480 
Journal of Agricultural Research 
Vol. XI, No. 10 
result of the pressure below. Definite cell layers of the pycnidiutn are 
laid down about this time or a little earlier. The pycnidium completes 
its full development soon thereafter (Pi. 43, B). 
The inner layer of the pycnidium is composed of hyalin cells from 
which the conidiophores arise. A slight cavity is formed at the center 
concomitant with the release of pressure by the rupture of the epidermis. 
Inclosed in this cavity numerous conidiophores are developed, extending 
toward the center from which the spores are cut off. As the spores 
increase in quantity, the cavity enlarges by the pushing back of the cells 
lining the cavity and probably by further lifting of the upper part of the 
pycnidium. It has not been possible to determine definitely when the 
opening from the pycnidial cavity to the outside through the beak is 
formed, but it probably takes place only after the pressure within has been 
greatly increased by the continued production of spores. It is believed, 
Fig. 1 —Diaporthe pkaseolorum: A, An early stage in the formation of a pycnidium. The epidermis (a) 
is not yet disturbed but the hyphse (6) have accumulated directly beneath it. Note the coursing of 
the hyphse through the parenchyma cells (c). X130. B, A more advanced stage of figure A. Note 
the bulging of the epidermis (a). X130- 
however, that their expulsion is largely brought about through the ab¬ 
sorption of water by the gelatinous mass in which the spores are em¬ 
bedded, a condition which has been found by Stewart (34) to occur for 
Guignardia aesculi (Pk.) Stewart. 
The writer does not wish to leave the impression that the pycnidia 
develop only in the stomatal chambers, but that this is a common and 
natural place for their formation there can be no doubt. Even in young 
pycnidia it is not always easy, and sometimes is impossible, to tell 
whether or not they were being developed just beneath a stoma. The 
bast cells of the pods of the Lima bean are very thick walled and are 
generally pressed so close together that it would be difficult, if not impos¬ 
sible, for the fungus to pass between them. These cells, however, are 
separated at certain places, and often just beneath a stoma, so it would 
be logical that they should form in such a cavity. From these facts it is 
evident that, in many cases at least, the pycnidium is developed beneath 
a stoma, and also at such other places where the bast cells are separated. 
