THALLOPHYTA 



395 



mucilaginous envelope. Their cell walls rupture at the constriction, and parting 

 in half allow the protoplasts to escape ; these then unite to form a zygospore. 

 The zygospores frequently present a very characteristic appearance, as their walls 

 are often beset with spines (Fig. 326 C). The four empty cell halves may be seen 

 close to the spore. In some Desmidiaceae the conjugating cells undergo a pre- 

 liminary division, the daughter cells uniting in pairs. 



The two sexual nuclei in the zygote do not fuse until germination of the latter 

 is about to commence. The resulting nucleus then undergoes division, presum- 

 ably with reduction, into four nuclei, two large and two small. Only two cells 

 are formed from the zygote, each of which has thus two nuclei of different sizes ; 

 the smaller nuclei disappear (Fig. 327). The production of two cells on germina- 

 tion thus appears to be derived from the division into four cells seen in Cylindro- 



8. .4, Conjugation of Spirogyra quinina (x 240). B, Spirogyra longata(x 150) ; z, zygospoie. 

 <_', Cell of Spirogyra jugoJis ; k, nucleus ; ch, chromatophores ; p, pyrenoid (x 256). 



cystis, and to stand to the latter as a reduced form. Two of the four chloroplasts 

 in the zygote disappear, while the other two undergo a division before germination. 

 3. Zygnemaceae. In this family, all of which are filamentous in character, 

 the genus Spirogyra, with its numerous species, is the best known. It is 

 commonly found in standing water, forming unattached masses of tangled green 

 filaments. The filaments exhibit no distinction of base and apex, and are 

 composed of simple rows of cells, which vary in length in different species. 

 Growth results from the division and elongation of the cells in one direction 

 only. Each cell has a large nucleus and one or several spiral green band-like 

 chromatophores (Fig. 328 (7). The cells of Zygnema contain two star-shaped 

 chromatophores. The cells of the filament may separate under certain circum- 

 stances. The cell wall is smooth and without pores. The filaments can undergo 

 movements. 



