46 Fresh-water Biology 



51 — Chloroplasts numerous, parietal, each with a pyrenoid (fig. 24), Cladophorales 



Cladophora 



— Chloroplasts single or several, large and of some definite shape, with pyrenoids. 



The entire contents of two cells unite to form a single zygote Conjugatae* 52 



52 — Thallus a thread of many similar cells, each a zygospore, produces only one germ 

 plant. Zygnemaceae. Chloroplasts of spiral bands (fig. 17), Spirogyra; chloroplast 

 consists of two stellate bodies for each cell (fig. 20) Zygnema; chloroplast an axial 



plate (figs. 18, 19) Mougeotla 



— Unicellular, rarely bound together in a loose thread. Desmidiaceae (see page 46). 



53— Organism yellowish green HETEROKONTAE . .54 



— Organism grayish or brownish or amber colored 55 



54 — Plants unicellular Chlorobotrys, Ophlocytlum (fig, 15) 



— Plants filamentous, cell wall firm, splitting into H-shaped pieces (fig. 31) Tribonema 

 ( = Conferva) 



55 — Unicellular organisms consisting of 2 silicious valves. BACILLIARIACEAE 



( = Diatoms) (See page 48) 



—Cells neither silicious nor 2 valved. (See PI. 22.) PHAEOPHYCEAE. A part of this 

 group is included by the zoologists in the class MASTIGOPHORA among the 

 Protozoa. (See page 54.) 



DESMIDIACEAE 



1 — Cell wall apparently not divided into 2 parts, and without pores 2 



— Cell wall showing 2 segments, and with a differentiated outer porous layer 4 



2 — Cells elongate, cylindrical and not constricted, forming loose filaments. Cell wall 

 with a differentiated outer layer, of which the small roughnesses and Spines form 

 a part. Chloroplasts axile (fig. 1), Gonatozygon; chloroplasts parietal and spirally 



twisted (fig. 16) Genicularia 



— Cells solitary, relatively short and mostly unconstricted 3 



3 — One chloroplast in each cell. Chloroplast spirally twisted, axile or parietal (fig. 2) 



Spirotaenia; chloroplast plane, axile, cells solitary (fig. 10) Mesotaenium 



— Two chloroplasts in each cell. Chloroplasts star shaped radiating from a central 

 pyrenoid (fig. 25), Cylindrocystis ; chloroplasts ridged with longitudinal serrated 

 ridges (fig. 12) Netrium 



4 — After division the cell remains free and solitary 5 



— After division the cells remain attached to form colonies 9 



5 — Cells not constricted. Cells of moderate length, straight, cylindrical (figs. 17, 23), 

 Penium; cells amost cylindrical, scarcely attenuated, chloroplasts single, Roya; 

 cells strongly attenuated towards each extremity, two chloroplasts in each cell 



(figs. 5, 6, 7, 8) Closterlum 



— Cells more or less constricted at the middle 6 



6 — Cells elongated and cylindrical, constriction slight. Base of semi-cells plicate (figs. 

 3, 4), Docidium; base of semi-cells plane (fig. 20), Pleurotaenium; apices of cells 



cleft, apical incision narrow (fig. 19) Tetmemorus 



— Cells relatively short, deeply constricted 7 



7 — Cells in vertical view radiating, triangular, quadrangular or radiate, rarely fusiform 



(figs. 13, 14) Staurastrum 



— Cells compressed (at right angles to the plane of the front view), in the vertical view 

 fusiform or elliptical 8 



"The 2 orders Schizogoniales and Microsporales are not represented in our plancton, 



{Continued on page 48) 



