106 FRESH- WATER ALG^ OF THE UNITED STATES. 



SYNZOOSPOBE^E et ASEXUALES. 



Sexual fertilization through the agency of spermatozoids and 

 oospheres does not take place in plants under this head ; cases 

 of copulation between two zoospores occurs now and then ; the 

 product is a zygospore (isospore according to Bostafinski), which 

 often lies dormant a long time before developing a young plant. 

 The more frequent propagation is asexual by zoospores which 

 are developed within the thallus, sometimes as microzoospores 

 with two cilise, and sometimes as macrozoospores with four cilire. 



Family VIILCONFERVACE^]. 



Chlorophylous green plants, usually filamentous, simple or 

 branched ; more rarely forming a menibranaceous thallus of a 

 single stratum. 



Subsection A. ULVACE^E. 



Membranaceous or foliaceous plants, rarely crustaceous, formed 

 of a single layer or stratum of cells, expanded, plane or crisped, 

 tubulose or vesiculose. 



Genus 14, PBASIOLA, Ag. 



Thallus at first filamentous, not unlike TJlothrix, then expanded, 

 leaf-like, plane, smooth or crispate, composed of angular cells ; 

 base sometimes loosely fibrillose. Vegetation by division of the 

 cells in two directions. 



* 



PRASIOLA CRISPA, Kg. 



Tufts dark green, usually dense, soft and elastic ; leaf-like 

 thallus plicately crisped, of variable size and form, often 

 bullate, 2-6 mm high and about the same breadth ; cells 

 oblong or quadrate arranged in distinct areolas, sometimes 

 confluent, cells 4-6 ^ diameter, often twice as long. 



Have seen very few specimens. Had them from damp 

 ground, rather foul places. In Europe the plant appears to 

 occur frequently and often in patches widely expanded. 



Syn. Ulva crispa, Hass. ; Ulva lactuca, var. Huds. ; Ulva terrestris, 

 Gray.; Praslola Rothii. ; P. orMcularis; P. Flotowii, Kg. 



Plate XCI, fig. 25, a frond, natural size ; figs. 26, 27, two 

 small portions of fronds with fibrillose margins, enlarged 

 about 400 diameters. 



