ECOLOGY AND TAXONOMY OF Holimeda 



183 



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Fig. 54. iif. ononile. Filament from a segment less than 24 hours old, showing a variety 

 of sizes of vesicles (v) near the wall, and amyloplasts (a) towards the centre. Approxi- 

 mately half the filament width is shown. Aragonite deposition has not begun, and 

 the fibrous coating of the wall can be seen. Stained with lead tartrate. Scale bar 

 is 1 [jiin. 



utricles are free in mature segments. What happens in the developing 

 segment has not been investigated. In other species where peripheral 

 utricles barely touch each other {gracilis, Section IV) the covering 

 lamella would provide the matrix to hold the utricles together and also 

 provide the continuous outer covering of the segment. 



2. Mature segment 



In these segments, which may be all the segments on many thalli, 

 the growth process is essentially complete, and the three regions of the 

 segment (peripheral utricles representing the exterior, the inner cortex 

 and the medulla) are fully delimited. Some subsequent elongation of 

 these regions may occur, with possibly the addition of another layer of 

 inner cortex in much older segments. There may be increased vacuola- 

 tion of the filaments, and calcification continues, although the full 

 extent of it varies with the species and with depth. 



The outer surface of a mature segment shows some physical differ- 

 ences from the walls of the filaments within them. The outer pilose layer 

 has been lost, and in cylindracea a second layer of covering lamella 

 develops beneath the original one, which then becomes detached 

 except in the corners where it appears to reinforce utricle adhesion 

 (Borowitzka and Larkum, 1977). 



