LAMINARIALES (HETEROGENERATAE) 169 



of large cells, and these are' surrounded by cortical cells which 

 become smaller and smaller towards the periphery, the outermost 

 layer giving rise to the branched hairs. 



The unilocular sporangia are on slightly raised portions of the 

 thallus and develop from cortical cells which undergo scarcely any 

 modification. Meiosis takes place in the sporangium, and the ripe 

 zoospores escape in a mass and germinate to give rise to dioecious 

 filamentous gametophytes which are heterothallic. The smaller 

 male plants produce terminal antheridia from each of which is 



Fig. 114. Desmarestia. A, plant with summer and winter appearance ( x ^). 

 B, apex showing cortication. C, transverse section stipe. D, female gametophyte. 

 = oogonium. E, male gametophyte. a = antheridium, e = empt>' antheridium. 

 F-J, stages in seedling germination. (A, after Newton; B, C, after Oltmanns; 

 D-J, after Schreiber.) 



liberated a single antherozoid, whilst the larger female plants 

 produce the sw^ollen oogonia. Each oogonium gives rise to a single 

 ovum which escapes, but as fertilization and germination take place 

 just outside the pore of the oogonium the young sporophyte develops 

 as far as the monosiphonous stage whilst still possessing a primitive 

 holdfast in the shape of the empty oogonium. Cortication, which is 

 best observed near the apex of old plants, commences in the young 

 plants after a few weeks, and further growth is maintained by an 

 intercalary growing zone some way behind the apex. It is only just 

 recently that the real life history of this genus has been established, 



