

 81.7 

 77.0 



Number . 

 14, 473 

 14,385 

 13,314 

 14,475 



Bushels. 



0.8 



1.0 



.5 



.3 



Pounds. 

 1.08 

 1.10 

 .45 

 .31 



Number of 

 trees. 



43 





2 



XX 



2 



XXX 



2 







Mean of x, xx, and xxx . 



80.9 



14,058 



.6 



.62 



7,052 



6 



1915 collection: 







53.2 

 47.1 



12, 156 

 14, 170 



1.1 

 .9 



.83 

 .37 



5,626 

 2,202 



11 



Mean of X,- XX, and xxx. . 



4 



The extent and condition of the seed crop on the standing trees on 

 the permanent sample plots were under observation from 1912 to 

 1916 in order to secure additional data on the effect of mistletoe 

 infection on the amount and periodicity of seed production. The 

 1912 seed crop was rather light; only a few of the trees bore any 

 large quantity of cones. In 1913 there was an average crop, with 

 practically none in 1914, while the 1915 seed crop was somewhat 

 below the average. 



The records of the 1912 and 1915 seed crops for 90 of the living 

 western yellow pines are classified in Table 19 by degree of mistletoe 

 infection and amount of the seed crop. The seed crops were arbi- 

 trarily classified by the observer as heavy, good, medium, light, or 

 none. An analysis of the data shows that the amount of seed pro- 

 duction varies inversely with the degree of mistletoe infection. As 

 the degree of mistletoe infection increases the amount of seed pro- 

 duced decreases until very little, if any, seed is produced by heavily 

 infected trees. 



These results are further substantiated by observations of the 

 writers covering a period of approximately 10 years. Trees heavily 

 infected with mistletoe produce small amounts of seed at such times 

 as there are generally heavy seed crops, and at other times practically 

 none. The few cones that may occasionally be found on trees 

 heavily infected with mistletoe are very often aborted, and are 

 frequently infested with a cone insect {ConopJithorus ponderosae) .^ 



8 Material determined by Office of Forest Insect Investigations, Bureau of Entomology, U. S. Dept. of 

 Agriculture. 



