CH.VIII. MORPHOLOGY AND COURSE OF DEVELOPMENT. MYXOMYCETES. 429 



The outer surface of the sclerotia is usually covered by a layer 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 LeVeiHe" * 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 20 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 

 i mm. 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 

 structure 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. 



