0f1 INTRODUCTION 5 



Dictyostelium, a genus of the Acrasieae. The reason for his 

 including this genus may be the fact, pointed out by de Bary, 

 that Brefeld in first describing the dense aggregations of 

 swarm-cells into the stalked spore-masses of Dictyostelium, 

 refers to them as being " plasmodia, that is, products of the 

 coalescence of swarm-cells " ; and it was not until after the 

 publication of Rostafmski's Monograph that Van Tieghem in 

 1880 and Brefeld in 1884 corrected this statement. 



Accepting the Mycetozoa as established by Rostafinski, 

 but excluding Dictyostelium on the ground of its not forming a 

 true plasmodium, we have a clearly defined group of organisms 

 separated from all others by the following combination of 

 characters : A spore provided with a firm wall produces on 

 germination an amoeboid swarm-cell which soon acquires a 

 flageUum. The swarm-cells multiply by division and sub- 

 sequently coalesce to form a plasmodium which exhibits a 

 rhythmic streaming. The plasmodium gives rise to fruits 

 which consist of supporting structures and spores. In the 

 Endosporeae these take the form of sporangia, each having a 

 wall within which the free spores are developed. A capillitium 

 or system of threads forming a scaffolding among the spores 

 is present in most genera. In the Exosporeae they consist 

 of sporophores bearing numerous spores on their surface. 



The fact referred to below that the swarm-cells ingest 

 bacteria, appears to strengthen the view that the group is 

 more nearly associated with the lower forms of animal than of 

 vegetable life, and the name of Mycetozoa appears to mark its 

 true position in the borderland between the two kingdoms. 



The Spore and Swarm-cell. — The spores of the Endosporeae 

 are mostly spherical, but occasionally they are ellipsoid. 

 Their size is so uniform in each species that their measurement 

 affords a valuable character for specific determination, though 

 in a few cases (as in Leocarpus fragilis and some others) 

 different gatherings vary considerably in the size of the spores. 

 The spore-wall is variously coloured in the different species. 

 It is described by Zopf as showing the chemical reaction of 

 cellulose, and consisting of a simple firm membrane.* The 

 spores of several species of Didymium and Trichia, when 

 crushed and stained, show the existence of two layers, the 

 inner more delicate and appearing less deeply stained than the 

 outer. In Physarum, Arcyria, and genera with thin-walled 

 spores, an inner layer has not been traced. The spore- wall 

 may be either smooth, warted or reticulated, and often shows 

 a thinner area where dehiscence takes place. The contents 

 of the spore consist of faintly granular protoplasm with a 

 single central nucleus. In abnormal developments, monstrous 



Die Pilztbiere," p. 53, in Schent " Handbuch der Botanik," iii. 2 (1884). 



A* 



