172 L, HILLIS-COLINVAUX 



growth in laboratory culture at approximately 27 °C, with light in- 

 tensities mostly of 310-560 ft-c for 12 hours, followed by 12 hours of 

 darkness. The only field study of some duration is that of Merten (1971), 

 worldng in Guam. She reported growth rates m macroloba over a period 

 of 10 months, and included some laboratory study of it as well. Like 

 ina'assata, macroloba is a R.hipsalian Halimeda, but the appearance 

 and size of its segments are very different (Figs 22 and 28, respectively). 

 The maximum length x width x thickness measurements for an in- 

 crassata segment are 10 x 14 x 0-7 5-1-0 mm. For macroloba they are 

 29 X 40 X 1 mm. The two species also have different habitats within the 

 reef, with macroloba seemingly restricted to water of — 12 m or shallower 

 and growing best in waters of — 2 m or less. 



In incrassata transplanted to the laboratory, the first obvious 

 growth is usually in the holdfast, as delicate new filaments extend out 

 into the surrounding medium. It can be readily observed in unplanted 

 thalli lying horizontally in sea water. In thalli so arranged most of the 

 first new rhizoidal growth is oriented downwards. 



Within the next few days there are also obvious signs of the develop- 

 ment of new segments, as white, conical protrusions appear from the 

 apical edges of terminal segments (Figs 16, 52, 53). These are the ex- 

 tensions of the medullary filaments which branch and ramify as they 

 grow. The branches develop into the cortical series of utricles of which 

 the outermost ones are contiguous. Within about 24 hours, the albino 

 protrusions from an apical segment develop a fairly complete, some- 

 what greenish segment. The outer surface, however, is spongy and 

 disconnected, for the peripheral utricles have not yet formed a con- 

 tinuous outer surface. This separateness of the peripheral utricles is 

 unlike the pattern encountered in mature segments of most species 

 (Sections III, IV). The very young filaments of a segment, therefore, 

 may have a different environment before and after being enclosed 

 within the segment since at the earliest stages of growth they are in 

 direct contact with seawater. The resulting closed or almost enclosed 

 spaces may be important in calcification (Colin vaux et al., 1965; Wilbur 

 et al., 1969; Bohm and Goreau, 1973; Borowitzka and Larkum, 

 1976a, b, c, 1977; Borowitzka, 1977). Within the next 12-24 hours or 

 somewhat longer, adhesion of peripheral utricles occurs in most species 

 at least, and it is at about this stage that the first granules of calcium 

 carbonate appear. 



The pattern of a segment, its length and width are essentially set 

 within the first couple of days. Subsequent development is mostly in 

 the calcification of the segment, with some change in thickness, de- 

 pending on the location of the segment within the thallus and the species. 



