CRYPTOGAMS 



323 



Sexual reproduction is effected either by the fusion of piano- 

 gametes (p. 319), or the sexual cells are differentiated as non-motile 

 egg-cells and motile spermatozoids. 



Ulothrix zonata, almost everywhere abundant in fresh water, may serve as a type 

 of the isogamous Conferwideae. The filaments of Ulothrix exhibit no pronounced 

 apical growth ; they are unbranched, attached by a rhizoid cell, and consist of single 

 rows of short cells (Fig. 243, A). 

 Each cell contains a nucleus and one 

 band -shaped, green chromatophore 

 in the form of an almost complete 

 hollow cylinder. Asexual repro- 

 duction is effected by means of 

 swarm -spores (1-8), which have 

 four cilia (C), and are formed by 

 division in any cell of the filament. 

 The swarm - spores escape through 

 a lateral opening (S) formed by 

 absorption of the cell wall, and, 

 after swarming, give rise to new 

 filaments. The sexual swarm-cells, 

 or planogametes, are formed in a 

 similar manner by the division of 

 the cells, but in much greater 

 numbers. They are also smaller, 

 and possess only two cilia. In other 

 respects they resemble the swarm- 

 spores, and possess a red eye-spot 

 and one chromatophore. By the 

 conjugation of the planogametes in 

 pairs, zygotes {F-S) are produced, 

 which, after drawing in their cilia, 

 round themselves off and become 

 invested with a cell wall. After 

 a shorter or longer period of rest 

 the zygotes are converted into uni- 

 cellular germ plants (J), and give 

 rise to several swarm -spores (K), 

 which in turn grow out into new 

 filaments. Ulothrix, like many fila- 

 mentous Algae, passes into a so- 

 called Palmella stage, in which, 

 under certain conditions, the sepa- 

 rate cells of the filaments give rise by 

 division to colonies of cells. The individual rounded cells thus produced have often 

 been mistaken for species of Trotococcoideae. In this manner, according to Chodat, 

 is formed the common Pleitrococcus vulgaris, which occurs as the green covering on 

 the trunks of trees, and consists of round cells which multiply by division, in 

 which, however, the formation of swarm-spores has been suppressed in the course 

 of adaptation to an aerial mode of existence. In its unicellular condition, accord- 

 ing to Chodat, the cells are round, and multiply by division ; they either remain 



Fig. 243. — Ulothrix zonata. A, Young filament with 

 rhizoid cell r ( x 300) ; B, portion of filament with 

 escaping swarm-spores ; C, single swarm-spore ; D, 

 formation and escape of gametes ; E, gametes ; F, G, 

 conjugation of two gametes ; H, zygote ; /, zygote 

 after period of rest ; K, zygote after division into 

 swarm-spores. (After Dodel-Port, B-KX4S2.) 



