106 PLANT MORPHOLOGY 



network innumerable spores are formed, each one being uninucleate. 

 The spores have cellulose walls and are scattered by the wind. In 

 their dispersal the wall erf the sporangium ruptures irregularly at the 

 apex and the capillitium performs hygroscopic movements. 



The myxomycetes display considerable variation with respect to the 

 development of the Plasmodium from a spore. Commonly the proto- 

 plast escapes from the spore wall and becomes a zoospore, developing 

 one long cilium and one very short one, both anteriorly attached. Some- 

 times two to eight zoospores are produced. The zoospore may ingest 



food and undergo multiplication by fission. 

 After a period of free swimming, the cilia are 

 retracted and the protoplast becomes amoe- 

 boid. These amoeboid cells, called myx- 

 amoebae, may also take in food and divide 

 repeatedly, or they may pass into a resting 

 stage. Finally, however, they fuse in pairs. 

 Then, instead of forming resting zygotes, a 

 number come together to form a multi- 

 nucleate Plasmodium. In the fusion of the 

 small amoeboid cells in pairs, the two nuclei 

 Fig. 83. Portion of capii- Unite, but there are no subsequent nuclear 

 litium of stemonitis with fusions. Consequently the nuclei of the plas- 



spores in its meshes, X 250. i • i • i 



modmm are diploid. They undergo repeated 

 divisions as the Plasmodium increases in size. Reduction of chromosomes 

 to the haploid number occurs just prior to the formation of spores in the 

 sporangium. 



Summary. The Myxomycetes combine features found in both plants 

 and animals. The body is a naked mass of multinucleate protoplasm 

 (a Plasmodium) that displays amoeboid movements and engulfs solid 

 food particles. In a quiescent state it gives rise to sporangia containing 

 numerous walled spores from which eventuall}^, although not directly, 

 a new Plasmodium is formed. Sexual reproduction occurs by a fusion 

 of similar amoeboid gametes. Certain aspects of the life history suggest 

 a relationship to the flagellates. Any possible connection with the true 

 fungi is very uncertain. 



Other Slime Fungi. In addition to the Myxomycetes, or slime molds, 

 there are two other groups of slime fungi that deserve some attention. 

 These are the Acrasieae and the Labyrinthuleae. Many mycologists 

 include all three groups in a separate assemblage, the Myxothallophyta, 

 and place them outside and below the fungi. They have certain impor- 

 tant characters in common: simple, naked, nucleated, amoeboid cells 

 resembling protozoans but plant-like in their reproduction by the forma- 

 tion of spores. The interrelationships of the three groups of slime fungi 



