316 



BOTANY 



1. 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 intertangled green filaments. The filaments exhibit no dis- 

 ii ij tinction of base and apex, and are composed of 



simple rows of cells, which vary in length in 

 different species. Growth results from the divi- 

 sion and elongation of the cells in one direction 

 only (<-/. Fig. 65, p. 64). Each cell has a large 

 nucleus situated either in the peripheral proto- 

 plasm or suspended in the centre of the cell by 

 protoplasmic threads extending from the parietal 

 protoplasm. The name of the genus, Spirogyra, 

 is due to the peculiar spiral form of its green 

 band-like chromatophores. These spiral bands lie 

 in the parietal protoplasm, and contain numerous 

 pyrenoids (p. 71). In Pig. 235 is represented a 

 species with three such spiral chromatophores ; in 

 other species their 

 number is some- j[ 



times less, some- 

 times more. The 

 chromatophores in 

 '"■ the other genera of 

 the Zygiiemareae ex- 

 hibit a variety of 

 ? form ; thus, in the 

 filaments of Zyg- 

 nema the chromato- 

 \\ phores are star- 



shaped. 



Fig. 235. —Cell from a /S„,t TT t« . mT „„ • 



filament of Spirogyra ; CONJUGATION, in 



i; nucleus ; eft, chro- the case of Spirogyra, 

 matophore; p, pyre- j s preC eded by the 

 noid. (X200.) , r , , J „ 



development of con- 

 verging lateral processes from the cells 

 of adjacent filaments. "When two pro- 

 cesses from opposite cells meet (Pig. 

 236, A), their walls become absorbed at 



, . . . . . , . , , , Fio. 236.—^, Conjugation of Spirogyra 



the point of contact, and the whole ,,»„„«( X240). B , Spirogyra Imgata 



protoplasmic contents of one cell, after (x iso); *, zygospore. 

 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 and form a zygospore invested with a thick wall, and filled 

 with fatty substances and reddish-brown mucous globules. It is the 



M, 



