ECOLOGY AND TAXONOMY OF Halimeda 



213 



This same mechanism, the development of new thalli by independent 

 and usually horizontal growth of rhizoidal filaments away from the 

 holdfast, may exist for rock growers as well. Certainly holdfasts and 

 filaments of many other algae grow across rock or penetrate it, with the 



Fig. 70. Two young Halimeda plants (arrowed), not yet emergent from the substrate, 

 developing from the larger thallus by rhizoidal runners. The white conelike masses 

 at their tips are similar to ones that form the first segments in similar young plants. 

 Development may be somewhat atj^pical because rhizoidal material, by being 

 next to aquarium wall, has been exposed to light. The diameter of the white 

 discs is 0-85 cm. 



siphonaceous alga Ostreobium, which appears to grow exclusively in 

 rock, being a notable example of the ability of some filaments to pene- 

 trate this substrate. 



There are also other methods of vegetative reproduction. Prostrate- 

 growing Halimedae, such as micronesica, opuntia and to some extent 



