176 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



H. RETICULATUM (I,.) Lagerheim, 1883, p. 71 ; Kiitzing, 

 1856, PL XXXV; Wolle, 1887, p. 169, PI. CUV, figs. 11-20; 

 P. B.-A., No. 65. Cells usually several diameters long, length 

 up to i cm., 100-200 /u, wide ; families i or 2 dm. long ; zoospores 

 to 20,000 in a cell, gametes to 100,000. Fig. 62. Generally 

 distributed in fresh water. Europe. 



A very attractive plant in appearance, the net-like frond be- 

 ing found from microscopic size to one or two dm. in length, the 

 cells and meshes varying correspondingly. 



2. PEDIASTRUM Meyeu, 1829, p. 772. 



Colonies unattached, disk-shaped, of round or star-shaped 

 outline, continuous or perforate, composed of marginal cells of 

 different shape from the interior cells ; cells multinucleate, with 

 net-form parietal chromatophore and one pyrenoid ; asexual 

 reproduction by biciliate zoospores, which escape from the cell 

 enclosed in a vesicular coating, within which they arrange 

 themselves to form a new colony ; or by the formation of a new 

 colony in a cell, without intervention of zoospores ; sexual re- 

 production by smaller gametes, more in a cell, copulating to 

 form an irregular Polyedrium-like zygote, within which a new 

 colony is formed, in the same way as in a cell of the mother 

 colony. 



Species of this genus are common in fresh water plankton, 

 and in most gatherings of miscellaneous algae from quiet water 

 in warm weather. Their regular disks, continuous or open- 

 work, with variously toothed or spiny marginal cells, are inter- 

 esting objects ; but while types of the species seem clearly dis- 

 tinct, in practice many intermediate forms will be found. 



A rather conservative course has been followed as regards 

 recognizing species ; but it should be noted that Chodat, 1902, 

 p. 224, includes as forms of P. Boryanum, the species P. ford- 

 patum, P. vagum and P. angulosum ; this may sometime be fully 

 justified, but for the present it will be more convenient to retain 

 them as species. 



KEY TO THE SPECIES OE PEDIASTRUM. 



i. Marginal cells undivided, cuspidate. 8. 



i. Marginal cells bilobed. 2. 



2. Lobes of marginal cells simple. 3. 



2. Lobes of marginal cells emarginate, bidentate or bifid. 7. 



3. Disk continuous. 4. 



3. Disk perforate. , 7. P. duplex. 



