6 



BULLETIN 360, r. S. DEPAETMEKT 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 

 Toiing- 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 

 liypocotyl and carefully examining the point where the disk was 

 attached, a barely perceptible pit or indentation is sometimes visible, 



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Fig. 4.- — Graphs showing the average annual growth (in inches) for 40 j-ears (1S74 

 to 1913, 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 nonmov- 

 able position of the seed, and irregularities of the stems, such as 

 leaf scales, exits of leaf traces, and leaf sheaths, particularly at 



