THE MYCETOZOA 47 



De Baiy concluded (2, pp. 43-51) that besides such a positive 

 vis a tergo, due to contraction of the protoplasm in the regions from 

 which the flow occurs, there is evidence of a negative pressure 

 exercised by the plasmodium in the regions towards which the flow 

 is going, and due to its expansion from the previous state of 

 contraction. 



While the conclusion appears probable that the streaming 

 movement is due, in part at any rate, to the contraction of the 

 outer portions of the protoplasm, we may bear in mind that such 

 an explanation appears inapplicable to other phenomena, which we 

 should expect to belong to the same category, such as those 

 exhibited by the pseudopodia of the Foraminifera, in which 

 streams of granules course along a filament of extreme tenuity in 

 opposite directions. 



When a piece of sclerotium resumes activity on being wetted, 

 it sends out a fan- like expansion over the substratum, and the 

 rhythmic flow is seen to be alternately away from the central mass 

 and back to it ; but as the fans extend farther over the substratum, 

 the flow in the several parts of the plasmodium becomes less and 

 less co-ordinated, in proportion as they separate from one another. 

 The several parts separate into distinct plasmodia, and distinct 

 plasmodia fuse with complete freedom. 



Reaction of Plasmodia to External Conditions. Experiments testing 

 the reaction of plasmodia to variations in external conditions have 

 led to some positive results, an interesting account of which is 

 given by Stahl (22). 



During the vegetative period of their existence plasmodia move 

 from the drier to the moister parts of their substratum, though at 

 the approach of the spore-producing stage the movement is in the 

 opposite direction, the organism seeking the driest part of its en- 

 vironment whereon to undergo its change into spores. Connected 

 apparently with the favourable influence of a moist atmosphere is 

 the phenomenon, familiar to tanners, of the " flowering of the tan- 

 heaps " at the approach of wet weather. This consists of the 

 emergence at the surface of the bright yello.w plasmodia of Fuligo 

 septica, commonly known as Flowers of Tan, which abound in the 

 heaps, and, except under such conditions (and at the approach of 

 sporulation), inhabit the deeper parts of the heap. 



When water is allowed to flow through the substratum, 

 plasmodia move in a direction opposite to the current, a tendency 

 which may be utilised for the purpose of isolating them for 

 experimental purposes. By arranging strips of filter paper, through 

 which water is flowing, so that their lower ends rest on the mass 

 containing the plasmodia, the latter will crawl up the filter paper, 

 and may thence be transferred, in the same manner, to glass slides. 



