14 L. HILLIS-COLINVAUX 



thawing, sonication or a variety of chemical treatments. Such rigescent 

 membranes may explain the resistance of siphonaceous chloroplasts to 

 digestion by opisthobranchiate mollusc grazers, and the survival of the 

 chloroplasts within the animals for a month or more. 



Thylakoids, or photosynthetic lamellae, are present in chloroplasts, 

 and in amyloplasts in which the starch grain does not completely fill 

 the boundary membrane (Borowitzka and Larkum, 1974b; personal 

 observation). They traverse the length of the plastid, and in so doing 

 may encircle a central starch grain if present. The lamellae appear to 

 be constructed of linearly arranged vesicles (Borowitzka and Larkum, 

 1974b) and are appressed into groups (Fig. 10). In chloroplasts, their 

 number, within limits, increases with age, whereas in amyloplasts the 

 starch grain may become so large that the development of thylakoids 

 is restricted, and the lamellae are displaced outwards until the plastid 

 membrane eventually ruptures. 



Synthesis and organization of the thylakoids involve a system of 

 concentric lamellae at one end of the plastid (Fig. 10), variously called 

 "concentric lamellae" (Descomps, 1965), "concentric lamellar system" 

 (Hori and Ueda, 1967; Hori, 1974), "thylakoid organizing body" or 

 "TOB" (Borowitzka and Larkum, 1974a, b), and "dome-shaped body" 

 (Turner and Friedmann, 1974). These concentric lamellae persist 

 throughout much or all of the life of the plastid, and are also present 

 in other siphonaceous heteroplastic algae such as Avrainvillea, Caulerpa, 

 Chlorodesmis and Penicillus (Hori and Ueda, 1967; Turner and 

 Friedmann, 1974). They also occur in amyloplasts in which starch does 

 not fill the entire space (Palandri, 1972b). 



Osmiophilic globules may be prominent in chloroplasts (Wilbur et 

 al., 1969) and DNA fibrils are present (Palandri, 1972b ; Borowitzka and 

 Larkum, 1974b). Pyrenoids or starch-storing bodies have not been 

 observed, but have been reported for some species of Caulerpa (Hori 

 and Ueda, 1967 ; Calvert et al., 1976), a genus which is also heteroplastic. 



2. Nuclei and other components 



Our knowledge of most other microcomponents of the filament 

 system of Halimeda is more restricted. The many small nuclei, each 

 containing several small electron-dense masses but usually only a 

 single nucleolus, are bounded by a double membrane possessing pores 

 (Palandri, 1972b). This author has also reported an association between 

 nuclear envelope and smooth endoplasmic reticulum. Data on nuclear 

 division are scant, and studies of the chromosomes of Halimeda do 

 not appear to have been published (Puiseux-Dao, 1966; Lewin, 1976). 



