BROWN ALGAE 



385 



A, B, st.). In Sarcophycus, however, the stalk is branched, and a succession 

 of oogonia may be produced, as is seen in the antheridia of Fucus. The 

 antheridial hair of Fucus may start precisely in the same way as the oogonium, 

 the terminal cell forming the antheridium. But the growth does not stop 

 there : the stalk-cell shoots out laterally and produces another antheridium, 

 and the process may be continued with irregular sympodial repetition (Fig. 324, 

 I.). This finds its biological explanation in the need for continued supply 

 of numerous spermatozoids, so as to secure fertilisation over a prolonged 

 period. The large number is further ensured by the divisions in each cell 

 being continued to 6 \. 



ft 1 



me? 



FIG. 325. 



Fucus. A, oogonium, the contents of which have divided into eight eggs. B 

 oogonium, from which the contents (C) have been extruded. D, E, liberation of 

 the eight eggs ; st = stalk ; ntes = middle end inner layers of the oogonial wall. 

 (After Thuret.) (From Strasburger.) 



The solitary oogonia, with their few ova, find their biological elucidation 

 in the facts that the ova are large, and have a strong chemctactic influence 

 on the motile spermatozoids. Their size gives a high degree of certainty 

 of successful germination if once fertilised. Their attractive influence secures 

 a high probability of fertilisation, notwithstanding their immobility, which 

 has followed on increase in size. But Fucus is not the last term of the series 

 of reduction of the oogonium. In Ascophyllum only four ova are matured in 

 each, in Pelvetia two, and in Himanthalia and others only one. Vestigia 

 of the atrophied eggs are found, which clearly indicate that their number 

 has been reduced. Thus the Brown Algae form a coherent series of sexual 

 differentiation. Their sexual cells probably originated from motile zoospores, 

 all alike. The first functional though not formal distinction of sex is seen in 

 Ectocarpus siliculosus (Fig. 321), and confirmed by difference of size of the 

 gametes in E. secundus (Fig. 322). Steps in loss of motility are seen in these 

 plants, and in Cutleria (Fig. 323), while in Fucus the large ovum is entirely non- 

 motile (Fig. 325). Finally, in Himanthaila it occupies the whole oogonium : 

 B.B. 2B 



