ECOLOGY AUB TAXONOMY OF Halimeda 177 



established above them ; then new segments are produced by the apical 

 branches of unburied portions (Section VII). 



Some species, or the thalli of some sites, may not lose segments as 

 part of their growth pattern. Merten (1971) does not mention segment 

 loss in the Guam macroloba populations, but one would expect few losses 

 if the life-span is four months. And Feldmann (1968), writmg briefly 

 of the Mediterranean tuna, speaks of ageing the thallus by the length of 

 the branches. "Les mdividus bien developpes, qui presentent souvent 

 8 a 10 articles successifs, doivent done etre ages d'une dizaine d'annees." 

 This does not imply much loss ofsegments, if the growth rate is as implied. 



Certainly dead whitish segments would be more apparent in the 

 laboratory because they remain on the thallus rather than being 

 removed by currents. It is possible, too, that they may be commoner in 

 laboratory culture. 



In a few incrassata I followed this pattern of gain and loss of seg- 

 ments several times for up to 2| years. During that time branches and 

 segments closer and closer to the base of the thallus were shed, until all 

 but a few basal segments had been lost. A stump, or markedly reduced 

 thallus, remained which was composed of whitish or yellowish, very 

 heavily calcified, segments. Holdfast filaments stopped growing, and 

 the thallus stump was loose in the substrate. Eventually the remaining 

 segments fell off, or the remnants of the alga toppled over, and that 

 individual had died. 



This pattern of growth is characterized by what could be called a 

 "perennating thallus", from which new segments and branches arise. 

 This perennating thallus not only consists of a well-anchored holdfast, 

 but also a few to several basal segments, together with a few vigorous 

 segments possibly of a younger generation (Figs 52, 53, first photo- 

 graphs). Very reduced, old, basal portions do not appear to be able to 

 develop new growth. 



1. Growth axis 



Depending upon the species, the addition of new segments may 

 result mostly in either a horizontal or a vertical extension of the thallus. 

 In Rhipsalian species the main growth axis is vertical, and a thallus that 

 is generally erect results. In gracilis the predommant axis is commonly 

 horizontal and a spreading or creeping habit is produced. In opuntia and 

 distorta both types of growth are common, and the resulting habit is 

 frequently a spreading cushion of many centimetres thickness. 



The size of the resulting thalli is partly determmed by this direction 

 of growth, with mature erect thalli such as incrassata generally bemg 



