178 L. HILLIS-COLINVAUX 



shorter than mature pendant thalli such as copiosa, some plants of 

 which are 1 m long (Goreau and Goreau, 1973). The confines of a 

 spreading thallus such as opuntia are often exceedingly large. Hence, 

 precise boundaries for this species may be difficult to determine, making 

 field counts meaningless. 



Halimeda, in its ability to continue adding to its thallus over a 

 relatively long period of time, resembles Udotea which periodically 

 resumes growth, adding onto the margins of its fan-shaped thallus 

 (Colmvaux et al., 1965) and Tydemania, which adds new glomeruli 

 (unpublished data). Both genera differ from Penicillus (Colinvaux et al., 

 1965) and Bhipocephalus (unpublished data) for which extensive new 

 growth from the mature photosynthetic portion of the thallus is 

 unusual. Although the filaments of the brush may elongate, another 

 capitulum is not formed and the original thallus dies within a few 

 months. 



2. Perennating structures and the systems of Raunkiaer and Feldmann 



Feldmann (1968) has modified for algae the Raunkiaer (1934) system 

 of classifying life-forms of terrestrial plants on the basis of the position 

 of the perennating bud, but did not develop the system to define 

 communities by life-form spectra as Raunkiaer did. This system may 

 have little interest to contemporary community analysts (Whittaker, 

 1975), and this doubtful utility becomes apparent when we attempt to 

 apply Feldmann's version to Halimeda. We find that a single species 

 can behave as two or more of the categories. 



A Halimeda with a perennating thallus as described earlier would fit 

 the hemiphanerophyte category in Feldmann's (1968) description of 

 life-forms, that is, only a part of the erect frond persisting. Some 

 Atlantic populations of tuna appear to fall into this category. The 

 Mediterranean tuna Feldmann considered a phanerophyte, that is, an 

 alga in which the entire thallus functions as the perennatmg structure, 

 and very young Halimeda thalli of many species after their initial bout 

 of growth would also fit this category. 



It is possible too, although there is no evidence as yet, that some 

 species, including tuna, may have rhizoidal structures functioning as 

 perennating structures. As such they would be classified as Jiemi- 

 cryptophytes. Feldmann placed the Mediterranean Caulerpa prolifera 

 and Udotea petiolata {= U. minima) in this last category because 

 they live through the winter without their leaf-like parts. Other 

 populations or individuals of tuna may be annuals, thereby fitting 

 another category. 



