210 - Multicellular Plants 



do not adhere to any standard type of life 

 cycle. The commonest type of cycle is ex- 

 hibited by Spirogyra (p. 168), and Ocdo- 

 gonium (Fig. 12-8). In both these algae, the 

 colony is composed of haploid cells; and any 

 cell of the filament can give rise to one or 

 more gametes. Accordingly, these plants are 

 equivalent to the gametophyte generation in 

 the higher plants. However, scarcely any 

 sporophyte generation can be recognized, be- 

 cause the zygote divides meiotically as soon 

 as it begins to germinate, and all the cells of 

 the new filament are haploid (Fig. 12-8). 



The opposite type of cycle is found in 

 Fucus (Fig. 1 2-9) and some other brown algae. 

 The body cells of this multicellular seaweed 



are all diploid, and only the gametes are 

 haploid. The sperm and eggs respectively are 

 formed by meiosis in hollow structures found 

 at the ends of many of the branches of the 

 sexually mature plants. After fertilization, 

 which occurs when the gametes are extruded 

 into the sea water, the diploid zygote multi- 

 plies mitotically, forming all the cells of the 

 new individual. 



An irregular alternation of generations, 

 foreshadowing the reproductive habits of 

 higher plants, occurs in a few Thallophyta — 

 as in the bread mold, Rhizopus (p. 211). The 

 spores of the mold are haploid, and in a 

 suitable medium, such as moist bread, each 

 haploid spore grows into a mycelium, called 



Fig. 12-9. Rockweed (Fucus) growing on intertidal rocks along the California 

 coast. (From Trie Plant World, by Fuller and Carothers. Holt, Rinehart and 

 Winston, Inc.) 



