214 TUFTS COLLEGE; STUDIES, VOL. II, No. 3 



gin more or less undulate; membrane 7-10 ju. thick, cells quad- 

 rate in cross section, 5-8 yu, high. Greenland (?), Mass., Wash. 



Arctic Sea. 



This species has the most delicate frond of all the genus, 

 except M.pidchrum, and grows in spring below low water mark 

 or in pools. It was founded by Kjellman on specimens found 

 unattached, without basal part ; Rosenvinge, 1893, p. 944, 

 identifies with it a plant from Greenland, with a filiform, tubu- 

 lar stipe of varying length. No such stipe has been seen in the 

 specimens from Massachusetts and Washington ; hundreds of 

 specimens of all stages of growth have been examined, without 

 finding any indications of a stipe. The structure of our plant 

 agrees with Kjellman's description and figures, and his name is 

 here retained ; it may be that the Greenland plant is distinct 

 from that of northern Europe.* 



4. ULVA Linnaeus, 1753, p. 1163. 



Frond membranaceous, flat, consisting of two layers of cells, 

 in any of which, except those in the thickened base, zoospores 

 or gametes may be formed, issuing through an opening in the 

 surface of the frond. Marine. 



KEY TO THE SPECIES OF ULVA. 



i. Frond divided into distinct segments. 3. U.fasciata. 



i. Frond entire or irregularly lobed or laciniate. 2. 



2. Frond minute, triangular or reniform, with distinct stipe. 



2. U. californica. 

 2. Frond ample, at first lanceolate, later of no definite outline. 



i. U. Lactuca. 



i. U. LACTUCA Linnaeus, 1753, p. 1163; Thuret and Bor- 

 net, 1878, p. 5, Pis. II, III ; U. Lactuca var. Lactuca Farlow, 

 1 88 1, p. 43. Frond very variable in shape, at first attached and 

 generally of a lanceolate or ovate-lanceolate form ; later of irreg- 

 ular shape, and often detached and floating. The cells usually 

 vertically elongate in cross section ; seen from the surface 

 irregularly angular, cfesely set ; thickness of the frond very 

 variable. Fig. 75. 



*There have been several reports of M. thermalis from interior points 

 of the United States, and in some cases the determination was confirmed 

 by J. G. Agardh. But the latter, 1882, p. no, expresses considerable 

 doubt as to whether what he understands as M. thermalis, is the same as 

 the Ulva thermalis of Meneghini, 1837, p. 21. All the American speci- 

 examined, including several in Herb. Farlow, prove to be species of 

 Enteromorpha. 



