WISCONSIN PHYTOPLANKTON 15 



The ligature-like constriction lies about a tenth of the distance from 

 the apex to the semi-cell base end and is the chief distinction between 

 the variety and the type. This constriction is variable in depth and 

 is much more prominent in some specimens than in others. The in- 

 terlocking of the apical tubercules is a very constant feature and the 

 cells are united in filaments that have a considerable degree of per- 

 manence, in fact if a break occurs in a filament it is more commonly 

 at the isthmus than at the point of contact between adjacent cells. 

 This union of cells to form filaments is of frequent occurrence in the 

 tropics but is very rare in temperate regions. The alga was found 

 in sufficient abundance to warrant the assumption that it can multi- 

 ply under plankton conditions. 



3. Pleurotaenium Ehrenbergii (De Brebisson) De Bary. PI. 54, 



Figs. 5-8. 



Unters. u. d. Fam. d. Conj. 75. 1858; W. & G. S. West, Monogr. Brit. Desm. 

 1: 205, pi. 29, figs. 9-11; pi. 30, fig. 1. 1904. 



Cells fairly large, length 15-30 times the breadth at the base, 

 slightly constricted, sinus a shallow depression ; semicells cylindrical, 

 with lateral margins slightly attenuated, the convergence being more 

 pronounced in the apical region ; basal inflation conspicuous and with 

 a single undulation above it that looks like a second basal inflation; 

 apex truncate and with angles slightly rounded, apex with a sub- 

 apical ring of 8-9 (5 generally visible in front view) conical papillae 

 with very blunt apices. Chloroplasts parietal, numerous, forming 

 undulate longitudinal bands; with several pyrenoids. (Tychoplank- 

 tont.) 



Zygospores spherical to subspherical, with smooth walls (W. & 

 G. S. West). 



Cells 350-650 /x long; 25-35 /^ broad at base of semicells, 15-24 /* 

 at apices ; isthmus 23-30 fi broad. 



Dutchmans (rrr). Little Doctor (rrr), Meta (rrr), Muskallonge (rrr). Oak 

 (rrr). Sand (rrr), Speese (rrr). 



The organism is subject to considerable variation in size, in relative 

 proportions between length and breadth as well as in the apical at- 

 tenuation. In typical specimens there is but a single undulation 

 above the basal inflation but it is not at all uncommon to have more 

 than one of these inflations. Specimens like those shown in Figures 

 7-8, with the undulations extending half way to the apex from the 

 bases of the semicells, approach the variety undulatum. 



var. UNDULATUM Schaarschmidt. PI. 54, Fig. 9. 



Magyar. Tudom. Akad. Math. s. Term6szettud. Kozelemenyek 18: 278, pi. 1, 

 fig. 21. 1882 (Eef. Nordst. Index Desm.) ; W. & G. S. West, Trans. Linn. Soc. 

 2nd. Ser. Bot. 6: 145, pi. 18, fig. 28. 1902. 



Lateral margins gently undulate from basal inflation to the semi- 

 cell apex. (Tychoplanktont). 



