ECOLOGY AND TAXONOMY OF Holimeda 13 



is coenocytic (Fig. 10). In mature, metabolically active segments the 

 cytoplasm of inner filaments is a thin peripheral layer with relatively 

 few chloroplasts, whereas in outer filaments, that is, in the utricles, it 

 occupies much of the volume, the vacuole thereby being less extensive. 

 The small, disc-shaped chloroplasts may be very numerous in the 

 peripheral portions of the organism, but are reported to move internally 

 at night (Stark et al., 1969). A gradient in the kinds and quantities of 

 organelles occurs in young growing filaments such as those forming a 

 new segment (Fig. 6), and across a segment from periphery to medulla 

 (Wilbur et al., 1969; Borowitzka and Larkum, 1977). The arrangement 

 at times suggests cytoplasmic streaming. 



1. Plastids 



Feldmann (1946), building on the earlier microscopic observations 

 of Askenasy (1888), Czurda (1928) and Chadefaud (1941), delimited 

 two basic types of plastids in a number of siphonaceous algae including 

 Halimeda. They were plastids with very large starch grains which he 

 called amyloplasts (Figs 55, 59), and those with no starch grains, the 

 chloroplasts (Figs 10, 59). He called this two-plastid system heteroplasty . 

 The plastic condition, however, is not so clear cut since starch is also 

 present in chloroplasts (Section III; Figs 10, 59; Wilbur et al, 1969; 

 Colombo and Orsenigo, 1977). 



Amyloplasts are the only plastids in the rhizoidal filaments (personal 

 observation), whereas chloroplasts predominate within photosynthe- 

 sizing mature, but not aged, segments, at least in the daytime (Wilbur 

 et al, 1969; Borowitzka and Larkum, 1974b; Colombo and Orsenigo, 

 1977). In the non-peripheral utricles of the segment both kinds of 

 plastids may occur (personal observation). The pigments contained 

 by the chloroplasts are chlorophylls a and 6, in a ratio usually of about 

 2 : 1 (Jeffrey, 1968), together with the principal carotenoids known for 

 other green algae, as well as two additional carotenoids siphonoxanthin 

 and siphonein (Kleinig, 1969). 



The chloroplasts of Halimeda are bounded by a double membrane, 

 although the outer one is not always intact. They are disc-shaped, 

 and when mature are approximately 2-5 y.m long, 1-3 [j.m broad 

 (Wilbur et al, 1969; Palandri, 1972b; Borowitzka and Larkum, 1974b). 

 Large amyloplasts have ruptured and lost much of their outer mem- 

 branes and are about twice the size or larger (Wilbur et al, 1969). The 

 membranes, before they break with age, may be as resistant to breaking 

 as are those of the related, but non-calcified Caulerpa, which Giles and 

 Sarafis (1974) were unable to rupture by blending, grinding, freezing, 



