208 



PART IV. CLASSIFICATION. 



by a number of the still motile male planogametes (Fig. 155), one of which 

 fuses with it. In Cutleria the larger planogamete soon comes to rest, and 

 then one of the smaller planogametes fuses with it. In Ectocarpus silicu- 

 losus the zygospore gives rise to a plant which resembles its parents : it 

 has been observed that, if the planogametes fail to conjugate, they are 

 capable of germinating independently. 



The Phseosporese are almost exclusively marine, the only fresh-water 

 forms being the genus Pleurooladia (Ectocarpacese) and two species of the 

 genus Lithoderma. The size of the plants included in this series varies 

 widely, from microscopic Ectocarpacese to gigantic tree-like Laminariacese, 

 such as Macrocystis, Nereocystis, etc., which may attain a length of 

 geveral hundred feet. In some of the 

 Laminarias, which have cylindrical 

 stalk-like region in their thalloid 

 shoot, secondary growth in thickness 

 takes place by means of a merismatic 

 layer. In these large forms too, the 

 conducting tissue is sometimes so far 

 developed as to form sieve-tubes; 

 though no woody tissue is developed, 

 nor is it required in view of the fact 

 that these plants live submerged. 



Series PH^EOGAM^E. The orders 

 comprised in this group are character- 

 ised by the oogamous sexual process. 



Order Fucaceae. Body differ- 

 entiated into root and shoot; shoot 

 usually thalloid, either cylindrical or 

 flattened ; differentiated into stem and 

 leaves in Sargassum ; growth in length 

 by a single apical cell; branching 

 generally dichotomous. No asexual 

 production of spores. Sexual organs, 

 unicellular antheridia and oogonia; 

 spermatozoids, ciliated, formed several 

 together in the antheridium : odspheres, 

 set free but not ciliated; one (Py- 

 cnophycus, Himanthalia), two (Pel- 

 vetia), four (Ascophyllum), or eight 

 (Fucus) formed in each oogonium. 

 Marine. 



The body consists of what may be termed cortical and medullary tissue. 

 The cortical tissue consists of closely-packed parenchymatous cells, the 

 external layer of which, the limiting layer, is for a time merismatic, and 

 plays an important part in the growth of the body. The medullary tissue 

 consists of filamentous rows of cells the walls of which are mucilaginous 

 and much swollen. The cortex is essentially the assimilatory tissue and 

 the medulla the conducting tissue. 



Fio. 155. Sexual process in Ecto- 

 carpus siliculosus : I a-/, female piano- 

 gamete coming to rest: II resting 

 female planogamete suspended from 

 the surface of the water, with numerous 

 motile male planogametes : III con- 

 jugation of a male and a female 

 planogamete. (x790: after Berthold.) 



