16 L, HILLIS-COLINVAUX 



The calcium oxalate monohydrate crystals observed in Penicillus 

 (see Friedmann et al., 1972; Turner and Friedmann, 1974) have not 

 been reported for Halimeda. 



D. Microstructure of rhizoidal filaments 



The rhizoidal filaments of the holdfasts have been little studied. 

 Hillis (1959) provided a very basic account of gross microscopic 

 structure. A preliminary examination of the fine structure oiincrassata 

 (personal observation) indicates that chloroplasts, as expected, are 

 absent, but amyloplasts, together with starch grains without apparent 

 plastid boundary membranes are common, at least in certain regions. 

 Vesicles of various sizes are present, particularly along the peripheral 

 edge of the cytoplasm, and both kinds of unidentified structures 

 mentioned above were prominent in much of the material examined. 



The walls of at least some of the rhizoidal filaments are layered, and 

 up to several times thicker than the filament walls of the mature 

 segments described above. No aragonite crystals were found associated 

 with them, but fine fibrils somewhat similar in appearance to those 

 associated with the walls of the filaments in segments (as described 

 above) were observed along the peripheral edge of the electron-dense 

 outer wall layer of young filaments. 



E. Summary: facets of the unusual structure and chemistry of Halimeda 



Halimeda is one of the largest and most complex of the green algae. 

 It is a filamentous alga which has been elaborated from a simple plan 

 of rows of tube-like filaments and is without cells. 



The resultant plant, which may sometimes reach 0-25 m in height 

 if erect, or sprawl to over 1 m in length, is composed of a photosynthetic 

 portion of many segments, the "strung leaves" of Parkinson, and a 

 holdfast system which ranges from a few short loose filaments to a 

 truly massive production which may extend to 13 cm or more. The 

 holdfast may exist in an anoxic environment. 



Such a plant is derived by the extensive growth, with limited and 

 controlled branching, of one or a few closely associated filaments. Aided 

 by their wall chemistry, the branch tips of the filaments, with growth 

 completed, stick together forming a closed or nearly closed internal 

 environment in which the precipitation of aragonitic calcium carbonate 

 occurs. 



The algal walls are made without cellulose and the component 

 filaments lack cross walls. The resultant "siphon", which contains the 



