208 L. HILLIS-COLINVAIJX 



36 hours after the first sign of gametangial development, and usually 

 within about an hour after Hghts go on in the controlled environment 

 room. The number discharged by even a single thallus has frequently 

 made the water of an 11-51 aquarium resemble thick pea soup, and 

 observation becomes extremely difficult. 



Gametes are released in a series of puffs, with the units of each puff 

 often held together by mucilaginous material (Hillis-Colinvaux, 1973). 

 Merten (1971) also reported a mucilaginous substance discharged from 

 macroloha gametangia for 15 of the 25 releases she observed. For the 

 remaining 10 a mucilaginous casing formed around the gametangia. 

 Discharge is not from individual pores in gametangial walls, but from 

 one, possibly two, central siphons within the gametangial cluster (Fig. 

 64) which represent the undifferentiated tip of the gametophore. It is 

 through such a structure that the gametes rush in the related genus 

 Chlorodesmis (Ducker, 1965). Discharged gametes are more distinctly 

 pear-shaped than when enclosed in gametangia. Characteristics of 

 macro- and microgametes of different species, as reported in the 

 literature, are given in Table XVI. 



Although I have observed fusion, I have not been able to induce 

 the resulting zygote to develop after it settled, nor is it clear that any 

 have done so when fertile plants have been left in my culture tanks. All 

 fresh plants in my cultures may be best explained as being outgrowths 

 from rhizoids in the sand, for which there is much direct evidence (see 

 below). Meinesz (1972b, 1973), however, managed to follow the develop- 

 ment of zygotes for about one year, though, even then, a recognizable 

 thallus had not been produced. 



Meinesz began his observations by bringing plants of both sexes of 

 tuna, which he found fertile in the wild, into his laboratory. Thalli 

 bearing the two kinds of gametangia were placed together in glass 

 dishes containing Millepore-filtered seawater of field temperature and 

 arranged in indirect sunlight. The bottoms of the dishes were covered 

 with glass slides. Air and circulation were provided by bubbler and 

 pump. After the gametes had discharged and fused, the glass slides 

 were placed in Petri dishes containing von Stosch's artificial seawater 

 prepared with filtered water. The Petri dishes were exposed to different 

 conditions of temperature and light, but the most successful were those 

 maintained at 20-23 °C in indirect sunlight. The seawater was changed 

 every 15 days, and the glass slides were moved to another container if 

 other algae or bacteria developed. When, after one year the resultant 

 growth was 0-5-1 cm tall, the glass slides were placed in a closed- 

 system seawater aquarium equipped with bubbler and filter (Meinesz, 

 1973). 



