THALLOPHYTA: FUNGI 



107 



are not well understood, but are not assumed to be close. They have 

 proliably been derived from protozoan ancestors and have evolved along 

 independent lines. 



Acrasieae. These simple forms are saprophytes on soil, decaying wood, 

 and animal refuse. The vegetative body is a myxamoeba, a naked cell 

 with a nucleus and a contractile vacuole. It reproduces by fission and 

 in the presence of unfavorable conditions may encyst. Eventually, a 

 number of myxamoebae come together without fusing to form a pseudo- 

 plasmodium in which each m3^\amoeba retains its individuality. Not 



S HRS 



Fig. 84. Three-dimensional graph showing the development of the fruiting body of 

 Dictyostelium discoideum in height, time, and position. A, aggregation of a mass of 

 individual myxamoebae; B to D, formation of the pseudoplasmodium; E toG, migration of 

 pseudoplasmodium; H to N, formation of fruiting body with disk, stalk, and spherical spore 

 mass. {From J. T. Bonner.) 



only is a multinucleate plasmodium lacking, but no zoospores are pro- 

 duced. The pseudoplasmodium assumes a definite form, usually elongat- 

 ing and varying in length from several tenths of a millimeter to a milli- 

 meter or more. 



The subsequent behavior of the pseudoplasmodium is remarkable 

 in that it migrates over the substratum, apparently by a gliding of the 

 amoebae over one another. After coming to rest, the pseudoplasmodium 

 is transformed into a fruiting body consisting of a basal disk, a vertical 

 stalk, and a terminal spherical region that is converted into a mass of 

 spores (Fig. 84). In some species the fertile region consists of a series 

 of spherical spore masses arranged at the ends of whorled branches. 

 All these complex changes are accomplished by movements of individual 

 myxamoebae to their proper place in the fruiting body, where each 

 becomes the type of cell appropriate for its position, such as a disk cell, 

 stalk cell, or spore. The spores have a cell wall. Upon germination, 

 each spore gives rise to a myxamoeba. 



