Hijdrodictyacece 



211 



and conjugate in pairs, the zygospore being polyhedral in form. 

 A new coenobium arises by the segmentation of the contents of 

 this polyhedral body. 



The species of this genus exhibit great variation in the 

 characters both of their central and marginal cells, the length of 

 the marginal processes, etc. ; especially is this the case in cultures. 

 Their natural habitat is in small ponds and ditches amongst other 

 water-plants, and not uncommonly in quiet bog-pools. Sometimes 

 they are numerous in the freshwater plankton. 



The two most abundant species are P. Boryanum (Turp.) Menegh. (fig. 85 

 F H and J L) and P. tetras (Ehrenb.) Ralfs (fig. 85 C and D), the former 

 sometimes reaching a relatively large size (200 p. in diameter). The marginal 

 processes of P. Boryanum are extremely variable and the cell-walls are often 

 granulated. P. tetras generally occurs in coenobia of 4 (diam. 10'5 18 /*) or 

 8 (diam. 22 29 /n) cells. P. duplex Meyen 1829 (fig. 85 E) ( = P. pertusum 

 Kiitz. 1845) is also a widely distributed species. P. simplex Meyen ( = P. 

 clathratum Lemm.) and P. integrum Nag. (fig. 85 A) are much rarer species. 

 P. Boryanum and P. duplex are the most abundant species in the plankton. 



Genus Euastropsis Lagerh., 1894 1 . In this genus the 

 coenobium is free-floating and consists of two flattened cells, 



Fig. 86. Euastropsis Richteri (Schmidle) Lagerh. A and B, from near Senens, 

 Cornwall ( x 760). C E, showing formation of young coenobia (after Lager - 

 heim ; highly magnified). 



which are closely applied along their straight inner margins, the 

 outer margins being widely notched. The entire coenobium has a 

 certain resemblance to a minute species of the genus Euastrum, 

 and was originally described as such 2 . There is a single parietal 

 chloroplast in each cell containing one pyrenoid. Lagerheim 

 describes the presence of one nucleus in each cell, but suggests 

 that there may be more. Reproduction occurs by oval, biciliated 



1 Lagerh. in Tromso Museums Aarshefter 17, 1894, pp. 1221, t. i, f. 827. 



2 Schmidle in Flora, 1894, p. 60, t. 7, f. 25. 



142 



