IOWA ACADEMY OF SCIENCES 
105 
nanus, E. Cerasi and E. institae^. Sargent- in his Silva reports Exoas- 
cus fiavus, now called Magnusiella -[lava, on species of Betula populifolia. 
No similar, fungus was found on any of the other Rock mountain species 
of Betula. 
Exoascus 
The genus was first described by FuckeP; Sadeback, who has made 
careful morphological and biological studies of these fungi, in his latest 
monograph retains the genus established by Fuckel. The perennial mycel- 
ium propagating the fungus from one season to the next, is copiously 
branched and sends ramifications between the cuticle and the remainder 
of the epidermal cells where the ascogenous layer, or hymenium, is 
developed. In the formation of the asci which may be densely crowded or 
somewhat scattered, the entire sub-cuticular mycelium is consumed; their 
development ruptures the cuticle and ensures the escape of the spores. 
The fungus causes deformation of leaves, fruit or stems. 
Exoascus nanus Joh'^. 
History. This fungus was described by Johanson- as occurring on 
Betula nana. There are several closely allied species, E. l)etuUnus and 
E, 'bacteriospermus (Job.) Sad., the latter having been collected at Lake 
of the Clouds, Mt. Washington by Myiabe and reported by Patterson. 
TubeuF states that the fungus causes hypertrophied branches of Betula 
nana. 
Genera Characters. .This species produces the so-called “witches 
brooms” of the western Birch {Betula occidentalis) . From one to a dozen 
©f these brooms may occur on a single tree. Hundreds of yellowish 
bunches may occur on a single branch. The statement that these bunches 
appear like foreign plants on the trees is very appropriate. Where this 
growth begins the branches are usually swollen. The newer growth of 
branches is also lighter in color but that is largely due to the yellowish, 
somewhat swollen leaves that have rolled edges. All of the leaves usually 
show the presence of the fungus, which when mature causes the lower 
surface of the leaf to appear covered with a white powder. 
Microscopic Characters. The perennial mycelium occurs in the buds, 
from whence the infection spreads with the formation of leaves the fol- 
lowing spring. I have not been able to study the development of the 
fungus, but in mature specimens the asci are not closely crowded, the ceils 
are short, rounded at. the apex, while the base of the ascus extends into 
the basal or stalk cell. The asci are quite uniform in size and measure 
11-13" X 18-3". The stalk cell is short but wider than the ascus. The 
normal number of ascospores is eight, but there are frequently more. 
They vary in size from 3 to 7". The elliptical are 4 x 5". 
1 . H. H. Hume. Proc. Davenport Acad. 7: 255. Contr. Bot. Dept. 1. S. C. 
•15 : 255. 
2. C. S. Sargent Silva North America. 9. 49. 
3. Eunm. Fung. Ann. Nat. Nass 15 : 29 Symb. Myc. 252. 
4. 1. C. : 108. 
5. 1. c. 179. 
