FUCUS VfiSICULOSUS. 303 



many of which cling to their bare surface (F). They attach 

 themselves obliquely by the pointed end and a portion* of the long 

 side, so that the hinder cilia remains free, and for some time can 

 be in motion. If a sufficient number be present, they can give 

 to the oosphere a rolling motion. This phenomenon is noticeable 

 in the processes of fertilisation of various sections of the animal 

 kingdom, e.g., in the Echinoderms, the Actinia, and Vermes ; but 

 is known amongst plants nowhere else than in the Fucaceae. 

 The movement lasts from ten to twenty minutes, then the 

 oosphere comes to a rest ; a spermatozoid has penetrated and 

 fertilised it. A cell-wall is then formed around the fertilised 

 oosphere, or oospore. If fertilised oospores are kept in a watch- 

 glass with sea-water, at the second, or, latest, the third day, 

 the first divisions of the oospore can be made out. Unfertilised 

 oospheres usually fall quickly to the bottom, without covering 

 wall or division. As antheridia and oogonia are found in the 

 same conceptacle, fertilisation of the oosphere by spermatozoids 

 of like origin can often result ; but, nevertheless, fertilisation 

 by spermatozoids from remote conceptacles is by no means 

 excluded, and may be facilitated by the fact that the spermato- 

 zoids are usually evacuated before the oospheres' of the same 

 conceptacle, and often when the latter are evacuated the former 

 are already swarming. 



Fucus vesiculosus. For the study of the sexual processes, 

 Fucus vesiculosus is still more favourable than F. platycarpus, and 

 it is even more common. The structure of the sexual organs is 

 much as in the latter plant, but one sex only, either antheridia or 

 oogonia, is found in the conceptacles of any plant (dioecious), 

 Plants hung up empty their sexual organs after a few hours. 

 The drops of mucilage which contain the spermatozoids are 

 recognisable, even to the naked eye, from their orange-red colour, 

 while those which contain the oogonia are coloured olive-green. 

 If a little of the orange-red mucilage is placed in a drop of sea- 

 water, this will usually be seen almost immediately to be filled 

 with actively moving spermatozoa. Thoroughly healthy sperma- 

 tozoa are fairly sensitive to light, and nearly always avoid it 

 (i.e., are negatively heliotropic, or apheliotropic), so that even 

 with slight illumination they usually collect at the room side, 

 rarely at the window side, of the drop. With strong light their 

 movement is fairly rectilinear, in the direction of the incident 

 rays. Single spermatozoa from time to time stop suddenly, and 



