226 SEXUAL REPRODUCTION (FUCUS) 



desiccation and contraction. The sea-water dissolves the mem- 

 branes still enveloping the sexual cells (Fig. 120, K), and the ova, 

 which have now assumed a spherical form, become fertilised by the 

 actively moving spermatozoids. The oospore secretes a thin mem- 

 brane and immediately, without a resting period, develops into 

 a new Fwcws-thallus. The young plant is at first spherical, but at 

 an early stage produces the basal holdfast (Fig. 120, J) and acquires 

 a strap-shaped form, and this is soon followed by branching. 



In Pelvetia, where both sexual organs occur in the same 

 conceptacle, the oogonium has an exceptionally thick wall and 

 produces only two eggs. Extrusion of the sexual cells takes 

 place in the same way as in Fucus, but the ova retain their 

 thick mucilaginous investment which the spermatozoids have 

 to penetrate, and which envelops the young plant during the 

 early stages of development. 



Fucus and Pelvetia differ from most other oogamous plants 

 in the number of eggs and in the fact that the latter are fertilised 

 outside the plant, in both of which respects these Algae appear 

 relatively unspecialised. Normally the female organ contains 

 but a single ovum (cf. (Edogonium, Vaucheria, and the higher 

 plants), and in this connection it is interesting to note the re- 

 duction of the eggs to two in Pelvetia, though here also the 

 nucleus divides into eight parts, six of which abort. 



The Conjugate (cf. p. 208) owe their name to a very special 

 type of sexual reproduction (conjugation) , in which neither gamete 

 is free-swimming. In the filamentous forms, such as Spirogyra 

 and Zygnema, two threads become ranged parallel to one another, 

 and their opposing cells develop finger-like protrusions which 

 grow towards each other till they meet and fuse (Fig. 122, B) ; 

 after this the separating wall breaks down, so that an open tube 

 (the conjugation canal, Fig. 122, c.c.) is established. In 

 Spirogyra and many species of Zygnema the development of 

 processes always commences a little sooner on the one filament 

 than on the other, and a similar difference is observed with 

 respect to the contraction of the protoplasts which now ensues. 

 The cells of the filament that first put out processes, in these 

 cases, act as males, since their contents commence to glide over, 

 through the conjugation canals, into the opposite cells (Fig. 

 122, B, b), with whose passive (female) protoplasts they fuse. 



