TAXONOMIC CHARACTERISTICS 7 



walls formed by the conjugating tube and the two gametangia. 

 This conjugation apparatus is relatively persistent and the zygo- 

 spore remains in the enclosure for days or weeks after its own 

 walls are mature. Zygospores at first have two nuclei and either 

 one set of chromatophores (Spirogyra) derived from the receptive 

 gametangium, or two sets (Zygfjema and Mougeotia) derived 

 from both gametangia, one of which disintegrates later. After the 

 union of the nuclei, reduction division occurs and four nuclei 

 result. Of these, three disintegrate and the remaining one becomes 

 the nucleus of the basal cell of the sporeling on germination. 



Only when one contemplates the many successive steps in 

 conjugation from the development of papillae and adhesion of 

 the gametangia onward to the maturation of the spore walls and 

 protoplast, does he realize how very complicated are the chemical, 

 physical, and biological processes that are integrated in zygospore 

 development (PI. I, Figs. 3, 8, 9, and 10). 



Parthenospores may be formed when the normal union of one 

 gamete with another fails to occur either by absence of an opposite 

 gamete, or by sudden changes in environmental conditions. In 

 some collections gametes are numerous; in others, rare. Partheno- 

 spores can usually be distinguished from aplanospores by their 

 position in the gametangial cells and their relation to adjacent 

 zygospores. It is customary to say that "they resemble the zygo- 

 spores of the same species but are smaller." Akinetes, aplano- 

 spores, zygospores, and parthenospores have all been seen germi- 

 nating by numerous students. New filaments grow from any of 

 these forms. 



The taxonomy of the Zygnemataceae has been centered on the 

 conjugation apparatus: its development, forms, and zygospores. 

 The evolutionary history of reproduction in the group, however, 

 must have begun with propagation and survival by akinetes. 

 These are frequently seen in other families of the Chlorophyta. 

 The development of aplanospores must have been a next step in 

 evolution. This is not a large step either, since it implies merely 

 the contraction of the protoplast and the growth of a spore wall. 

 This type of spore also occurs in many other families of the 

 Chlorophyta. 



When aplanosporangia became differentiated chemically and 

 hormones with plus and minus properties were released which 

 brought about local growth of cell walls followed by adhesions 



