202 



L. HILLIS-COLINVATJX 



Fig. 63. Cloning of incrassata. The two large thalli have lived in the aquarium for 

 approximately 14 weeks. The tallest of the five young thalli, produced by "riinners" 

 in the sand, is 23 days old. The others are less than 10 days old. Scale bar is 1 cm. 



A. Sexual processes in Halimeda 



Feldmann (1951) provided the first modern description of the 

 contents of the small globules or gametangia borne externally on a 

 segment. His material was the large-segmented Mediterranean tuna, 

 and he noted that mature gametangia were of two different colours, 

 brown and green, which occurred on different thalli. The gametes 

 released by the two were different also. Those from the brown game- 

 tangia were slightly larger (macrogametes), and usually contamed three 

 small chloroplasts and a posterior eyespot. Those from the green 

 gametangia (microgametes) possessed the same number of chloroplasts 

 but lacked an eyespot. He noted, too, that when the gametangia were 

 first formed almost all the contents of the coenocytic filaments of the 

 segment, especially the chloroplasts and amyloplasts, moved into the 

 gametangia. The filaments of the segment, therefore, were essentially 

 empty, and the thallus, as a result, appeared white. 



These observations show that the sexual process involves : 



(i) growth of gametangia on stalks or gametophores (Fig 64) ; 



(ii) the transfer of the free organic matter of the thallus, 

 includhig reserves, to the gametangia (holocarpy), a process that 

 turns the parent thallus white ; 



(iii) release of gametes of different sizes ; 



(iv) fusion of gametes ; 



(v) death of parent thallus ; 



(vi) development of the zygote. 



