128 A TEXTBOOK OF THEORETICAL BOTANY 



a fresh diploid plant, like the parents in appearance, but on which eventually 

 tetrasporangia will be produced. 



It will be seen, then, that Dictyota exhibits a typical alternation of genera- 

 tions between a diploid tetrasporic plant on the one hand, which is regarded 

 as the sporophyte, and two haploid sexual plants, the male and the female, 

 which constitute the gametophytes. The life-history of Dictyota, then, may 

 be graphically represented by the diagram (Fig. 112). 



Laminariales 



The Laminariales are Phaeophyceae in which there is a marked alternation 

 of dissimilar generations in which the sporophyte is large and conspicuous 

 while the gametophyte is small and filamentous. 



Although species of the Laminariales are widely distributed in temperate 

 seas, the bulk of them occur in the colder waters of the North Pacific and 

 North Atlantic oceans. Several species are common just below low-tide 

 mark around the British coast. 



The thallus is large, sometimes exceptionally so, for the order includes 

 the largest of all the seaweeds, such as Lessonia and Macrocystis ; the latter 

 reaches 200 ft. in length and grows at a depth of 60 to 100 ft. In other 

 species, however, the thallus may be much smaller and more comparable 

 with other brown seaweeds. The largest British species is Saccorhiza biilbosa, 

 the " Sea Furbelows," which may grow up to 15 ft. in length. 



The gametophyte is small and filamentous, and has only been studied 

 critically in a few species. The sex organs are borne on separate plants and 

 consist of antheridia and oogonia. The former liberate antherozoids, but the 

 oosphere after fertilization remains within or attached to the oogonium. 

 The sporophytic plant may be either annual or perennial. 



We shall consider only one genus of this order — Laminaria. 



Laminaria 



There are a number of common British species, all of which agree in 

 general structure and life history, but as they difl^er from one another very 

 markedly in external form it will be desirable to outline briefly these difi^er- 

 ences at the outset so far as the common species are concerned. 



Laminaria saccharina (Tangles) (Figs. 113 and 114). — In this species the 

 thallus consists of an undivided frond with a wavy margin, arising from a 

 rather short, thin, round stalk, which is attached by a rhizoidal holdfast. 

 It is perennial, grows up to 6 ft. long and occurs from low-water mark to 

 a depth of 10 fathoms.* 



Laminaria digitata (Kelp) (Fig. 115). — In this species there is a smooth, 

 thick stalk which widens out gradually into a broad frond which is divided 

 palmately into a number of separate fingers. It is a considerably larger species 

 than the last and the degree of dissection of the lamina varies considerably. 



* A fathom = 6 feet. 



