6 BULLETIN 360, U. S. DEPARTMENT OF AGRICULTURE. 
No evidence is at hand to show that the primary sinker of these 
parasites can penetrate other than the more tender epidermis of 
young parts of the host. Germinating mistletoe seeds located on 
the smooth bark of the Douglas fir or on the irregularities of older 
stems of yellow pine or larch have never been observed, even after 
a protracted contact of the disk of the hypocotyl with the surface 
of the branch, to penetrate the bark. Removing the exhausted 
hypocotyl and carefully examining the point where the disk was 
attached, a barely perceptible pit or indentation is sometimes visible, 
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Fic. 4.—Graphs showing the average annual growth (in inches) for 40 years (1874 
to 1915, inclusive) of 40 trees of Douglas fir heavily infected with mistletoe, com- 
pared with 40 uninfected trees of the same species for the same period. A, Heavily 
infected trees: Average-age class, 97 years; average height, 62 feet; average di- 
ameter, breast high, 17.3 inches. B, Uninfected trees: Average-age class, 97 years; 
average height, 73 feet; average diameter, breast high, 22.2 inches. 
possibly indicating the presence of a solvent, which, however, is 
ineffective upon more mature bark. There is as yet no proof to sup- 
port the theory of the presence of a digestive substance which 
enables the sinker to penetrate the bark more readily. If this were 
true, infection could possibly occur on older tissues, provided they 
were not too thick and the food supply in the seed did not become 
exhausted. As it is, mechanical force, supported by the nonmoy- 
able position of the seed, and irregularities of the stems, such as 
leaf scales, exits of leaf traces, and leaf sheaths, particularly at 
