SIPHONALES 



93 



The width of these pores in the case of C. Bursa is said to vary with 

 the season. Detachable propagules develop on the vesicles and 

 form a method of vegetative reproduction, whilst sexual reproduc- 

 tion is by means of gametes, which are produced in ovoid game- 

 tangia that arise from the vesicles as lateral outgrowths, each being 

 cut off by a septum. The plants are anisogamous, the macrogametes 

 being formed in green and the microgametes in yellow gametangia. 

 Some of the species are dioecious whilst others are monoecious, 

 and in two of the latter the male and female gametangia are borne 

 on the same utricles. The gametes fuse or else develop partheno- 

 genetically, but in either case a single thread-like protonema 

 develops which has a lobed basal portion, and it is from this that the 

 adult develops through the growth of numerous ramifications of 

 the one primary filament. Meiosis occurs at gametogenesis and the 

 plants are therefore wholly diploid and comparable to Fucus (cf. 

 p. 192). At gametogenesis some of the nuclei in the gametangia 

 degenerate whilst the remainder divide twice. 



CoDiACEAE : Halimeda {Halimeda, daughter of Halimedon, King of 

 the sea). Fig. 64. 

 The genus is known from Tertiary times onwards, and it has 

 played a considerable part in the formation of coral reefs where the 



Fig. 64. Halimeda. A, plant oi H. simulans (x 22). B, H. dtscoidea, longitudinal 

 section showing structure ( x 20). C, central filament: two fuse and subsequently 

 divide into three ( x 20). D, cuticle of H. opuntia ( x 132-5). E, H. scabra, ter- 

 mination of filaments ( x 100). F, fruiting plant. G, sporangia. (A, D, E, after 

 Taylor; B, C, after Howe; F, G, after Oltmanns.) 



species are very abundant. The plants are borne on a short basal 

 stalk that arises from a prostrate system of creeping rhizoids. The 



