CH.VIIL— MORPHOLOGY AND COURSE OF DEVELOPMENT. — MYXOMYCETES. 427 



It consists of a sticky substance which swells in water and contracts in alcohol and 

 is scarcely coloured by iodine, and must therefore be different from protoplasm. It 

 passively follows the movements of the plasmodium. Portions of it often remain 

 adhering in the form of thin films of mucilage to spots from which a plasmodium has 

 moved away. This envelope is often very thin round the rapidly swelling extremities 

 of the branches, and cannot be distinguished at all round slender pseudopodia, 

 having been either pierced through by them or extenuated by their advance till it is 

 no longer perceptible. 



The plasmodia of the Stemoniteae, Trichiaceae, Ceratieae and Lycogala have 

 in the main the same structure and power of movement as those of the Physareae, but 

 they never have the granules of calcium carbonate and therefore usually appear to be 

 much more finely granular than in the Physareae. The dark-blue or violet-brown 

 plasmodia of the Cribrariae and Dictydium contain large brown granules of some 

 organic substance, but have as yet been very insufficiently examined. The plasmodia 

 of Lycogala which live in rotten wood are surrounded by a thick colourless membrane; 

 I observed a similar membrane some time ago in Arcyria punicea. It is not yet ascer- 

 tained how this membrane behaves in the movements. I was unable to see it in former 

 years in specimens of Lycogala grown in water. I found the plasmodia of Stemonit^s ^ 

 fusca, when it issues from the substratum to form its sporangia, surrounded by a stout 

 envelope, the inner and thicker layer of which is coloured a dark blue by iodine 

 while an outer thin layer remains colourless. All the plasmodia last mentioned are 

 inconspicuous bodies, the stouter branches of which in Arcyria punicea are not more 

 than 1 6 /x in thickness, in Lycogala not more than 24 fx. They live for the most part in 

 the interior of rotten parts of plants, especially rotten wood, and are not visible to 

 the naked eye till they come to the surface to form sporangia. 



Section CXX. Transitory resting-states. Those stages of the development 

 in Myxomycetes which have the power of motion are able to pass into resting-states 

 and to return again under favourable conditions to the state of movement. Three 

 resting-forms are at present known : microcysts, thick-walled cysts (Cienkowski) and 

 sclcrolia. 



It appears from cultures of Chondrioderma difforme that these transitory 

 resting-states are not necessary members of the course of development. Their 

 formation would seem to take place only when the development of the swarm-cells into 

 plasmodia or of the plasmodia into sporangia is interrupted by insufficiency of food, 

 by slow desiccation, or by slow cooling to below a certain minimum. But there are 

 a number of observations which also point to other at present unknown causes. 

 The state of movement is restored when the bodies after desiccation are again placed 

 in water of the proper temperature. 



The term microcyst was given by Cienkowski to the resting- state of the swarm- 

 cells. Under the conditions above mentioned these cells assume the form of spheres 

 which are smaller than the spores and are surrounded by a very delicate colourless 

 membrane, as in Perichaena liceoides according to Cienkowski, or are without a 

 membrane but provided with a very firm marginal layer. In other respects their 

 structure remains the same as that of motile swarm-cells, only that the vacuoles in 

 many cases disappear and the protoplasm becomes more dense. The swarm-cells of 

 Didymium praccox and D. difforme encysted in this way and perfectly dry retain 



