Mar. 3, 1923 
Origin of the Central and Ostiolar Cavities in Pycnidia 749 
it is more often very irregular, dipping down here and there rather 
deepty (PL 3( D). Superficially the pictures presented certainly suggest 
that the rupture is caused by forces from within, either that the lower 
two-thirds of tissue has shrunken while the outer third designed as the 
future wall has remained rigid or that the latter part has been raised 
bodily by addition of new cells at the ends of the arch, the rest of the 
tissue remaining dormant. But the early stages of these large, dome¬ 
shaped pycnidia develop in nature beneath the tough and tightly stretched 
cuticle, for example, of canes of Rubus spp., where if the rupture were 
caused in the manner just suggested, there would occur more or less 
buckling of the outer wall under such unequal strains as must exist. 
At such a stage (PI. 3, D) no buckling occurs, which means that the 
wall must be supported by the pressure from below, furnished by disor¬ 
ganization of the intercalary cells, as indicated. Mature pycnidia on 
leaves in dry weather are found collapsed, indicating that the thick 
arched wall in itself is not strong enough to prevent buckling after the 
pressure has been withdrawn by the shrinkage of the mucilaginous 
substance within. A concave pycnidium usually indicates that it will 
be found full of spores. Figures D and E are from a section of the 
same pycnidium at slightly different planes. Figure E shows somewhat 
better that the break occurs in radially growing hyphae, but Figure D 
suggests that disorganization accompanied by the swelling of cells is 
responsible for the rupture of the tissue. The five or six large bodies 
mixed in with the tissue just pulling apart (at the center above) are the 
remains of what just previously were very minute cells of the parallel 
hyphae and which have now swollen to many times their original size. 
Many such cells are visible in the section shown at F and also at H, 
which is still more highly magnified. Above are large masses of small 
intercalary cells detached from the hyphae which bore them and are 
now caught between two regions of disorganization. In the section 
shown at G, the sporophores are fully formed along the entire base; but 
the mass above these consists only of rows of disorganizing cells, above 
which is a clear substance, difficult to stain. In this condition the 
structure might be mistaken for a mature pycnidium nearly filled with 
spores. Compare this with the section shown at I, which is from a very 
small pycnidium in which the first spores have just been formed. The 
spreading, or nearly parallel, rows of cells forming the upper wall are 
still distinct. . The cavity may now enlarge to some extent, and there 
is little question that growth at the border accounts for some increase 
in its size. It is noteworthy that there is always more tissue below 
the line of rupture in a young pycnidium than there is below the true 
sporophores in a mature structure (PI. 3, C, D, and I). The cells that 
are first forced apart in the destruction of tissue and that remain below 
are homologues of the sporophores, but as noted they provide the mate¬ 
rial which by swelling forces the wall farther out, incidentally later 
furnishing food for the development of spores. 
It has been shown that the rupture in the tissue is initiated by the 
disorganization of certain intercalary cells in oriented hyphae and that 
this occurs not at a period of shrinkage but while expansion is still going 
on. Pressure is provided by the swelling of the cells tom loose from the 
wall above or isolated from the vertically growing hyphae below. This 
force maintains the even curve of the wall as it lifts the resistant portions 
