362 



Conjugate 



In some species of the genus Pleurot&nium, such as PL trochiscum 

 (fig. 227 A) and PL doliiforme (fig. 227 B), the cell-wall possesses large 

 internal pits. These are thin places in the cell-wall and except near the 

 poles of the cell are mostly of a rectangular shape. 



Every Desmid, like every Diatom, consists of two halves, one of which is 



Fig. 226. A, part of colony of Cosmocladium constrictum (Arch.) Josh., x 475. B, colony of 

 C. pulchellum Breb., x 475. (7, zygospore of C. perissum Hoy & Biss., x 475. D F, 

 Oocardium stratum Nag. x730; F shows the nature of the chloroplasts. G, Cosmocladium 

 saxonicum De Bary showing disposition of pores in cell-wall, x 1200. H, pore-organs and 

 mucous secretion in Cosmarium turgidum Breb. ; the upper figure is in optical section ; 

 x 1600. I, section of cell-wall of Xanthidium armatum (Breb.) Rabenh. showing the pore- 

 organs and secreted mucus, x 1600. J, apex of semicell of Tetmemorus granulatus (Brb.) 

 Ealfs showing the pore-organs, x 800. (D J, after LutkeiniUler ; G J, specimens treated 

 with fuchsin and potassium acetate.) chl, chloroplast ; o, outer layer; i, inner layer of 

 cell wall; py, pyrenoid; a, front view; b, end or vertical view; c, side view of semicell. 



at least one generation older than the other, and may be many generations 

 older. These halves are technically known as the ' semicells.' In the Placo- 

 derm Desmids the wall is composed of two pieces joined at the isthmus, the 

 suture consisting of the overlapping bevelled edges of the two halves ; but in 

 the Saccoderm Desmids the wall is all in one piece and no suture is discernible 



