138 A. E. HILTON ON SPORANGIAL CHARACTERS OF MYCETOZOA 



variety, especially after the liberation of the spores. For ex- 

 ample, the enclosing wall of Diderma floriforme splits into strips 

 which curl backward and simulate the petals of a flower. In 

 the lovely genus Arcyria, the capillitia are expanding nets of 

 wonderful delicacy and beauty. Alwisia bomharda, found in 

 Ceylon and Jamaica, is a remarkable species with a capillitium 

 of stiff fibres, quaintly resembling a shaving-brush. Neverthe- 

 less, despite all diversities of ultimate shapes, the initial forms 

 from which they spring are not difficult to account for. Owing 

 to the surface tension of the plasm of which it consists, a 

 sporangium, in the first instance, is necessarily of a rounded 

 contour. Indeed, some sporangia are almost perfect spheres ; 

 the surface tension, in such cases, finding almost complete 

 expression, as in the familiar instance of a mercury globule, or 

 a soap-bubble floating in air. At the surfaces of liquids or 

 semi-fluids, the tension tends to produce spherical forms because 

 it behaves like a stretched elastic membrane seeking to con- 

 tract ; and a sphere has the smallest surface for the volume it 

 contains. 



It will help us to comprehend the aberrations from the spherical 

 form which occur in the majority of the Mycetozoa if we bear 

 in mind other facts concerning this well-known force, which 

 plays so important a part in biology. Surface tension, as now 

 understood, is a manifestation of cohesive force ; that is, of 

 the mutual attraction between molecules. Interiorly, this at- 

 traction is equal in all directions, and therefore balanced ; but 

 molecules at surfaces are subject to the unbalanced pull of 

 interior molecules, and this produces the tension which results 

 in sphericity. The force varies with circumstances. The 

 denser the body the greater the strain at the surface ; the more 

 fluid it is, the less the strain. Changing temperatures, causing 

 expansions or contractions, produce similar effects. Cold 

 increases surface tension ; heat lessens it ; so also do electrical 

 charges. It is therefore easy to understand that changing 

 conditions of warmth and humidity, or electricity in the atmo- 



