juiy i 5 , 1915 Wallrothiella Arceuthobii 375 



in the direction of the most constant winds. On either side of the most 

 heavily infected area the trees did not support infected plants, although 

 the mistletoe was abundant. Furthermore, large compact brooms 

 always bore the greater number of infected plants on the windward side. 



The writer has recently determined that insects to a certainty play 

 a role in the pollination of these mistletoes. Hymenopterous insects 

 are chiefly in evidence, but those of other orders are also known to 

 promote pollination. During 1914 grasshoppers in great numbers 

 came out of the Hangman Creek Valley near Spokane and fed upon 

 blooming staminate plants of the large mistletoe growing in profusion 

 on yellow pine of the bench lands. These insects seemed to select only 

 the flowers of the staminate plants for food; but, swarming over the 

 pistillate plants, they deposited some of the pollen that adhered to their 

 bodies. It is as easily possible that the spores of the mistletoe fungus 

 are in a minor degree transported in a like manner. Rain dropping from 

 infected to uninfected plants or running down the pendent branches 

 and dropping off at the tips of the mistletoe plants is probably a factor 

 in distributing the disease on any one tree or broom. It so happened 

 at the field station that a number of newly collected infected capsules 

 were left overnight and a portion of the following day on a glass slide 

 under the microscope. An examination of the slide showed that a num- 

 ber of spores had been expelled and lay in a ring about ~% mm. away from 

 the apex of the perithecium. Evidently there is a slight expulsion of the 

 spores under favorable conditions. This came as a surprise, as the stiff 

 ends of the hyphae forming the perithecial wall seem to open with diffi- 

 culty. A number of perithecia collected from fallen capsules in the 

 spring still contained numerous spores. The early disappearance of the 

 ascus within the perithecium precludes any expulsion from this source. 

 The force must arise from the continual maturing and crowding of the 

 spores toward the outward end of the perithecium. Under favorable 

 conditions this pressure may become sufficient to force the spores out 

 through the aperture. It has already been indicated that a pressure 

 seems to exist within the perithecium. This force, though weak, may 

 still be sufficient to cause the spores to land on capsules of the same plant 

 that escaped previous infection. 



The spores of the fungus are beginning to ripen and to be expelled 

 from the perithecia in the latitude of northern Idaho about the end of 

 November and are capable of germinating immediately. The method 

 of penetration of the germ tube of the spore into the developing fruit of 

 its host has not as yet been observed. Since a considerable period 

 elapses between pollination and the time actual fertilization takes place 

 in the host, it is quite possible that the germination of the fungus spore 

 coincides with the advance of the pollen tube toward the embryo sac. 

 This would enable the germ tube of the spore to travel toward the ovule 

 of its host by a line of least resistance. In early spring, or at the time 



