1920] BetcJiell-Gardner : Chlorophyceae 235 



18. Monostroma Tlinr. 



Frond at the beginning a closed sack or tube, at times splitting 

 very early or again retaining the saccate or tubular shape until late, 

 in almost all cases, however, finally becoming a flattened or crisped 

 membrane of a single layer usually parench>Tnatous but occasionally 

 of gloeocapsoid cells, except at the base where thickening occurs by 

 the descent of elongated rhizoidal cells forming several layers; vege- 

 tative multiplication by gemmation or proliferation, non-sexual repro- 

 duction by 2- or 4-ciliated zoospores and sexual reproduction by 

 2-ciliated isoplanogametes all originating in unchanged cells; zygote 

 usually germinating immediately. 



Thuret, Note sur la Syn. des Ulv., 1854, p. 13. 



The genus Monostroma comprises those members of the Ulvaceae 

 which, at maturity, form an expanded membrane of a single layer 

 of cells. Certain species of Monostroma develop nearly to full size as 

 sacks which then split open by one or more slits and become expanded 

 membranes. There are still other species, however, which seem to 

 consist of expanded membranes of a single layer of cells almost, if not 

 quite, from the beginning. We have some reason to suppose that these 

 species are tubular or saccate only in their very youngest stages, 

 splitting early and becoming, therefore, one-layered almost from the 

 beginning. 



In some of the species of Monostroma the cells are closely placed, 

 with thin, or even thicker, firm walls, giving a parenchymatous appear- 

 ance, while in others the intercellular substance is ample and more 

 or less gelatinous causing the cells to stand off from one another, 

 usually in small groups (2-4 or more) after the fashion of Gloeocapsa 

 or Chroococcus. The relative sizes of the cells in different dimensions, 

 as well as the abundance or scarcity of intercellular substance, added 

 to the size, development, and shape of the frond, furnish characters 

 which may be used for the separation of the species. 



The species of Monostroma are marine, and also found in brackish 

 and fresh water. At times what appears to be the same species may 

 be found in both salt and fresh water. The thirty-five species credited 

 to the genus are known only with certainty from the Northern Hemi- 

 sphere, where they occupy, for the most part, the colder waters, intrud- 

 ing into warmer zones only in winter and spring when the temperature 

 of the water is lowered. 



