186 L. HILLIS-COLINVATJX 



(Mirande, 1913) which clots at the site of the injury. Burr and West 

 (1971b) have studied this process in Bryopsis where proteinaceous 

 schizogenous bodies are involved. Wounding has not been examined in 

 Halimeda. The spherical bodies may be involved in this process. 



5. Summary of uUrastructural changes 



In the developing segment a gradient of organelles and activities 

 occurs in the tips of the growing medullary filaments and their lateral 

 branches. At the extreme tip is a region of small vesicles. This region 

 may be up to about 4 [i,m long. Behind it is a mitochondrial region, 

 followed by a region of small vacuoles. 



By the time utricle adhesion takes place, which in some species 

 occurs by the fusion of the osmiophilic covering lamellae of adjacent 

 walls, the utricles contam many mature chloroplasts, as well as some 

 amyloplasts. 



Migration of organelles from the preceding segment and their 

 participation in the development of the new segment have not been 

 studied. 



When utricle adhesion occurs growth is essentially complete. 

 Thereafter utricle surfaces on the outside of the plant slough off much 

 of the pilose layer of their walls, utricle layers may elongate somewhat, 

 and calcium carbonate deposits build up. 



In very old segments the walls may be much thickened, up to at 

 least 2 ptm, and stratified. In some specimens or species the peripheral 

 utricles do not adhere in these segments although they adhere eke- 

 where on the plant (Hillis, 1959). In the limited material examined 

 cytoplasmic components of old basal segments are much reduced. 



C. Calcification 



Somewhat over 100 genera of algae calcify, and there are calcareous 

 representatives in most algal divisions (Table XV). Two species of 

 calcium carbonate are deposited, aragonite and. calcite, and the type is 

 constant for the species. Aragonite rather than calcite precipitation is 

 avoured by high temperatures, high pH and the presence of sodium 

 succinate, chondroitin sulphate and the cations of strontium, barium 

 and lead (Milliman, 1974). The precipitation of calcite but not aragonite 

 is inhibited by the presence of magnesium in solution, while sulphate 

 ions may inhibit precipitation as aragonite (Milliman, 1974). 



In Halimeda, as the new segment is completing its development 

 calcification begins, and the process continues for much or all of the life 

 of the segment. X-ray diffraction studies (McConnell and Colinvaux, 



