July is, 1915 W allrothiella Arceuthobii 373 



genus in the West which are most similar to the eastern-spruce mistle- 

 toe may have some bearing on the taxonomic position of this group 

 of mistletoes. The occurrence of the fungus on R. americana, a very 

 definitely associated and characteristic species with no affinity whatever 

 with the mistletoes of the Pseudotsuga-Abies-Picea group, indicates a 

 cosmopolitan character for the disease. To determine this point, the 

 fungus has been introduced into clumps of the yellow-pine and larch 

 mistletoes. These experiments are now under way. 



MORPHOLOGY 



Photographs of W. arceuthobii have not been published. For this 

 reason detail enlargements from the original negatives of infected and 

 uninfected fruits of R. americana are reproduced in Plate LVI, figures 

 i and 2. Reproductions of photographs of the fungus (natural 

 size) on all its western hosts, so far as known, are likewise shown 

 (PI. L-V, fig. 1-5). These illustrations indicate very clearly the inter- 

 esting habitat of the fungus. A study of the enlargements (PI. LVI, 

 fig. 2) shows the shiny black perithecia densely crowded at the apex of 

 the fruit. Varying numbers, sometimes as many as 40 or more, have 

 been counted springing from the brownish black stroma within the seed 

 capsule. The general shape of the perithecia is that of an oblong cylinder. 

 Usually, however, they are slightly enlarged at the free ends and very 

 abruptly rounded. The hyphae composing that part of the stroma from 

 which the perithecia take their origin are densely compacted, brown or 

 black, with thick walls. Deeper within the capsule, the brown color is 

 not so conspicuous, although the mycelium is generally brownish. The 

 outer walls of the perithecia are uniformly smooth; very rarely a 4- to 

 6-celled projection is present. The crowded condition of the perithecia 

 often gives them the appearance of being partially embedded in the 

 stroma. The wall between two perithecia when densely crowded may 

 be very thin and appears to be occasionally ruptured on the escape of 

 the spores from the asci. The asci show considerable variation in shape, 

 owing principally to their crowded condition, but when free are uni- 

 formly pear- or club-shaped, with fairly long pedicels. Probably in no 

 other species of the order is the early disappearance of the wall of the 

 ascus so characteristic. Before the spores have reached maturity or at 

 least before they have assumed the normal color of mature spores, the 

 ascus wall disappears. The ascus is probably ruptured at the apex by 

 the pressure of the developing spores within. That a considerable pres- 

 sure must be exerted against the walls of the ascus is shown by the fact 

 that the spores when free are normally spherical, but within the ascus 

 they are often bluntly angular. They often persist in clumps after their 

 escape from the ascus. The asci vary but very little in size. The 

 measurements (Zeiss filar micrometer with No. 12 compensating ocular 



