362 



Conjugate 



In some species of the genus Pleurottenium, 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., x475. B, colony of 

 C. pukhellum Brb., x475. (7, zygospore of C. perissum Koy & Biss., x475. D F, 

 Oocardium stratum Nag. x 730 ; 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 Br6b. ; the upper figure is in optical section ; 

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

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

 Ealfs showing the pore-organs, x 800. (D J, after Liitkemuller ; 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; &, 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 



