SPtROGYiU. 205 



This section may be subdivided into two parts, the Zygneminae, 

 and the Mesocarpinae. 



SUBSECTION I. ZYGNEMINJE. 



Chlorophyl arranged in three ways ; in bands on the inside of 

 the cell walls, in twin stellate nuclei, and in axillary laminae. 



The process of copulation is the union of two filaments ; they 

 come into close proximity, then the cells put out short tubercle- 

 like projections by which the cells of one unite with the cells 

 of the other filament ; the ends open and thus a tubular passage 

 is formed between each of two cells of a longer or shorter series 

 of cells. Such copulation is termed scalariform. In some forms 

 the cells become geniculate and unite at the angle ; this mode is 

 termed geniculate copulation. A third mode is termed lateral 

 copulation, signifying that the contents of two adjoining cells 

 unite, producing the zygospore. In the other two modes the 

 chlorophylous cell-contents pass out through the connecting 

 tube from one cell to unite with that of the other, or through an 

 opening at the point of junction to mingle with that of its mate, 

 thus producing the zygospore; sexual distinction between the 

 two filaments conjugated, has not been observed. 



Genus 70, SPIBOGYBA, Link. 



Cells with one to several parietal chlorophyl bands spirally 

 winding to the right. Copulation scalariform (Spirogyra, Kg.) 

 or lateral (Rliynclionema, Kg.). Zygospores always within the 

 wall of one of the united cells. Copulating cells usually shorter 

 than the sterile ones, and often more or less swollen. 



More recent observations have proved that the two modes of 

 copulation are not constant, that both ways may be found in the 

 same filament, according to circumstances, hence Rhynchonema 

 is omitted as a generic character. The most essential and 

 reliable characters are derived from the zygospore. The length 

 and breadth of the cells, the character, and number of spirals, 

 are features not to be overlooked, but in themselves not sufficient 

 to determine species. 



One of the most recent and complete monographs of Sfpirogyra^ 

 embracing almost all of our species is by Mons. Paul Petit, 

 Spirogyra des Environs de Paris, 1880. 



In accord with his arrangement of the species of this genus, 

 the following division is made of our United States species : 



