ZYGOSPORES. 



121 



Two small branches from the vegetative threads, or my- 

 celium, unite end to end (Fig. 238, /) ; continue to grow 

 larger (Fig. 238, 77) ; the terminal portion of each becomes 

 separated by a partition wall (Fig. 288, 777) ; the common 

 wall between these cell-ends becomes absorbed (Fig. 238, 

 IV) ; their contents fuse ; a firm, thick cell-wall is secreted ; 

 and thus is formed the zygospore (Fig. 238, F), or rest- 

 ing-spore, as it is sometimes called, for it often remains 

 quiescent and retains its vitality for a great length of time. 

 i6i. The Hydro- 



dictyon, or Water-net, 

 a fresh-water Alga, is 

 a representative of the 

 division, Zygosporese. 

 It sometimes occurs in 

 abundance in ponds 

 and slow streams. It 

 is composed of a multi- 

 tude of cells, arranged 

 so as to form a tubular 

 net, whose meshes in the full-grown plant — when it may 

 be eight or ten inches long — are plainly visible to the 

 naked eye. It reproduces by the protoplasm of certain 

 cells breaking up into several thousand small masses, or 

 daughter-cells, which presently unite bo as to form a minia- 

 ture net ; the wall of the mother-cell becomes absorbed, 

 and the little plant is set free. It is said also to repro- 

 duce by means of swarm-spores. The Desmids (Desmi- 

 diacem) are motile unicellular fresh-water Algse (Figs. 

 239, 240), belonging also to this division. The cells are 



Fig. 238. Successive stages (7, 11, III, IV, V) in the fonnation of a zygospore ia 

 case of Mucor siolonijera. 



