196 L. HILLIS-COLINVAUX 



as calcite or aragonite, but the two species forms are not mixed within 

 plants. In the genus Halimeda aragonite is deposited, and the crystals 

 are predominantly needle-shaped. Carbonate deposition seems to be an 

 important function in the metabolism of Halimeda. 



The deposition of aragonite in Halimeda occurs within the 

 segment but outside the filaments that comprise it, that is, it takes 

 place within the closed or essentially closed spaces of the Halimeda 

 plant. 



The first crystals form in the spaces between the peripheral utricles, 

 at the filament wall surface (inter utricular and not intercellular space), 

 when the segments are about one to two days old. A distinct organic 

 matrix has not been observed, and crystal orientation is random. The 

 deposits increase rapidly with age until most of the space within a 

 segment is filled. Secondary crystal formation occurs, the aragonite 

 needles acting as nuclei. 



The spaces within the segments are essentially closed since the 

 peripheral utricles of most species adhere laterally at their outer edges. 

 The seawater environment of the segment is not in free communication 

 with the surrounding seawater except in the very youngest segments, 

 and possibly the very oldest segments of some thalli. Such an environ- 

 ment could favour crystallization. 



The mucilages of the filament walls contain a calcium-binding 

 polysaccharide fraction. Its calcium complexing strength, however, is 

 low, and conclusive evidence linking it with calcium carbonate deposition 

 has not been obtained. The fibrous matting layer of the filament walls 

 appears to be polysaccharide. 



Two- and three- or more compartment systems have been proposed 

 as representing the pathway of calcium from seawater to aragonite 

 deposit. Other workers have concluded that calcification in Halimeda is 

 primarily a function of the uptake of carbon dioxide during photo- 

 synthesis from the enclosed spaces of the segment. Theoretical calcula- 

 tions indicate a resultant increase in pH and in the concentration of 

 carbonate ions within the spaces, which in turn stimulate the rate of 

 aragonite precipitation. 



These combined studies on calcification in Halimeda have added 

 much in a relatively short time to our appreciation of calcification in 

 this alga. They have also left unanswered questions which have in- 

 terested more than one group of workers, such as the precise pH within 

 the spaces of the segment at different metabolic phases and the impor- 

 tance of crystal nuclei in initiating aragonite deposition. We need 

 more information on calcium within the filaments, and also some 

 comparison between results of physiological experiments using intact 



