ECOLOGY AND TAXONOMY OF Halimeda 



1. Holdfast 



The filaments of the holdfast are not organized into a regular shape 

 such as a segment. They generally branch irregularly to form a mass 

 of threads, which may remain loose, as in much of the opuntia 

 material, may become tangled into a mat as in H. tuna, or may 

 adhere to fine particles of loose substrate and with these particles 

 produce an impressive bulbous structure to 13 cm or more in length 

 (Figs 2, 4, 5). These large holdfasts sometimes grow in a reducing 

 environment. 



In at least some species the main rhizoidal filaments have much- 

 thickened walls, whereas the walls of the finer branches are not 

 noticeably thickened. 



2. Segment 



In contrast to the holdfasts, the filaments within segments show a 

 definite pattern of organization. In most species several central, so- 

 called medullary or axial, filaments run the length of the segment 

 (Figs 3, 6) and the entire length of the branch, "stringing" the segments 

 together. These filaments form the central axis, core or medulla. The 

 exception, cryptica, has a core of but a single filament (Colinvaux and 

 Graham, 1964), which is two or more times the diameter of such 

 filaments in other species (personal observation). 



The medullary filaments generally branch trichotomously, with the 

 resultant branches becoming displaced laterally and rebranching one 

 to three times. This lateral branch system is the cortex (Figs 3, 6). 

 The branches themselves, called utricles, are relatively short, and 

 become progressively shorter towards the periphery of the segment. 

 They also may be swollen, except at their bases, where in most species 

 they are usually constricted and the wall thickened. The outermost or 

 peripheral branches are called primary nitrides, the ones immediately to 

 the inside are secondary utricles, and so on until cortex disappears into 

 medulla (Figs 6, 7). 



The primary utricles of most species touch at their peripheral edges 

 and adhere in mature segments. In surface view, therefore, the 

 peripheral utricles generally appear polygonal, like a section of honey- 

 comb (Fig. 8). They give the impression of cells and in the older 

 literature sometimes, erroneously, were called cells, although they are 

 but the adhering tips of coenocytic filaments. 



The cortex of the segment is not uniformly developed over the 

 entire plant but varies with the relative age of the segment, and is 



