L. HILLIS-COLINVAUX 



.Holdfast of a ""TapSas*- — Holdfast of branched 

 few loose filaments; >^-^^^^;-^^^ filaments which form 

 usually several on a mat which fixes onto 



a plant ""ocks 



SPRAWLER ROCK-GROWER 



(eg. H. o puntia ) (eg. H. tuna) 



Segment 



Node (juncture of Segments) 



Holdfast of branched filaments and adhering 

 particles of sand 



SAND -GROWER 



(eg. H. incrassata ) 



Fig. 2. Basic parts of a Halimeda, and the three main types of holdfast. ThalU which 

 sprawl over rock or sand usually have a number of places of attachment, provided 

 by a few loose filaments from between segments or sometimes from the ends of seg- 

 ments. In rock-growers generally the branched filaments form a mat which holds 

 to rock, whereas in sand-growing species the branched holdfast filaments adhere to 

 fine particles of substrate, and the whole forms a sizeable "root". (Drawings by 

 D. Dennis, The Ohio State University.) 



A. The basic plan 



The constructional unit of the entire alga, both holdfast and 

 segment, is the filament (Fig. 3), which has a diameter of approximately 

 0-05-0- 1 mm. These filaments, with a resemblance to fungal hyphae, 

 are matted and branched in such a way as to build plants which range 

 in size from 5 cm or less in H. lacrimosa, to over 1 m in sprawling forms 

 of H. opuntia. 



The filament itself is unusual. Unlike those of most other plants it 

 lacks cross walls which would divide it into a linear row of cells. The 

 Halimeda organism, which is constructed out of a mass of these 

 filaments, may therefore be considered one giant multinucleate cell. 

 This filament without cross walls, or coenocytic filament, occurs also in 

 a few other algae such as Penicillus, Tydemania, Udotea and Caulerpa, 

 all of them green algae or members of the Chlorophyta, and in the 

 Phycomycete fungi. These green algae, with their siphon-like filaments. 



