BROWN ALGAE 



385 



A, B. sf.). In Sarcophycus, however, the stalk is bran. hod. and ;i succi-»sion 

 of oogonia may be produced, as is seen in the anthrridia of lucus. The 

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

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

 tliere : 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 prohmgetl 

 period. The large number is further ensured by the divisions in e;»cli cell 

 being continued to 6 <. 



Fig. 325. 



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



oogonium, from which the contents (C) have been e.xtruded. 1), H, liberation of 



the eight eggs ; s/ = stalk ; «;«^middU^ — ^nrf = inner layers of the fmgonial w.dl. 



(After Thuret.) (From Strasburger.) 



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

 in the facts that the ova are large, and have a strong chciiy tactic inrtucncc 

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

 of successful germination if once fertilised. Their attractive influence .•wxures 

 a high probability of fertilisation, notwithstanding their immobility, which 

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

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

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

 of the atrophied eggs are found, which clearly indicate that their numU-r 

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

 difterentiation. Their sexual cells probably originated from motile roos{K:)res. 

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

 Ectocarpus siliculosus (Fig. 321), and confirmed by diflerence of sire ol the 

 gametes in E. secundiis (Fig. 322). Steps in loss of motilitv are seen in thcv 

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

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



n.H. 2 15 



