ECOLOGY AND TAXONOMY OF Halimeda 171 



nuclear domain. Furthermore, the shape imposed on a filament becomes 

 fixed in a matrix of mineral calcium carbonate. Clearly, there are 

 interesting questions to ask about the patterns of growth in Halimedae 

 that concern control of growth, rates of growth, maintenance of 

 structure and the process of calcification. 



One of the reasons Acetabularia, a relative ot Halimeda with a single 

 nucleus instead of many, has yielded so many data about the nucleo- 

 cytoplasmic control of growth is that it could be cultured. Halimeda can 

 be easily grown too, if initial care is taken in obtaining clean starting 

 plants. But nucleo-cytoplasmic and chemical aspects of its growth have 

 not yet been approached. Our information on growth is at the thallus, 

 filament and ultrastructural levels. 



The classical accounts of growth are basically three. Askenasy (1888) 

 provided the first short description when he noted that after a "rest 

 period" the nodal filaments of an apical segment produced filamentous 

 extensions which branched many times. The branches in turn branched 

 and rebranched to the tips or peripheral utricles, which adhered, 

 forming an outer surface and, as a result a new segment. 



Barton's (1901) account of the development of a new segment varied 

 from that of Askenasy only in the branching pattern of the medullary 

 filaments. The initial branching was trichotomous, with the middle 

 branch continuing as part of the medulla while the two side branches 

 divided and redivided until they terminated in the peripheral utricles. 

 A demonstration of this pattern can be seen in cryptica (Figs 3, 15) with 

 its single medullary filament. 



The first field datum on rate of growth was obtained about this time 

 too, and appears to have been originally published by Barton (1901). 

 This was the observation by Finckh (1904), at Funafuti, that a branch 

 of Halimeda growing through a hole in a submerged board of wood on 

 the reefs added three inches of height and thickness in six weeks, or 

 14'38 g of calcareous matter. Thereafter, until these algae were 

 grown in the laboratory (Colinvaux et al., 1965) little more was known 

 about growth of Halimeda or, indeed, of any of the calcareous 

 Caulerpales. 



A. Macroscopic growth 



Much of the following account uses hitherto unpublished data and 

 some reported by Colinvaux et al. (1965). It is based mostly on the 

 species incrassata, simulans and monile from the Caribbean, and 

 although many of the developments described have been observed in 

 the field, all of the quantitative data, particularly those on rates, are of 



