GROUP I. THALLOPHYTA : ALGM : PILEOPHYCEjE. 



263 



sporangia, borne on distinct individuals ; each sporangium sets free four 

 spores (tetraspores), which are destitute of a cell- wall and of cilia. The sexual 

 organs are unicellular oogonia and multicellular antheridia ; the oogonia are 

 generally in groups (sori), and each gives rise to a single oosphe re, which is set 

 free as a naked unciliated cell ; the antheridia are always in groups (sori), and 

 give rise to a large number of small, apparently non-motile, spermatozoids, 

 which have no chromatophores. The process of fertilisation has not been 



FIG. 188. Fucus vesiculosus, about half nat. size : b air-bladders; / fertile branch. 



observed. The male and female organs are either borne on the same plant 

 (e.g. Padina), or on distinct plants (Dictyota, Taonia). 



The tetraspores and the oospores germinate alike. The spore divides into 

 two cells, one of which grows out into the filamentous primary root, the other 

 grows out directly into the shoot in Dictyota and Zonaria ; but in Taonia, 

 Padina, and Dictyopteris, the development is heteroblastic, since the latter cell 

 gives rise to a rounded multicellular embryo, from which the adult shoot grows 

 out as a branch. In the embryonic shoot of Taonia and Dictyopteris there is 

 a single apical cell, but eventually it gives place to a number of initial cells. 



