84 



PLANT MORPHOLOGY 



rounds itself with a cell wall and divides at once to produce a new vege- 

 tative thallus. The reduction of chromosomes occurs when the nucleus 

 of the young antheridium and that of the young oogonium divide. Thus 

 from the four-nucleate stage to maturity the sex organs are haploid, the 

 diploid condition arising at fertilization. 



Although Fucus has no alternation of gametophyte and sporophyte 

 plant bodies, there is a brief haploid phase and a prolonged diploid phase. 

 Some botanists interpret the vegetative body of the Fucales as a sporo- 

 phyte, the antheridia as microsporangia, 

 and the sperms as microspores (small 

 zoospores). They interpret the oogonia 

 as megasporangia and the eggs as mega- 

 spores (large aplanospores). Then, to 

 explain the sexual fusion, the microspores 

 and megaspores are said to function di- 

 rectly as gametes. This interpretation 

 implies that a gametophyte generation 

 was once well developed and has become 

 so reduced that it comprises only the 

 haploid nuclei in the gametangia and 

 the gametes themselves. 



Sargassum. This is a very large genus 

 whose 250 species are widely distributed 

 throughout tropical and subtropical seas, 

 especially in the Southern Hemisphere. 

 The vegetative body is more highly de- 

 veloped than that of Fucus, having 

 distinct branches, leaf-like blades, and 

 often small stalked air bladders as well (Fig. 72). Sargassum may live 

 either in an attached or a floating condition. Like other rockweeds, it 

 grows chiefly along seacoasts, but frequently plants are torn loose from 

 the rocks and carried for hundreds of miles out to sea. The Sargasso Sea 

 is a vast eddy lying west of the Canary Islands. Here great floating 

 masses of "gulfweed," transported by the Gulf Stream from the West 

 Indies and tropical America, accumulate and propagate themselves 

 by fragmentation of the thallus. 



Summary. The Fucales have a coarse, ribbon-like thallus that grows 

 by means of an apical cell. Spores are not formed. The order displays 

 well-developed heterogamy. The sex organs are unicellular, the anther- 

 idia producing numerous biciliate sperms, the oogonia producing one, 

 two, four, or eight nonmotile eggs that escape before fertilization. The 

 sex organs are borne in internal cavities (conceptacles). The Fucales are 

 without a distinct alternation of generations. 



Fig. 72. Small portion of a plant 

 of Sargassum, showing differentiation 

 into stem, leaf-like blades, and 

 berry-like air bladders, natural size. 



