264 PART IV. CLASSIFICATION. 



Asexual reproduction is effected by means of spores, either zoo- 

 spores (as in the Phseosporese), or non-motile spores (as in some 

 Phseogamte). The spores are developed either singly, or more 

 commonly several together, in unicellular (and also necessarily 

 unilocular) sporangia. 



Sexual reproduction is either isogamous or oogamous : when 

 isogamous, it may be effected by aplanogametes (Diatomacese), 

 but more commonly by planogametes (Phseosporese) which usually 

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

 Cntleriacese) 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 sexual plants may be monoecious 

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

 gametangia, sometimes unicellular (Diatomacese) but more com- 

 monly multicellular (Phseosporese) : 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, Cutleriacese) 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 antheridium 

 is sometimes multicellular, but it is unicellular in the Fucacese ; 

 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 

 and the planogametes contain chromatophores, and have two cilia 

 inserted laterally ; the spermatozoids, however, have no chromato- 

 phores, nor have the smaller planogametes in those cases in which 

 the conjugating planogametes differ in size ; the oosphere has no 

 receptive spot. 



The following groups of the Phseophycese will be considered : 

 Unicellular Forms : 



Order Diatomaceae: sexual reproduction isogamous by aplanoga- 

 metes. 



Multicellular Forms : 



Series (a) PHJEOSPORE^E : sexual reproduction isogamous by planoga- 

 metes ; asexual by zoospores. 

 Order Ectocarpaceae (Ectocarpus, Sphacelaria, Cladostephus, etc.). 



