EUPHYCOPHYTA lOI 



branches, although a fine pore is left for intercommunication. The 

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

 season. Detachable propagules develop on the vesicles (Fig. 56L) 

 and form a method of vegetative reproduction, whilst sexual repro- 

 duction is by means of gametes, which are produced in ovoid 

 gametangia that arise from the vesicles as lateral outgrowths, each 

 being cut off by a septum. The plants are anisogamous, the macro- 

 gametes 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 gamet- 

 angia are borne on the same utricles. In C. elongatum it would 

 appear that the determination of the sex may be a seasonal pheno- 

 menon, females appearing first, then hermaphrodites and finally 

 males. The gametes fuse or else develop parthenogenetically, but 

 in either case a single thread-Hke 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 there- 

 fore wholly diploid and comparable to Fucus (cf. p. 192). In C. 

 tomentosum the 2n number of chrosomes is 20 whilst in C decorti- 

 catum it appears to be 40 so that there is evidence of polyploidy. 



Codiaceae: Halimeda (daughter of Halimedon, King of the Sea). 



Fig- 57 

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

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

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

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

 branched aerial thallus is composed of fiat, cordate or reniform 

 segments which are strongly calcified on the outside, the segments 

 being separated from each other by non-calcified constrictions. 

 Branching in some species is restricted to one plane and the size of 

 the segments varies greatly from species to species. The segments 

 are composed of interwoven threads with lateral branches that 

 develop perpendicularly and produce a surface of hexagonal facets 

 through fusion of the swollen ends. Sporangia develop at the ends 

 of forked threads which vary greatly in their mode of branching : 

 these threads, which are cut off from the parent thallus by basal 

 plugs, arise from the surface of the segments or, more frequently, 

 are confined to the edges. The sporangia produce bifiagellate 



