CONJUGATAE 



1 1 



filament into shorter lengths. The second method is by the sexual formation 

 of spores, which are called zygospores or zygotes. In order that this 



may take place it is necessary for 

 two filaments to lie parallel to one 

 another and not very far apart. 

 Then short protuberances are put 

 out from adjacent cells of each, 

 which meet together between the 

 filaments. At the point of junction 

 the adjacent walls break down, and 

 through the opening the contents 

 of one cell pass over into the other 

 cell ; the united mass then surrounds 

 itself with a fairly thick wall, thus 

 constituting the zygospore, which 

 generally remains dormant for some 

 time before germinating to form a 

 new Spirogyra plant. The whole 

 process is known as conjugation ; 

 hence the name of the class. 



Closely related to Spirogyra are 

 the beautiful unicellular algae called 

 Desmids, one of which (Staurastrum) is illustrated in Plate 2, Fig. H. 



Desmids are chiefly remarkable for their diversity of form and their 

 wonderful symmetry. All are microscopic, some large enough to be, although 

 with difficulty, visible to the naked eye. The cells are often beautifully 



5. Spirogyra. A. Copulation of Sp. 

 quinina ; z, zygospore, 240/1. B. Sp. 

 longata, 150/1. C. Sp. juga/is ; k, nu- 

 cleus ; ch, chromatophore ; />, pyrenoids. 

 250/1. (From Strasburger, Textbook.) 



Fig. 6. Cosmarium Botrytis Menegh. 1—3, stages of division ; 4, temporary membranes 

 of two sister-cells shed during the formation of the final cell-wall. (After De Bary) 



ornamented with spines, ridges, etc., and are usually symmetrical about three 

 planes. In one of these planes, called the equatorial plane, there is a median 

 constriction known as the isthmus. Each cell contains a central nucleus 

 in the isthmus, and each semi-cell contains one, or sometimes more than one, 

 bright green chloroplast, usually in the form of a thick axile band, with 

 radiating ridges. 



