504 Harper. — Cell- Division in Sporangia and Asci. 
sporangium are filled with a characteristically foamy proto- 
plasm. Between this foamy plasma and the spore-plasma 
there is at first a very gradual transition (Fig. 33). Later, 
the difference in density is more sharply defined, the tran- 
sition being sudden, on the lower surface of the dense spore- 
plasma, to a very open foamy structure. Just along this line 
of changed structure a series of larger vacuoles become 
arranged which gradually flatten in a curve, outlining the 
surface of the columella (Figs. 34, 35). It is perfectly evident 
that these are ordinary vacuoles which have grown larger and 
begun to flatten. At first it is not easy, in many cases, to 
say whether a particular vacuole is to take part in the 
formation of the series or not. As they grow larger they 
mark very sharply the boundary of the dense plasma. One 
wall of any one of these vacuoles will be dense plasma, while 
the opposite wall abuts on a number of smaller vacuoles. 
The nuclei 1 are somewhat more numerous in the spore- 
plasma, but not strikingly so. The mechanism of the flatten- 
ing of these vacuoles is not easy to understand, except in 
so far as the denser resistent spore-plasma might be supposed 
to resist the action of surface-tension of the cell-sap on the 
side adjacent to it. 
These vacuoles become more and more flattened and tend to 
fuse side by side (Fig. 35). Especially large vacuoles are to be 
seen at the edges of the crescent-shaped section of the spore- 
plasma. These come to lie closer and closer to the cell- wall at 
that point, and finally break through the plasma-membrane, 
thus bringing the cell-sap in direct contact with the sporangial 
wall through which it doubtless filters out and is evaporated. 
I have detected no cleavage-furrows cutting in to meet these 
vacuoles at the boundary of the sporangium, as was the case 
in Pilobolus . The remaining vacuoles in the series now fuse, 
thus separating spore-plasma and columella-plasma by a wide 
irregular cleft filled with cell-sap (Fig. 36). The turgor of 
1 In the figures given of Sporodinia the magnification is not sufficient to show the 
structure of the very minute nuclei clearly, and hence I have represented them merely 
as dots. 
