CH.VIII.—MORPHOLOGYF AND COURSE OF DEVELOPMENT.—MYXOMYCETES. 429 
The outer surface of the sclerotia is usually covered by alayer of the same 
homogeneous substance with a capacity for swelling which is found between the cells 
in Aethalium. Upon it there are also in many cases (Fuligo, Didymium) scales or 
grains or crystals of calcium carbonate which must have been excreted during the 
formation of the sclerotia. 
If a mature and dry sclerotium is placed in water it at once swells up, and its 
cells coalesce once more into a motile plasmodium often in from six to fifteen hours, 
in older specimens after a longer interval which may last some days. Where 
membranes of cellulose are present they are first dissolved. The process begins at 
the surface and advances towards the centre. 
If single cells of a sclerotium are watched, contractile vacuoles are seen to form 
in them a few hours after they are moistened, and protrusion of motile branches and 
pseudopodia and the creeping forward movement all begin as in plasmodia. “Where 
moving cells meet and. touch they coalesce; if moving cells encounter cells that 
are still at rest, they absorb them. In this way a large plasmodium is ‘gradually 
formed containing many sclerotium-cells which it has engulphed. These phenomena, 
which were first observed by Cienkowski in Didymium difforme, explain the formation 
of the plasmodium from the compact sclerotium. In plasmodia recently formed 
from sclerotia in which the cells have not separated from one another we always see 
a number of unaltered or evidently dead sclerotium-cells carried along by the stream 
of granules; these become presently less frequent and at length entirely disappear ; 
they must therefore be either dissolved or they coalesce with the substance of: the 
plasmodium. 
Sclerotia are known to retain their vitality in a dry state for 6-8 months. 
Fuligo and Didymium Serpula are known from several direct observations to persist 
during the cold and dry season of the year in the condition of a sclerotium, and pass 
again into the motile state with damper and warmer weather. Vitality did not last 
more than 7-8 months in most of the observed cases, though sclerotia of Didymium 
Serpula lived more than a year (others only 7 months), and Léveillé? quotes an obser- 
vation to the effect that. a sclerotium of a Myxomycete had been known to return to 
the motile condition after having been kept for zo years. 
Section CXXI Development of sporophores and sporangia. The de- 
velopment of the plasmodia closes with the formation of spores within receptacles, 
sporangia, or on the outside of sporophores. The latter are confined to the 
Ceratieae, the former are common to the rest of the Myxomycetes. We may 
therefore with Rostafinski distinguish the Ceratieae as exosporous, all other 
Myxomycetes being endosporous. 
The sporangia of the endosporous forms are vesicles, which are usually about 
ımm. in height but may considerably exceed that size, and rise with or without a 
stalk above the substratum or lie upon it in the form of round or flat tubes. Their 
stricture when they are fully formed will be more minutely described in section 
CXXII. Their development from the plasmodium is divided into the successive 
sections of forming, development of wall, separation of the spore-plasm, and lastly 

1 Ann. d. sc. nat. ser. 2, XX, p. 216. 
