Thraustotheca , a Peculiar Water-Mould. 161 
or spherical sporangia, although it is sometimes possible from the arrange¬ 
ment of the spores to predict where they will be set free, the dehiscence is 
quite irregular. Moreover, the wall often varies in thickness, and the 
weakest spots are naturally the first to give way. Occasionally, where the 
thickened basal collar merges into the normal sporangium wall, there 
appears to be a line of weakness along which rupture takes place. In the 
case of intercalary sporangia, the thick septa at the ends prevent any 
longitudinal expansion, and discharge is invariably lateral (Fig. 37). 
During sporangium dehiscence, a distinct adhesion and mutual 
attraction is noticeable among the spores. Frequently sporangiospores 
remain sticking to hyphae with which they have come in contact when 
discharged. Microchemical tests fail to demonstrate any intersporal slime 
such as Swingle ( 28 ) found excreted from the spores of Rhizopus ; and 
since this adhesive quality is soon lost, it is probably due only to the 
viscidity of the recently formed cellulose walls. At the moment of dis¬ 
charge, however, the spores seem to exert a mutual attraction ; since they 
do not immediately scatter, but press together, sliding over each other and 
changing their relative positions, but still keeping in contact. It is of 
interest to note that the contiguous surfaces of such spores are often 
decidedly flattened, as if through the exertion of mutual pressure. 
The fragility of the sporangium wall has been over-emphasized by 
previous investigators, probably because they did not observe the actual 
bursting of the sporangium, and hence inferred that the spores were freed 
by disintegration of the wall. In pure culture the sporangium walls are as 
persistent as would be the walls of any other Saprolegniaceous sporangium 
that had undergone a like distension and rupture. 
It seems advisable at this point to call attention to the confusion 
which often arises from an unrestricted use of the terms ‘ zoospore 5 or 
‘spore’ in the case of diplanetic Saprolegniaceae. To avoid any misunder¬ 
standing in the case of Thratistotheea the term ‘ sporangiospore * will be 
used for the non-motile spores formed in the sporangium: the motile 
spores emerging from these will” be called ‘ zoospores ’, and the encysted 
spores into which these in turn metamorphose will be called * cysto- 
spores \ 
The liberated sporangiospores (Fig. 13) vary from a rounded polygonal 
to a practically spherical shape ; their average diameter is about 12*5 /x ; 
and the distinct wall is about 0-5 /x in thickness. The content consists of 
a number of microsomes scattered irregularly through a finely granular, 
almost hyaline matrix, a less densely granular central region indicating the 
position of the nucleus. 
Further History of the Sporangiospores . The sporangiospores may 
either emit motile zoospores, give rise to hyphae of germination, or form 
dwarf sporangia, according to certain conditions of the environment. In 
M 
