266 



PART III. THE CLASSIFICATION OF PLANTS. 



the Khodophyceas, becomes altogether illusory ; for in the Dictyotaceae these 

 spores are developed on the sporophyte, whilst the " tetraspores " of the 

 Khodophyceae are gonidia developed by the gametophyte. All marine. 



Order 10. Fucaceae. Body differentiated 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 gonidia or of spores, and therefore no 

 alternation of generations. Sexual organs, unicellular antheridia and oogonia ; 

 spermatozoids, ciliated, formed several together in the antheridium ; oospheres, 

 set free but not ciliated ; one (Pycnophycus, Himanthalia, Halidrys, Sargassum, 

 Cystoseira), two (Pelvetia), 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 ex- 

 ternal 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. 



In some genera (e.g. Fucus vesiculosus, Ascophyllum, Halidrys, Cystoseira, 

 Sargassum) there are large intercellular spaces, filled with air, which project on 



the surface, and are known as 

 air-bladders ; they serve as 

 floats. In Halidrys and Sar- 

 gassum the air-bladders are 

 borne on special branches. 



The sexual organs are in 

 all cases borne in depressions 

 of the surface known as con- 

 ceptacles (Fig. 189.) The 

 conceptacles are commonly 

 confined to special portions of 

 the thallus ; either to the tips 

 of the branches (e.g. Fucus, 

 Cystoseira) or to special 

 branches, the gametophores 

 (e.g. Himanthalia, Ascophyl- 

 lum, Sargassum). From the 

 inner surface of the concep- 

 tacle there arise a number 

 of hairs (paraphyses) among 

 which the sexual organs are 

 borne. The oogonia (Fig. 190) 

 are nearly spherical, and are 

 borne on a short stalk con- 

 sisting of a single cell ; the 

 antheridia (Fig. 190) are the lateral branches of some of the hairs. The plants 

 may be monoecious (e.g. Fucus platycarpus, Halidrys, Pelvetia, Cystoseira, 



FIG. 192. Himanthalia lorea (much reduced), a 

 Vegetative part of body ; b branched gametophore. 



