194 



THE 



parts, which duplicate one another even in the details of pro- 

 toplasmic structure, and the nucleus lies in the middle region, 

 with the chromatophores arranged symmetrically in the halves. 



The desmids multiply rapidly by 

 cell division, each daughter cell 

 taking one half of the old cell 

 wall and adding to it a repro- 

 duction of the other half (Fig. 

 186, B). The gametes are naked 

 protoplasts, which escape hy the 

 breaking apart of the halves 

 of the cells and fuse in pairs, 

 forming thick-walled zygospores 

 (Fig. 186, A). In some common 

 forms (Closterium and Cosma- 

 rium) the zygospore on germi- 

 nating produces two desmids. 



225. The pond scums. Some 

 of the commonest and most 

 beautiful of filamentous green 

 algae, such as Spirogyra and Zyg- 

 nema, belong here. The com- 

 plex chromatophores with their 

 A, Spirogyra, illustrating stages in the q i, flrT .i v diff PTPn Hatpd nvrpnoidd 

 conjugation between cells of differ- S ar P 1V C a P Vr( 



ent filaments, two zygospores shown give an especial interest to the 



above; since the cells in the filament n j i i j- A - i_ 



on the left are shorter than those on cells and hel P to distinguish 



the right, some of them must be left the genera. Thus the chroma- 



out in the pairing of the gametes. -, f a i 



B, the conjugation between adjacent tophores of Spirogyra are spiral 



cells of the same filament in Spirogyra bands (FigS. 168,187); Zygnema 

 quadrata; C, Zygnema (Zygogonium) , , , , ' 



pectinatum, showing zygospores has two star-shaped chromato- 



formed in the conjugating tubes be- phores, and Mougeotid has a 

 tween two filaments . , . . , .. . . 



broad, thin band in the center 



of the cell. The method of sexual reproduction is very charac- 

 teristic, but exceptional among the algae. Generally the cells of 

 different filaments unite or conjugate with one another by the 



FIG. 187. Pond, scums 



