346 



BOTANY 



The conjugation of the protoplasts takes place, in the case of the Desini- 

 diaceae, outside their cell-walls. Two cells approach each other, and surround 

 themselves with a mucilaginous envelope. Their cell-walls rupture at the con- 

 striction, 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. 271 C). The four empty 

 cell halves may be seen close to the spore. 

 In some Desmidiaceae the conjugating 

 cells undergo a preliminary 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 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. The pro- 

 duction of two cells on germination thus 

 appears to be derived from the division 

 into four cells seen in Mcsotaenium, and to 

 stand to the latter as a reduced form. 



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 differ- 

 ent 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. 273 C), 

 Fio. 272.-Oosfcrti<i. A, Zygote before ger- The cells of Zygnema contain two star- 

 mination showing the two nuclei not yet gh d chromato hores . 

 united ; B, germinating zygote with the *L r . 



nuclei united ; C, division into two cells 



membrane; F, Closterium moniliforme , 

 mature plant. (A-E after KLEBAHX.) 



CONJUGATION, in the case of Spirogyra, 



each containing one larger and one smaller is preceded by the development of con- 

 nucleus ; D, further state of germination ; verging lateral processes from the cells 

 E, young plants escaping from the cell- of ad j acent filaments. When two pro- 

 cesses from opposite cells meet (Fig. 

 273 A), their walls become absorbed at the 

 point of contact, and the whole protoplasmic contents of one cell, after contracting 

 from the cell wall, passes through the canal which is thus formed into the opposite 

 cell. The protoplasm and nuclei of the conjugating protoplasts then fuse 

 together while the chloroplasts do not unite, but those of the entering protoplast 

 disorganise. The resulting cell forms the zygospore invested with a thick wall, 

 and filled with fatty substances and reddish-brown mucilage-spheres. This form 



