256 PART III. THE CLASSIFICATION OF PLANTS. 



forms are known, but it is not clear whether the asexual forms are 

 sporophytes or are merely potential gametophytes : in the Cut- 

 leriacese, however, it is highly probable that the asexual form is 

 really a sporophyte, and this view may, for the present, be ex- 

 tended to the other more doubtful cases. 



In all but the lowest forms there is a distinction between re- 

 productive and vegetative cells, the former developing into more 

 or less highly differentiated reproductive organs. 



A sexual reproduction is effected by means of spores (or possibly 

 gonidia), either zoospores (as in Syngeneticae and Phaeosporeas) or 

 non-motile spores (as in Tilopteridaceas and Dictyotaceae). The 

 spores are developed either singly, or more commqrily several to- 

 gether, in unicellular (and also necessarily unilocular) sporangia. 



Sexual reproduction is either isogamous or oogamous : when 

 isogamous, it may be effected by apian ogametes (Diatomaceae), 

 but more commonly by planogametes (Pheeosporeas) which usually 

 resemble each other ; but in some cases (e.g. species of Ectocarpus, 

 Cutleriaceae) they are of two kinds, differing in size and in the 

 duration of their movement, the one which is smaller and more 

 active being the male : when oogamous, it is effected by means of 

 spermatozoids and oospheres, and is peculiar in that the oospheres, 

 though not ciliated, are extruded from the female organ before 

 fertilisation takes place. The gametophytes may be monoecious 

 or dioecious. The sexual organs, in the isogamous forms, are 

 gametangia, sometimes unicellular (Diatomaceae) but more com- 

 monly multicellular (Phaeosporeae) ; in the latter case each cell of 

 the gametangium gives rise either to a single planogamete or 

 to several : they are in most cases all alike, though some (e.g. in 

 species of Ectocarpus, Cutleriaceae) consist of smaller and more 

 numerous cells than the others and give rise to the smaller 

 planogametes. In the oogamous forms, the oogonium is unicel- 

 lular, giving rise to one or more (2-8) oospheres : the antheri- 

 dium is multicellular in the Tilopteridaceee and Dictyotaceae, but 

 unicellular in the Fucaceae ; in the former case each cell gives rise 

 to a single spermatozoid, in the latter several spermatozoids are 

 developed in the one cell. 



Of the motile reproductive cells of this sub-class, the zoospores 

 (or zoogonidia) and the planogametes, contain chromatophores, 

 and have two cilia inserted laterally ; the spermatozoids, however, 

 have no chromatophores, nor have the smaller planogametes in 

 those cases in which the conjugating planogametes differ in size ; 

 the oosphere has no receptive spot. 



