416 



BOTANY 



PART II 



and the asexual generation (sporophyte) do not, however, show differences in 

 structure. 



Order 4. Laminariaceae ( 46 ) 



The regular alternation of generations of these plants, which are the largest of the 

 Brown Algae, corresponds with that in Ferns in that the gametophyte in contrast to 

 the sporophyte is very small. The large sporophyte bears club-shaped or cylindrical 

 sporangia forming an extensive superficial layer. Each surface cell of the thallus is 

 prolonged as a club-shaped sterile cell or paraphysis beside which the sporangia arise 

 as shorter cells. The reduction division takes place in the sporangia. From the 



biciliate swarm-spores minute fila- 

 mentous male and female gameto- 

 phytes develop (Fig. 358). The 

 former are abundantly branched, while 

 the latter consist of few cells and in 

 extreme cases may be reduced to a 

 single cell. The male gametophytes 

 bear the antheridia beside or behind 

 one another at the tips of the branches. 

 Each antheridium gives rise to one 

 spermatozoid. Any cell of the female 

 gametophyte may form an oogonium, 

 from an opening at the summit of 

 which the naked egg -cell emerges. 

 This remains in front of the opening 

 and after fertilisation (which has not 

 been observed) proceeds to grow into 

 the young sporophyte (Fig. 358 E. F]. 

 The oogonia and antheridia are homo- 

 logous with the gametangia of the 

 Phaeosporeae. 



PASCHEII has observed on culti- 

 vated young sporophytesof Laminaria. 

 saccharina an extremely early pro- 

 duction of sporangia. Thus the sporo- 

 phyte, which is usually of large size, may under particular external conditions 

 undergo profound reduction in size. A point of view is thus attained from which 

 the striking dimorphism of the two generations may be explained. 



Order 5. Fucaceae ( 47 ) 



Asexual reproduction is wanting in this order, while sexual reproduction is 

 distinctly oogamous. The oogonia and antheridia of Fucus are formed in special 

 flask-shaped depressions termed CONCEPTACLES, which are crowded together below 

 the surface in the swollen tips of the dichotomously-branched thallus (cf. F. scrratus, 

 Fig. 352). The conceptacles of F. platycarpus (rig- 359) contain both oogonia and 

 antheridia, while F. vesiculosus, on the contrary, is dioecious. From the inner wall of 

 the conceptacles, between the oogonia and antheridia, spring numerous unbranched 

 sterile hairs or PARAPHYSES, some of which protrude in tufts from the mouth of the 

 conceptacle (Fig. 359 p). The antheridia are oval in shape, and are formed in 

 clusters on special short and much-branched filaments (Figs. 359 a, 360, 1). The 

 contents of each antheridium separate into sixty-four spermatozoids, which are dis- 



a o o 



FIG. 359. Fucus platycarpus. Monoecious con- 

 ceptacle with oogonia of different ages (o), and 

 clusters of antherid ia (a) ; p, paraphy ses. ( x circa 

 25. After THURET.) 



