THE GREEN ALGAE OF NORTH AMERICA 207 



into the sea, occupying the space between high and low water 

 marks ; so that twice each day its medium is changed from sea 

 water to quite fresh water, and back again. It appears to be 

 common from Maine to New Jersey, and is found at salt springs 

 at Sussex, New Brunswick, 20 miles from the sea. 

 5. MONOSTROMA Thuret, 1854, p. 13. 



Frond at first a closed tube or sac, which later opens or splits, 

 forming a membranous expansion, of a single layer of cells, ex- 

 cept at the base, where it is thickened, and may consist of sev- 

 eral layers of elongated cells. Two- or four-ciliate zoospores 

 and biciliate gametes formed in any of the cells of the monostro- 

 matic part, issuing through an opening at the surface of the 

 frond. 



In some of the species of this genus the saccate form has not 

 been observed, but it probably occurs in all. Its persistence 

 varies much, from M. latissimum, in which the frond forms a 

 flat expansion when only two or three mm. high, to M. groen- 

 landicum, in which the greater part of the frond continues tubu- 

 lar through its whole life, only the upper part opening at the 

 time of the formation of the spores. Some of the smaller species 

 do not exceed i dm. in length; others, like M.fuscum, may 

 reach 5 dm. It has representatives in all oceans, and several 

 species inhabit by preference brackish water ; one species lives 

 in fresh water exclusively, and some of the marine and brackish 

 species occasionally occur also in fresh water. 



KEY TO THE SPECIES OF MONOSTROMA. 



i. Frond always tubular. 2. 



i. Frond tubular only in an early stage of growth. 3. 



2. Tube filiform. i. M, groenlandicum. 



2. Tube intestine-like, collapsing. 



4. M. arcticum var. intestiniforme. 

 3. Frond saccate till plant is well developed : then splitting part or all 



of the way to the base. 4- 



3. Frond saccate only in the early stages or not at all. 5. 



4. Segments rather broad, irregularly divided. 2. M. Grevillei. 



4. Segments narrowly linear, simple, forked or palmate. 



3. M. Lactuca. 



5. Frond dark to dull green, not adhering to paper, n. M, fuscum. 

 5. Frond light or bright green, adhering generally to paper. 6. 



6. Mature frond divided into distinct segments. 7. 



6. Frond broadly lanceolate to orbicular, not divided into segments. 



8. 



