180 L. HILLIS-COLINVAUX 



These thalli may persist, however, as offspring from asexual reproduc- 

 tion (Section VII). Spreading forms such as opuntia appear to persist as 

 senile portions of the plant fall away only to he replaced in the spreading 

 mass by younger fronds which make their own attachments to the sub- 

 strate with rhizoids from between younger segments (Section V). 

 Halimeda thalli may appear senile and dehisce many of their segments 

 only to regenerate later from the remaining segments and possibly 

 from buried rhizoidal filaments (Section VII). These patterns of growth, 

 life and death obviously raise questions of the strategies of Halimeda 

 species. One of the most striking of the aspects of growth in Halimeda is 

 that individual thalli have life-spans; they die like so many other 

 organisms, after a life-span typical to the species. 



B. UUrastructural events 



The basic ultrastructure of Halimeda has been described in Section 

 I. The approach in this subsection is to consider aspects of the ultra- 

 structure of the filaments that are associated with the growth of a seg- 

 ment. Two of the studies, Wilbur et al. (1969) and Borowitzka and 

 Larkum (1977), were undertaken initially to provide data on calci- 

 fication. Borowitzka and Larkum (1974b) provide additional informa- 

 tion on chloroplast development, and Colombo and Orsenigo (1977) 

 give some information on distribution of organelles within the mature 

 segment. 



From the descriptions of the preceding section three stages of 

 segments can be delimited. They are : 



the young developing segment; 

 the mature segment; 

 the old basal segment. 



Some of the ultrastructural features of these three ages of segment are 

 given in Table XIV. 



1. Young developing segment 



Embodied in the young segment are all the stages in the develop- 

 ment of a new segment, from the first filamentous extensions of the 

 medullary filaments of the preceding segment to the estabhshment of 

 lateral branch filaments (cortex), to the peripheral filaments (utricles) 

 making contact and eventually adhering, at least m most species. This 

 whole process takes about 24-48 hours, and seems to start early in the 

 day or light cycle. In about the last 12 hours, which corresponds to the 



