with substiates. The cells are long or short cylinders, but in some 

 species the cells become inflated during the reproductive process 

 (conjugation). The cell wall has 1 or 2 layers of firm cellulose and 

 an outer pectose layer of varying thickness and consistency. The 

 mucilaginous sheath present in most forms is responsible for the 

 fact that these plants very seldom become hosts for epiphytes 

 although frequently parasitized by Lagenidium and Myzocijtium 

 (Phycomycetes). The end walls of the cells are separated by a middle 

 lamella and may be either plane or characteristically infolded 

 (replicate). One of the most characteristic features of this order 

 is the relatively large size of the chloroplasts, usually 1 or 2 in each 

 cell, although in Spirogyra there may be several (up to 16). 

 Pyrenoids in the chloroplasts are large and conspicuous, but in 

 Mougeotiopsis pyrenoids are lacking. The chloroplasts may be axial 

 plates, stellate masses or cushions, or parietal spiral ribbons. In asex- 

 ual reproduction aplanospores and akinetes are sometimes formed. 

 It is in sexual reproduction that plants in this order show their most 

 distinctive characteristics. Either all or part of the vegetative cell 

 content serves as a gamete, union being achieved by one or both 

 gametes passing through a tube which forms between the cells of 

 two filaments (scalariform conjugation) or between cells of the 

 same filament (lateral conjugation). In a few species, filaments 

 become adjoined or are juxtaposed, and gametic union is achieved 

 without the use of tubes. The resulting zygospore has a wall which 

 is usually 3-layered and thick. It may be smooth or the outer layers 

 may be decorated with scrobiculations, pits, or reticulations, features 

 which are specific and of taxonbmic importance. The gametangial 

 cells after conjugation may be empty or may become filled with 

 pectic compounds which are usually deposited in layers. 



Motile reproductive cells do not occur. This, together with other 

 fundamental details of the life history, behavior of the germinating 

 zygospore, etc., set the Zygnematales well apart from other orders of 

 the Chlorophyta, so that in some systems of classification they have 

 been placed outside the Chlorophyceae. 



Of the 3 families which comprise this order ( Zygnemataceae, Me- 



sotaeniaceae, Desmidiaceae*) only the first is considered in this 



book. 



FAMILY ZYGNEMATACEAE 



Characteristics as described for the order; cells short- or long- 

 cylindric with plane or folded end walls, usually unbranched and 

 without basal-distal differentiation; chloroplasts few, large, and, 

 except for one genus, with conspicuous pyrenoids. 



•See appendix for desmid genera. 



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