214 L. HILLTS-COLINVAUX 



renscJiii, sometimes produce medullary filaments which remain un- 

 corticated. These filaments grow to a length of several segments and 

 are frequently intertwined, forming coarse, orange-coloured ropes 

 (Fig. 46) which I have sometimes referred to as "rope-like extensions" 

 (Hillis, 1959; Section VI). Some thalli have a number of these exten- 

 sions, produced on as many branches, which are frequently firmly fixed 

 to rock at various points along their length. Sometimes noticeably 

 younger thalli are attached to an older plant by these extensions, 

 indicating that such filaments not only anchor a thallus, but initiate 

 new ones (Fig. 46). 



Another mechanism for vegetative reproduction lies in the produc- 

 tion of rhizoidal filaments between segments. These filaments, at least 

 initially, provide additional anchorage, but they also permit division of 

 the alga into separate thalli as the older, more basal segments of the 

 branch disintegrate. This method of growth and clone development 

 (Fig. 71) appeared to be important in maintaining some of the large 

 patches of macrophysa and opuntia at Enewetak. 



On a few occasions I have observed how cloning might occur when 

 a thallus is partially buried in shifting sand. Although half and some- 

 times more of the thalli observed were buried, and the buried portions 

 had lost their green colour and started to disintegrate, extensive new 

 growth occurred from the branch tips. The new segments had all the 

 signs of vigour, possessed the muted green colour characteristic of many 

 growing Halimedae and were unepiphytized. For these plants it seems 

 likely that independent holdfasts would develop near the bases of the 

 actively growing branches, perhaps as described above for macrophysa. 

 A number of separate thalli would result. 



In addition, vigorous branches of Halimeda which have been 

 separated from the main thallus can develop holdfast systems, thereby 

 functioning as reproductive structures (Hillis-Colinvaux, 1972, 1973). 

 Such Halimeda "fragments" might be produced in the reef by waves, 

 storms or animals, and they might be transported in currents. Under 

 favourable conditions they could become established and develop into 

 complete thalli. 



Vegetative reproduction seems to be the way in which many in- 

 dividual Halimeda plants are produced. Many of the familiar aggrega- 

 tions of Halimedae, whether on rocks or in patches of sand, are probably 

 clones (Figs 63, 72). And the commonest method of cloning is by the 

 outgrowth of rhizoidal filaments, either from holdfasts or from the tips 

 of segments. It is a matter of note, however, that the individual plants 

 so produced have limited lives. The ageing thallus becomes coated with 

 epiphytes whereas the segments of a young plant growing from the 



