4 6 



MORPHOLOGY 



In either case the zoospores produced are laterally biciliate (fig. 121) 



and develop new filaments directly. 



Gametangium. The gametangium is a many-celled structure, and, 



like the sporangium, is developed either from ordinary cells of the fila- 

 ment or at the end of a short lateral branch 

 (fig. 122). The small cells composing it are 

 cubical, being packed together closely, and each 

 one produces a biciliate, zoospore-like gamete. 

 Upon escaping, the gametes pair and fuse, and 

 a zygospore is formed. This gametangium is of 

 great interest on account of the variations that 

 occur, leading sometimes to uncertainty as to 

 whether the structure should be called a 

 gametangium or a sporangium ; in fact, it is 

 often called a plurilocular sporangium, in dis- 

 tinction from the true unilocular sporangium. 

 In some cases the gametangium is reported to 

 produce both zoospores and gametes, judged 

 by their behavior. The question might be 

 raised whether the so-called zoospores in this 

 case are not parthenogenetic gametes (see p. 

 40). Other cases are reported in which the 

 gametangium is said to produce motile cells of 

 three sizes, the medium-sized ones germinating 

 directly, and the largest and smallest ones 

 pairing. In this case, we should have not only 

 zoospores, but also dissimilar gametes. In 

 another case it is reported that the larger 

 gamete comes to rest and is fertilized in this 

 quiescent condition ; in which case heterogamy 

 would be very apparent. In the same genus, 

 therefore, there occur both zygospore and 

 oospore formation, and more or less confusion 

 of gametes and zoospores. The group would 

 seem to be an unusually favorable one for ob- 

 serving both sex origin and sex differentiation. 



FTG. 123. Laminaria: Sphacelaria. In general structure and reproductive 

 showing bladelike body, habits this form resembles Ectocarpus, but it may be 

 stipe, and holdfast. used to illustrate the appearance and powers of an apical 



