THE GREEN ALGAE OF NORTH AMERICA 105 



exceed 10 p.. The larger forms are distinguished from Z. cruci- 

 atum chiefly by the form of the spores, ovoid rather than glo- 

 bose ; also by the somewhat coarser marking of the membrane. 

 2. SPIROGYRA Link, 1820, p. 5. 



Cells cylindrical, once to many times as long as broad ; dis- 

 sepiments either smooth and even or with ring-like projections ; 

 chromatophores one or more in a cell, in the form of parietal, 

 more or less spirally bent, broad or narrow bands, each contain- 

 ing several pyrenoids ; nucleus in the middle of the cell. Con- 

 jugation lateral or scalariform ; spore formed in one of the two 

 conjugating cells; median membrane colored, smooth or pitted ; 

 germinating spore producing a more or less clavate filament. 



A genus of many species, of world-wide distribution ; the 

 specific distinctions not always clear, based on the character of 

 the dissepiments, the number and breadth of the chromato- 

 phores, the size and form of the spore, the character of its 

 median membrane, the inflation of the fertile cell, and, too often, 

 the dimensions of the filaments ; this last character being too un- 

 certain to have much weight, except when associated uniformly 

 with other characters. The size and shape of the spore is of 

 more importance, but it must be kept in mind that a change of 

 position in the cell may give a totally different aspect to the 

 spore when seen under the microscope. An ovoid spore 

 appears circular when its axis is in the line of vision ; and a 

 lenticular spore varies from circular to a quite narrow oval, 

 according to the angle at which it is seen. Good plates are of 

 the utmost use in determining species of Spirogyra ; the best 

 work available is Petit, 1880. Of the 3.7 species there figured, 

 32 are given in the following pages, and there is no reason why 

 the remaining five should not be expected in America. In* ad- 

 dition to the zygospores, parthenospores are sometimes formed, 

 the usual process not extending to the union of the tubes from 

 the two cells ; spores formed in one or both of the filament cells. 

 Aplanospores occasionally formed, as in Zygnema. 



The following key may be of assistance in determining 

 species, but in a rather general way. 



KEY TO THE SPECIES OF SPIROGYRA. 



r. Cell conjugating directly, not by a tube. 38. 5. stictica. 



i. Cell emitting a tube. 2. 



