4 THE MYXOMYCETES 



The Plasmodiophoracese includes Plasmodiophora and several other 

 genera parasitic on vascular plants. They show several points of 

 resemblance to the Myxomycetes, especially in the character of the 

 zoospore with its apical flagellum. On the other hand they entirely 

 lack the secreted sporangial wall and the capillitial threads found in 

 most of the true slime molds and have been held to possess a distinctive 

 type of nuclear division. Schroeter (1886, p. 133) erected the order 

 Phytomyxini to contain them and his classification has been exten- 

 sively followed. The tendency among recent authors, e. g., Gaumann 

 (1926), Gaumann and Dodge (1928) and Fitzpatrick (1930) has been 

 to include them among the lower Phycomycetes. 



The Labyrinthuleae is a curious group of organisms of uncertain 

 position, containing two genera and a small number of species, and 

 characterized by an assimilative phase known as a net Plasmodium, 

 which is thought by Zopf (1885) to be intermediate in character be- 

 tween the aggregate amoeboid mass of the Acrasieae, and the Plasmo- 

 dium of the true slime molds. Much more study is required before 

 these organisms can be placed in a satisfactory relationship with other 

 forms, but at the present time the evidence does not warrant including 

 them in the Myxomycetes. 



Jahn, in one of the most recent general treatments of the Myxomy- 

 cetes (1928), excludes the Acrasieae and Plasmodiophoraceae from the 

 group, but includes the order Hydromyxales, a small group of aquatic 

 organisms, of which Vampyrella is the best known genus. The order is 

 divided into two families. In the less specialized, the Plakopodaceae, 

 there is no suggestion of a plasmodium, but it is included because of its 

 probable relationship with the other family, the Vampyrellaceae. The 

 latter possesses a multinucleate, naked assimilative phase which Jahn 

 regards as a plasmodium similar to that of the slime molds. The 

 manner of life and the reproduction are so different from those of the 

 slime molds that it seems best to exclude this group also, pending fuller 

 information. 



This leaves, then, two major divisions in the Myxomycetes, the 

 Exosporeas and the Myxogastres or Endosporeae. The structure and 

 life-histories of these two groups, here ranked as subclasses, is suffi- 

 ciently different to make it desirable that they be considered separately. 



1. Exosporece. Represented by a single genus, Ceratiomyxa. Several 

 species have been described, but the fructification is so variable that it 

 seems better to include them all as varieties and forms of a single 

 species, C. fruticulosa. This is extremely common, found on decaying 

 wood throughout the greater part of the world. The plasmodium is at 

 first colorless, then white, or occasionally pinkish or bluish, and lives 



