232 BRITISH DESMIDIACEJE. 



gelatinous material, which is secreted by a similar group of 

 pores in the middle part of the apex. 



Aplanospores have been observed in this species in material 

 from Burma (f. bidentula and tridentula) ; vide W. & G. S. West, 

 'Alg. Burma,' 1907, p. 225, t. 12, f. 11-15. Their formation is 

 preceded by the dissociation of the filaments into individual 

 cells. The cells then increase their volume by a growth in length, 

 becoming ultimately as long as broad. The chloroplasts soon 

 show signs of disintegration, and then the entire protoplasmic 

 mass assumes a spherical shape, becomes invested with a thick 

 cell-wall, and forms a globular aplanospore. The increase in 

 the length of the cell is often unequal, resulting in an asymmetrical 

 mother-cell. 



The zygospore is usually developed in the middle of the con- 

 jugation-tube between the two conjugating cells, but occasionally 

 it may be formed nearer to the one gametangium than the other, or 

 even entirely within one of them. In this case the gametangium 

 nearer to or containing the zygote is considered $ (cf. Desmidium 

 cylindricum, p. 251). 



Delponte has distinguished 2 distinct forms of H. dissiliens. 



A. var. minor Delp. Filaments usually naked, cells nearly 

 as long as broad or slightly broader. Length of cells 18 /< ; 

 breadth 21 ju. 



B. var. major Delp. Filaments usually with a sheath, cells 

 1 J times broader than long, or even broader still. Length 27'2 ^<; 

 breadth 36 //. 



The only British record for var. minor is Capel Curig, N. Wales, 

 and for var. major, Sutton Park, Warwicks., but it is almost 

 certain that both are more widely distributed. The sheathless 

 form of H. dissiliens does not commonly occur however. 



The vertical view of the species varies in appearance, in 

 consequence of which several forms are recognised. The typical 

 form has an exactly circular vertical view, without projections 

 of any kind, and is known as forma circularis Jacobs. ('Desm. 

 Danem.' 1875, p. 212). Formae bidentula and tridentula are as 

 follows :- 



Forma bidentula Nordst. (PL CLXI, figs. 20, 26.) 



H. dissiliens f. bidentula Xordst. Xorges Desm. 1873, p. 48, t. 1, f. 22 ; 

 Lagerh. Bidr. Amerik. Desm.-fl. 1885, p. 228 ; Boldt, Desm. Gronl. 1888, 

 p. 43 ; Nordst, Freshw. Alg. X. Zealand, 1888, p. 27 ; Hauptfl. Zellm. 

 u. Hiillgallerte Desm. 1888, p. 8 ; t. 1, f. 5, 9-13, 14, 18, 22, 23, 26-29 

 and 30 ; West, Freshw. Alg. W. Ireland, 1892, p. 116 ; AVest, Alg. Engl. 



