42 MANUAL OF BOTANY 



some being microscopic while others have a plant body several 

 hundreds of feet in length and of considerable thickness. In 

 the simplest cases the body is unicellular, or forms a colony 

 resembling that of many of the Cyanophyceae. The colony may 

 be attached or not. In other cases the plants, which vary a 

 good deal in size, are attached to rocks by specially differentiated 

 roots, or by haptera. Epiphytic forms occur, filamentous 

 patches being seen upon other Algae ; these are the free por- 

 tions of the epiphyte, rhizoidal appendages having penetrated 

 the tissues of the host. 



The brown colouring matter, which is a mixture of phyco- 

 phaeine and phycoxanthine, is localised in definite plastids or 

 chromatophores, instead of being diffused through the proto- 

 plasm of the cells. The anatomical or histological differentia- 

 tion is sometimes considerable, a great variety of tissues being 

 found in tlie thallus. 



Asexual reproductive cells are found in some of the members 

 of this sub-class, which may be spores or gonidia ; sometimes 

 these are motile, sometimes not. Some members of the group 

 possess only sexual reproductive cells. 



The sexual organs are sometimes antheridia, containing 

 antherozoids, and oogonia, producing oosjpheres. Fertilisation 

 never takes place in the oogonia, the gametes of both sexes being 

 discharged with the water. The antherozoids are ciliated, the 

 cilia being two in number, attached at the side of the gamete 

 and pointing one towards each end. 



More frequently the gametes are alike, or nearly so, when the 

 organ producing them is called a gametangium. They are then 

 always ciliated. 



The oogonium is always unicellular and produces one or 

 several oospheres ; the antheridium may be unicellular or 

 multicellular. In the latter case each cell produces only a 

 single antherozoid. 



There is a good deal of variety in the morphological differ- 

 entiation of the group. Some show only a thallus ; others 

 present the appearance of root and shoot, and in many the latter 

 is distinctly leafy. In some, special branches bear the reproduc- 

 tive organs. 



Alternation of generations can only be traced in some forms ; 

 the sporophyte phase is generally absent ; it is represented in 

 some of the Cutleriaceae, though sometimes the two forms are 

 very similar in appearance and can only be distinguished by 

 noticing what kind of reproductive cells they respectively produce. 



