48 



with oxide of iron, and the margins may be smooth or undulate. 

 The multiplication is by longitudinal division or by the formation 

 of globose resting-cells (or cysts) which are furnished with a 

 peculiar projecting process (fig. 7 B and C). The cells occur singly 

 or joined into dense, spreading colonies. The daughter-cells effect 

 a lodgement above the inner rim of the mother-receptacle and 

 then secrete a similar receptacle for themselves. Senn 1 has written 

 a good account of this genus, and Lemmermann 2 has published a 

 monograph of it, discriminating between fourteen species. 



Three species, D. Sertularia Ehrenb. (fig. 7 C), D. sociale Ehrenb. and 

 D. cylindricwm Imhof, and varieties of them, are abundant throughout the 

 British Isles, the first-named one being the most widely distributed. Species 

 of this genus are very abundant in the freshwater plankton, the colonies of 

 each species exhibiting a characteristic type of branching. D. protuberans 

 Lemrn. and D.' elongatum Imhof are generally distributed but not abundant. 



Family 4. PHJEOCAPSACE^E. 



The plants are unicellular, forming colonies, the cells of which 

 are embedded in a mass of mucilage. The cells are spherical or 

 ellipsoidal and division takes place in all directions. The repro- 

 duction is by zoogonidia and zoogametes. 



Genus Phseococcus Borzi, 1892 3 . The cells are ellipsoidal or 

 oblong-ellipsoidal, 6 11 /u, in diameter, 

 and occur in twos, fours, eights or mul- 

 tiples of these numbers, in hyaline 

 gelatinous integuments which some- 

 times show a delicate concentric struc- 

 ture. There are two yellow-brown 

 chromatophores in each cell and usually 

 a red pigment-spot. The zoogonidia 

 are ovoid or subpyriform. 



Fig. 8. PhcKOcoccus palu- 

 dosus West & G. S. West, from 

 Eldwick, W. Yorks. ( x 410). z, 

 zoogonidia. 



P. dementi (Menegh.) Borzi has not been 

 observed from Britain, but P. paludosm West 

 & G. S. West occurs in moorland ditches. 



1 Senn, 'Flagellata' in Engler and Prantl Natvirl. Pflanzenfam. I Theil. la Abth. 

 '- Lemmermami, in Berichte Deutsch. Botan. Gesellsch. 1900, Bd xviii, pp. 500 

 524, t. xvii u. xix. 



a Borzi in Atti del Congr. Botan. Internaz. Geneva, 1892, pp. 463 471, t. xviii. 



