TH A LLOPH Y TA : A WA E 



59 



accomplished by the degeneration of all its nuclei except one, as some 

 observers have claimed, or, as others contend, by the passage back into 

 the filament of all but one nucleus before the wall is formed at the base 

 of the oogonium. Arising close to the oogonium is a longer and more 

 slender branch, its curved tip being cut off by a wall to form an anther- 

 idium. In some species both kinds of sex organs are borne on the same 

 branch, the antheridium being terminal and surrounded by two or more 

 oogonia (Fig. 47) . The antheridium produces many small biciliate sperms 

 that are liberated into the water through a terminal pore. A sperm 



*#5^^^ 





.$, 





'^ 





^ 



. *'■■ 



Fig. 47. Sex organs of Vaucheria 

 geminata, two oogonia and an anther- 

 idium borne on the same branch, 

 X250. 



Fig. 48. Thallus of Codium fragile, one- 

 half natural size. 



enters an oogonium through a terminal pore in its wall and fuses with the 

 egg to produce a heavy-walled zygote. After remaining dormant, it 

 gives rise to a new filament directly. The reduction of chromosomes 

 probably occurs during germination of the zygote. 



Codium. Codium is a widely distributed marine alga that grows on 

 rocks between tide lines. The thallus is dark green and spongy, consist- 

 ing of thick cylindrical branches composed of a dense mass of interwoven 

 filaments (Fig. 48) . It is anchored by means of a basal disk-Uke holdfast. 

 Like other members of the order, the vegetative body is without cross 

 walls. The cytoplasm is peripheral and has numerous small nuclei and 

 chloroplasts. There is no asexual reproduction by means of spores. 

 Two kinds of gametangia are produced, generally on different plants. 

 They arise on the sides of large club-shaped branches that form a sort of 

 cortex, and are cut off by a basal wall. The male gametangium, which is 

 smaller than the female one, hberates many thousands of biciliate male 

 gametes. In the female gametangium some of the nuclei degenerate, 

 while others enlarge. Several hundred biciliate female gametes are 



