ECOLOGY AND TAXONOMY OF Holimeda 197 



growing Halimedae, Halimedae recently collected from their reef 

 habitats, and branches removed from these coenocytic plants. 



The importance of calcification to the life of the plant is not under- 

 stood. What is the energy cost? What are the benefits that pay for this 

 cost? Is the process important to the cycle of senescence in Halimeda'i 

 Being conducted within the closed spaces of the plant, the process can 

 scarcely be a passive consequence of manipulating carbonate ions. But 

 we do not know why it is done. 



Halimeda is an admirable plant to grow in culture and in which to 

 study the control of chemistry outside a biological membrane. And m 

 mastering the mechanism used by Halimeda to control the medium 

 between its filaments we will learn something of how the whole organ- 

 ism maintains homeostasis without the benefit of cellular epithelium 

 or, indeed, even an intact boundary layer. 



VII. Reproduction 



Much of the reproduction oi Halimeda may be by vegetative cloning, 

 the details of which have now been followed in laboratory culture and 

 are understood in outline. The complete sexual cycle, however, is still 

 not known, with no observations of the development of a zygote uito 

 the familiar Halimeda thallus. A sexual episode appears to be an out- 

 pouring of the energy reserves of an entire thallus into the gametes, and 

 involves the death of the parent plant. This is a breeding strategy 

 reminiscent of an annual weed, though here it is followed by a plant that 

 we now know can maintain itself for an indefinite number of generations 

 between sexual episodes by vegetative means. The breeding strategy 

 has much in common with that described by Wilhams (1975) as the 

 "strawberry-coral model", though, as we shall see, the Halimeda 

 version presents some unusual twists. 



There was little information about reproduction in Halimeda when 

 Fritsch (1935, 1948) wrote his treatise on the algae, or Dawson (1966) 

 his book on marine botany. It was known, however, that plants were 

 occasionally collected bearing tiny globules clustered on stalks along 

 the margins of the segments or over their surfaces (Figs 61, 62). The 

 clusters looked like miniature bunches of greenish to blackish grapes, 

 and were sometimes sparse and restricted to one or a few segments ; at 

 other times they were dense and covered much of the thallus. The 

 general form of these structures in a number of species had been 



