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TEXTBOOK OF BOTANY 



has the reduced number of chromosomes, and spore formation (except 

 zygotes ) results only in vegetative multiplication of the gamete-bearing 

 phase of the life cycle of the plant. 



Fig. 289. Diagram illustrating the life history of a Laminaria. On the mature 

 thallus (A) an enlongated patch of sporogenous tissue develops, an enlarged 

 portion of which is represented in (B) with three sporangia from which motile 

 spores (C) later escape. Reduction division occurs during the formation of these 

 spores, and from one half of them male gametophytes (D) develop, and from 

 the other half female gametophytes. Sperms are liberated from the male gameto- 

 phytes and after swimming in the water unite with the egg cells in the female ga- 

 metophyte. From the zygote a young sporophyte grows at first attached to the 

 oogonium wall ( F ) , but later it is set free and ultimately becomes attached to some 

 other substrate (G). 



Covering the rocks in the intertidal zones are the widely distributed 

 "rockweeds" or "bladder wracks," represented on our coasts by Fiiciis 

 (Fig. 3). They occur in tropical, temperate, and arctic seas, but the 

 species or genera are usually characteristic in each zone. Most of the 

 genera grow permanently attached to the rocks, although certain species 

 of Sargassum are free-floating. Fucits is a dichotomously branched plant, 

 attached by holdfasts and kept afloat at high tide by bladder-like struc- 

 tures filled with air. It has no asexual propagation except by fragmenta- 

 tion, and has no alternation of spore-bearing and gamete-bearing phases. 

 The reproductive cycle is similar to that of animals. Spemis and eggs 

 are formed by the first cell division immediately following reduction 



