204 L. HrLLIS-COLINVAUX 



cylindracea, lacunalis f. lata, macroloba, macrophysa, micronesica and 

 velasquezii. Gametangia are therefore known for 60% of the species. 



The stalks supporting the gametangia (gametophores) arise from the 

 segment in three ways (HiUis, 1959): 



(i) as continuations of the main medullary filaments subsequent 

 to the nodal fusion characteristic of the species (Fig. 64, Nos. 3, 5, 8, 

 10, 12); 



(ii) as lateral outgrowths without fusion from medullary 

 filaments (Fig. 64, Nos. 2, 4, 9) ; 



(iii) as extensions from peripheral or secondary utricles (Fig. 64, 

 Nos. 1, 6, 7, 11). 



Contents of the gametophore are continuous with those of the filaments 

 of the supporting segment, and true plugs or septa such as noted by 

 Howe for scahra (1905a) have not subsequently been reported for this 

 or other species (Feldmann, 1951; HilHs, 1959; Colinvaux et al., 1965; 

 Graham, 1975). 



Significant taxonomic differences among species may occur in the 

 dimensions of gametophores particularly the length, the extent of 

 branching of gametophores, and in the size, shape and number of mature 

 gametangia. In cryptica the gametangia are borne predominantly on 

 the shaded under surface, rarely on the margins or at the nodes, and 

 not on the upper surface (Graham, 1975). Non-reproductive charac- 

 teristics, however, are more consistently available, and therefore are 

 considerably more useful for species identification. 



In laboratory aquaria I have found that when a Halimeda becomes 

 fertile, the thallus changes, often dramatically, from green to white 

 overnight or during the "dark" phase of growth, and clusters of game- 

 tangia develop on the surface of the segments or along their margins, 

 from white filaments protruding beyond the surface of the segment. At 

 first the developing gametangial structures are white, but they soon 

 become pale green, then darken further as they mature (Fig. 61 ; 

 Hillis-Colinvaux, 1973). 



Electron micrographs of young, pale green gametangia show 

 numerous plastids with sizeable starch grains (Fig. 65), at least in the 

 vicinity of the wall which has the familiar matting of fine fibrils over 

 its outer boundary. Electron-dense bodies (Section I) and granules are 

 common, and spherical bodies are also present. Gametes have not yet 

 formed. 



The development of gametangia was once thought to be a rare 

 event since few herbarium specimens have them. This paucity of fertile 

 material in herbaria, however, is certainly a function of the brief 



