26 BULLETIN 1112, U. S. DEPARTMENT OF AGEICULTURE. 



although the mistletoe may stunulate growth in that portion of the 

 host adjacent to the parasite, causing the abnormal development of 

 certain limbs and branches into witches' brooms, the excess of carbo- 

 hydrates produced by the additional foliage of the mistletoe-stimu- 

 lated branches is consumed by the parasite, thereby reducing the 

 amount available for the seeds. The effect upon the host in all cases 

 is insidious, slowly depriving the tree of its vitality instead of causing 

 injuries in the nature of a shock, as in the case of burns and other 

 mechanical injuries. In this and the following study, no informa- 

 tion was available as to the influence upon seed production of the 

 condition of other trees which may have supplied the pollen for fer- 

 tilizing the seed tree. Because of the lack of pollination experiments, 

 the conclusions are necessarily based wholly upon the condition of 

 the tree from which the seed was collected. 



In order to check this theory further, additional seed collections 

 were made by the Fort VaUey Forest Experiment Station in 1913 and 

 1915 from mistletoe-infected and healthy western yellow pines. 

 Aside from the difference in degree of mistletoe infection, the trees 

 were otherwise normal and very similar in age and crown develop- 

 ment. The seed extractions and germination tests were conducted 

 separately for each tree. Five hundred seeds from each lot were 

 germinated in sand in the greenhouse at a temperature ranging from 

 40° to 90° F., with a mean daily temperature of approximately 65° F. 

 Daily counts were made during the progress of the germination tests, 

 which continued for approximately 50 days. 



The pertinent data on the seed'production of 64 felled western yellow 

 pine trees for the 1913 and 1915 collections are classified in Table 18, 

 according to the degree of mistletoe infection. The best criterion by 

 which to judge the relative seed production is believed to be the repro- 

 ductive value of each tree or the total number of viable seeds produced 

 per tree, which represents a convenient summation of the fundamental 

 variables of seed production. This is obtained by multiplying the 

 yield per tree in pounds of clean seed by the number of clean seeds 

 per pound, and in turn multiplying the product by the final germina- 

 tion per cent. It will be noted that, with the exception of the lightly 

 infected class upon which the mistletoe appears to have a slight stim- 

 ulating effect, there is a decrease in the percentage of germination, 

 the yield of cones, and clean seed per tree and consequently a marked 

 decrease in the reproductive value per tree with an increase in the 

 amount of mistletoe infection. The reproductive value of the trees 

 in the intermediate infection class is about 40 per cent of that of the 

 healthy class, while the reproductive value of the trees in the heavily 

 infected class falls to only about 25 per cent of that of the healthy 

 class. This is not surprising when the effects of the parasitism are 

 fully understood. The data for the mistletoe-infected classes of 



