286 G. R. BISBY 
lining tubular capillary spaces. In Trichia, however, the capillitial 
cavities are of relatively large diameter; while in the two species 
described in this paper, they are of very narrow diameter. The fact 
that in Trichia and similar cases, the capillitium starts in vacuoles in 
the interior of the protoplasm, while in Stemonitis and Physarella 
these spaces apparently originate as invaginations of the external 
plasma membrane or that lining the capillitial or columellar cavities, is 
undoubtedly of fundamental importance. 
Cleavage in these two forms under consideration is decidedly 
progressive. This shows particularly strikingly in cross sections of 
young sporangia of Stemonitis, in which cleavage will often be seen 
to involve at one time only a small sector of the peripheral protoplasm. 
I am unable, however, to add anything new to the valuable observa- 
tions made along these lines by Professor Harper ('00, '14&). 
I am indebted to Dr. Olive, under whose supervision this work was 
carried on, for suggestions and criticisms freely given during the course 
of these investigations. 
Summary 
1 . The capillitium of Physarella mirahilis and of Stemonitis fusca 
is formed within tubular capillary spaces. In both cases these spaces 
are formed as invaginations into the protoplasm, and are not considered 
to be vacuoles. From the plasma membrane lining the tubular spaces 
occurs a progressive deposition of substance to forrn the walls of the 
capillitial threads. This deposit is continuous with that forming the 
sporangial wall; and, in Stemonitis, as well with that which forms 
the columella wall, to which the radiating, tubular threads are attached. 
In Physarella, a continuous wall also exists, bounding capillitial threads 
and lime knots, when they are in connection with each other. 
2. The protoplasm remains in contact with the capillitial threads 
until a short time prior to cleavage. Cleavage furrows then appear, 
which follow the threads and which are thus determined to some extent 
by the capillitium. 
3. The mature capillitial threads of these two forms are smooth 
(in Physarella an occasional spinous process occurs), and a careful 
examination shows that a majority of the threads are hollow and not 
solid as is usually stated. Where seeming solidity does occur, this is 
interpreted as due either to an actual collapse of the tube when in a 
plastic condition, or else in reality to the obscurity of the lumen 
owing to the fineness or opacity of the threads. 
