INTRODUCTION 5 



within the wood; upon what it feeds, or how it grows, little is known, 

 although it has recently been demonstrated that it may feed to some 

 extent, at least, upon bacteria. When ready to fruit the Plasmodium 

 appears at the surface and proceeds to form erect sporophores. These 

 may be poroid or clavate, or dendroid and elaborately branched, with 

 all intermediate conditions. While the sporophores are in process of 

 formation, according to Jahn the bulk of the nuclei undergo reduction 

 division, the rest degenerating, as in the Myxogastres. When the 

 sporophores are mature, the surface becomes marked off into poly- 

 hedral, uninucleate protospores, each of which becomes elevated upon 

 a slender stalk and is transformed at the tip into a smooth-walled, 

 elliptical spore. Within the spore, the nucleus divides twice mitoti- 

 cally, so that at maturity the spores are 4-nucleate. Upon germination 

 the contents of the spore emerge as an amoeboid body which may pro- 

 duce short pseudopodia. The nuclei shortly divide again and the whole 

 mass becomes separated into eight uninucleate portions each of which 

 develops a flagellum and swims off as a pyriform swarm-cell. The 

 nuclei of the younger sporophores seem to be diploid, those of the 

 swarm-cells haploid. According to Jahn (1908), fusion occurs at the 

 time the plasmodium emerges from the wood. Olive (1907) claims that 

 fusion occurs just before the formation of the protospores and regards 

 the first two divisions in the spore as constituting the reduction divi- 

 sions. H. C. Gilbert has recently demonstrated, however, that fusion 

 occurs between the swarm-cells, and supports Olive's view as to the 

 place of reduction division. 



2. Myxogastres. This subclass includes all of the slime molds except 

 Ceratiomyxa, — some four hundred species. The fructification is char- 

 acterized by a wall which surrounds the spores and other internal 

 structures. The wall may be thick or thin and relatively permanent or 

 quickly fugacious, but it is always present. The spores are uninucleate 

 when formed normally, but the nucleus may divide before germination. 

 They are small, more often under than over 10 \x in diameter, and en- 

 closed in a cellulose membrane. This is rarely smooth, usually finely or 

 coarsely echinulate or verrucose, or reticulate. The spore markings are 

 rather constant in a given species and hence serve as useful taxonomic 

 characters. In spite of their small size, the spores are extremely tena- 

 cious of life. E. C. Smith, who has investigated this phase of their 

 life-history intensively, reports germination of the spores of a number 

 of species, from collections left in the herbarium for from ten to thirty- 

 two years (1929) ; later, in correspondence, he has extended the upper 

 limit to forty-four years. 



Germination of most species takes place fairly readily in tap water or 



