ECOLOGY AND TAXONOMY OF Halimeda 



11 



carried out by Mirande (1913) who, through differential staining 

 techniques, deduced the walls to be composed mainly of what he called 

 callose, which is a polymer of [3-1,3-linked glucose residues (Aspinall 

 and Kessler, 1957). Cellulose, until recently thought to be present in 

 the walls of all plants except fungi, has not been found in Halimeda, 

 although it has been demonstrated in some of its siphonaceous relatives 

 (Feldman, 1946; Huitzing and Rietema, 1975). 



"# *. 



Fig. 9. The wall of a youiig filament from H. monile, showing the outer layer of fine 

 fibrils (top), the osmiopliilic covering lamella (appears dark) and main part of 

 wall. Calcification has not yet started in this segment, but the first aragonite crystals 

 form in the vicinity of the fine fibrils, and therefore outside the filament. The plasma 

 membrane has separated from part of the wall. Scale bar is 0-1 [xm. 



The outermost region of the filament wall contains one or more 

 osmiophilic layers (Fig. 9) which sometimes separate from the rest of 

 the wall. Their precise chemical nature as yet is unknown. Although 

 this region may be equivalent to the "cuticle" of Hanic and Craigie 

 (1969), a word they employed with reservation, it seems more appro- 

 priate to use the earlier terminology of Brand (1901) and call it a 

 "covering lamella" (Decklamella). This lacks the chemical connotation 

 of cuticle, and clearly indicates the region of the filament wall involved. 

 Numerous fine fibrils, approximately 5-10 nm in diameter and 200 nm 

 long (Wilbur et al., 1969; Borowitzka and Larkum, 1977), often with 

 knob-like ends, project from this covering lamella into the lumen of 

 the segment. This fibrillar or pilose layer, which contains polysaccharide 



