GROUP I. THALLOPHYTA : ALG& : PILEOPHYCEJ]. 



261 



celled gametangia are considered to be the male, and the large-celled the 

 female organs. The plants may be monoecious or dioecious (Cutleria). 



The zoospores and the planogametes are generally all very much alike : in 

 Cutleria, however, and in those species of Ectocarpus which have two kinds of 

 gametangia, the one kind of planogamete (female) is considerably larger than 

 the other (male), and has a shorter period of motility ; the smaller planoga- 

 metes are developed in the small-celled gametangia. A sexual process has been 

 observed in but few cases (Ectocarpns siliculosus, Scytosiphon lomentariu*, 

 Cutleria). In the two former the planogametes are externally similar, but 

 they behave differently in the process of conjugation, some coming to rest 

 earlier than others, thus indicating that they are female. When the female 

 planogamete is at rest, it is approached 

 by a number of the still motile male 

 planogametes (Fig. 187), one of which 

 fuses with it. In Cutleria the larger 

 planogamete soon comes to rest, and then 

 one of the smaller planogametes fuses 

 with it. In Ectocarpus siliculosus it has 

 been observed that, if the planogametes 

 fail to conjugate, they are capable of 

 germinating independently ; in Cutleria, 

 also, the unfertilised female planogamete 

 has been observed to germinate indepen- 

 dently; these interesting cases of parthe- 

 nogenesis indicate the incomplete de- 

 velopment of sexuality in this group. 



Inasmuch as the germination of the 

 zygospore has only been observed in the 

 case of Ectocarpus siliculosus and of the 

 Cutleriacese, it is only with reference to 

 these plants that any definite statement 

 can be made as to the life-history obtain- 

 ing in this group. In E. siliculosus the 

 zygospore gives rise to a plant which re- 

 sembles its parents, so that here there is 

 110 indication of an alternation of genera- 

 tions. In the Cutleriaceaa, on the other 

 hand, the zygospore gives rise to an 

 asexual form which is probably a true 

 sporophyte ; in the one genus, Zanar- 



FIG. 187. Sexual procpps in F.c^o- 

 carpus siliculosus : I a-/, female plano- 

 Kamete coming to rest : II resting 

 female planogamete suspended from 

 the surface of the water, with numerous 

 motile male planogametes : III con- 

 jugation of a male and a female 

 planogamete. ( x 790 : after Berthold.) 



dinia, this asexual form closely resembles the sexual ; in the other genus, 

 Cutleria, the asexual form is very different from the sexual in appearance, and 

 has long been regarded as a distinct genus under the name Aglaozonia. With 

 regard to the other genera of the group, it is probable that in those forms 

 (?..g. many Ectoc*rpaceae) in which the same individual bears at one time 

 sexual, and at another, asexual organs, there is no alternation of generations ; 

 and further, that in those forms (e.g. many Sphacelarieee) in which the 

 asexual and the sexual organs are never borne by the same individual, there is 



