ECOLOGY AND TAXONOMY OF Holimeda 205 



Fig. 65. H. incrassata. Section through inaniature, green gametangium in early organiza- 

 tional stages of niicrogametes, showing scattered chloroplasts often with starch 

 and oil globules, electron-dense bodies and granules. Fine fibrils on exterior of 

 gametangial wall can be seen in upper left-hand corner. Stamed with lead tartrate. 

 Scale bar is 1 [xm. 



existence of plants in the fertile condition. In my cultures gametes 

 are usually released about 36 hours after the white gametangial stalks 

 appear, leaving behind a white, disintegrating thallus which would be 

 unlikely to be taken by collectors. Fertile plants can often be found in 

 the field by diligent examination of large populations, but this proce- 

 dure only became practicable with the advent of scuba. It remains true, 

 however, that only a small proportion of a population may be found to 

 carry gametangia at any one time (Beth, 1962), and that several asexual 

 generations may pass before gametangia are produced. 



2. Development and fusion of gametes 



Gametangia in a late stage of development are either brownish or a 

 very dark green and have the often-remarked "bunch of grapes" 

 appearance. Electron micrographs of the dark green gametangia at an 

 estimated 12-16 hours before the gametes would be released (Figs 66, 

 67) show that cleavage into microgametes is well advanced, although 

 not yet complete. The microgametes contain a large, somewhat pos- 

 teriorly oriented chloroplast, often with one or more starch grains. The 

 single nucleus is generally above the plastid, and there are often three 

 mitochondria. Two whiplash flagella are insei'ted into an anterior 

 cytoplasmic papilla. The flagella have the standard 9 -f 2 arrangement 

 of microtubules. Small vacuoles and occasionally a large lipid body are 



