284 
Journal of Agricultural Research 
Vol. XI, No. 6 
prepared slides are sufficiently transparent for a thorough exploration 
with oil-immersion lenses. The practicability of the method has been 
demonstrated by constant use during the past year in the diagnosis of 
suspected cases of blister-rust and in the study of the anatomical fea¬ 
tures of parasite and host. 
MYCELIUM OF THE PARASITE 
The characters which distinguish the hyphae of Cronartium ribicola 
from other hyphae occasionally found in white-pine bark are their com¬ 
paratively large size, fairly uniform diameter, and possession of haus¬ 
toria. They are larger than any other parasitic hyphae which the writer 
has observed in pine bark. While there is, of course, some difference in 
the width of individual hyphae one rarely finds them deviating widely 
from the average width of 3 to 3.5 p . The haustoria are particularly 
striking. They invade practically every cell in the infected region in 
young bark. In older bark they are most prominent in the cells of the 
medullary rays; and for this reason, as mentioned above, radial sections 
are particularly satisfactory for study. At the point where the haus¬ 
toria pass through the cell wall, they are constricted. Once inside the 
cell their width increases until they average a little more than 4 jjl in 
diameter (PI. 31, B). Prominent bulges, which occur quite frequently, 
give them a characteristically irregular outline. When young and 
short they are straight; as they mature they usually develop pronounced 
bends, which may be as great as right angles. The host nuclei appear 
to attract the haustoria. Frequently their tips press dents in the sides 
of the host nuclei (fig. 1, a), recalling the occurrence of this well-known 
phenomenon in many other rusts; but, so far as the writer has been able 
to observe, the nucleus does not appear to suffer greatly from this close 
association with the parasite. In fact, the cells in which the haustoria 
are present seem to be quite as healthy as the cells which have remained 
free from attack. Usually the haustoria are unbranched; but in some 
instances, especially in the cells of the medullary rays, they branch 
(fig. 1, b). In no case have more than three tips been observed. All 
the cells of the mycelium, including the haustoria, are uninucleate until 
cell fusion occurs at the base of the secium. 
The Peronosporales and the Erysiphaceae are the only other groups of 
parasitic fungi beside the Uredinales which possess prominent haustoria. 
No members of the first two groups are known to parasitize the bark of 
Pinus strobus . Moreover, the second group is composed of leaf parasites 
whose mycelium is almost entirely exterior. Even if by any chance 
material containing mycelium from either of these two groups should 
be introduced into the mount along with the bark sections, there would 
be no danger of confusion. The fact that the members of the first group 
have coenocytic hyphae and that the members of the second group possess 
