RIcE: INTERNAL SORI OF PUCCINIA SORGHI 41 
leaf. The mesophyll is heavily infected with crowded inter- 
cellular hyphae from which the sorus arises, and at its base 
there is a space in the midst of the hyphal mass enclosing a few 
uredospores (Fic. 1). This pocket extends through a series of 
about one hundred sections, 7.5 vu. thick, and from one to three 
spores lie in the pocket in each section. In the few cases where 
the point of origin of the spores is indicated, the attachment of 
the uredospores is on the side of the sorus above. 
The second instance of internal uredospores was found in 
1922 in a leaf sheath which, as the result of inoculation from 
spores placed between stem and sheath, showed a heavy infection. 
A succession of erumpent sori occurs between the veins on the 
inner surface of the sheath. These sori themselves are internal 
in so far as the chance for dissemination of spores is concerned 
but, in addition, intercellular strands of hyphae connect with 
young sori developing on the outer surface of the sheath, and 
in the midst of these intercellular hyphae there is an internal 
sorus appearing as a mass of close-packed angular uredospores. 
The mesophyll cells near the internal sorus show small haustoria 
and the cells are many of them hypertrophied, a condition not 
usually apparent in infected corn leaves. The pocket seems to 
have been formed by the breaking down of a host cell. This 
pocket formation suggests the hyphal arrangement described by 
Richards (18) for Uromyces Caladii, and by Nemec (14) for 
Uromyces Betae although there is in these cases no internal spore 
formation. Richards describes hyphae which, starting in large 
intercellular cavities, push out and surround neighboring cells, 
cutting them off from their fellows. Such isolated cells, he says, 
are usually completely broken down and absorbed. Nemec 
describes intercellular hyphae of Uromyces Betae which fill not 
only the original intercellular spaces but also spaces made by 
dissolving the middle lamella until many host cells appear 
completely enclosed by a hyphal felt. It is also of interest to 
compare the internal cysts of the corn rust with the close-packed 
masses of intercellular hyphae which Clinton (6) figures for 
Cronartium ribicola in needles of Pinus Strobus. These sclerotia, 
as he names them, are a tangle of hyphal threads, which swell 
the intercellular space to abnormal proportions. Such host cells 
as are enclosed in them are finally disintegrated. There is no 
hollow center in these masses, as in the case of the internal sori 
