CHAPTER FIVE 



THE GENUS ZYGOGONIUM KtJTZING 1843 



The discovery and description of 14 species belonging to this 

 genus have demonstrated the distinctive characteristics of the 

 genus clearly and have justified its separation from the genus 

 Zygnema. The small pillow^-shaped, or compressed-globular, 

 chromatophores of the Zygogoniums are quite different from 

 those of the Zygnemas. Under certain conditions these chroma- 

 tophores may have a fevi^ short irregular appendages. In all species 

 that have been described, how^ever, the chromatophores occupy 

 but a small fraction of the cell lumen. The small nucleus lies in 

 the bridge between the chromatophores. 



The filaments are often branched, sometimes with horizontal 

 branches and rhizoids in the soil surface, and with erect branches 

 arising from them. A regular feature of the species growing on 

 soil is the formation of thick, lamellate vegetative cell walls and 

 akinetes. Indeed in situations which frequendy are dry, the fila- 

 ments appear to be chains of akinetes. 



A common mode of reproduction is by aplanospores. These 

 globose or cylindric-ovoid spores are formed within vegetative 

 cell walls and are usually much smaller than the cells. Cyto- 

 plasmic residues are present regularly in the cell lumen outside 

 the spore, a feature not found in Zygnema. 



Reproduction by zygospores is less frequently seen. Conjuga- 

 tion may be scalariform or lateral. In either event there are 

 cytoplasmic residues left in the gametangia after the union of the 

 gametes. All the 14 species that have been found reproducing 

 sexually have the zygospores in sporangia formed by the greatly 

 enlarged conjugating tubes. The two gametes unite directly in 

 the tube, and a sporangium wall is immediately formed around 

 them which separates them from the gametangia. The spores are 

 discharged by the breaking of a distinct equatorial suture in the 

 sporangium wall. 



All the species are amphibious and grow on wet, acid mineral 



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