210 L. HILLIS- COLIN VAUX 



Meinesz (1972b, 1973) reported that after one week of growth under 

 these conditions, the zygotes had a diameter of 5 pim, and contained a 

 single nucleus and four to five chloroplasts. In seven months they 

 developed into spheres of 100-150 [i,m diameter containing a single large 

 nucleus of 7-10 [j,m diameter and over 500 lenticular chloroplasts with 

 one or two large grains of starch. Amyloplasts were absent. This stage 

 he called a "protosphere", the term protonema being inappropriate. 



Then a number of changes occurred, spread over about two weeks. 

 The coenocytic condition was established by division of the large 

 nucleus to form nuclei of 2-3 y.m diameter. The chloroplasts lost their 

 starch, and thereafter appeared like rice grains. Amyloplasts differ- 

 entiated. A positively phototrophic filament of about 30 [j,m diameter 

 grew out of the sphere, and a few days later a negatively phototrophic 

 filament developed from the opposite end. 



After about five additional months of cultui'e there were a number 

 of erect and creeping filaments. The erect ones were about 90-130 {j.m 

 in diameter and 2 cm tall, and at the tips of some were slight dichoto- 

 mous or trichotomous for kings. The creeping filaments proliferated in 

 all directions, and sometimes produced additional erect filaments as 

 well. The diameters of the creeping filaments, which were often con- 

 stricted, were variable. A scheme of these changes is given in Fig. 68. 

 Subsequent development was not observed. 



Meinesz' observations thus end with the production of a filamentous 

 mat, and the stages that must be passed between this structure and an 

 adult thallus are still unknown. We have a similar ignorance of the 

 sexual development of many taxa in the whole order Caulerpales, not 

 just in Halimeda. It may be that a mature thallus can grow directly 

 from one or more of the filaments of the mat. On the other hand, new 

 structures might develop which could be precursal to the Halimeda 

 structures with which we are familiar. Various workers have suggested 

 that some of the more filamentous Caulerpales, such as species of 

 Chlorodesmis and Pseudochhrodesmis, may be part of the cycle of genera 

 such as Halimeda (Taylor et al., 1953; Papenfuss, 1962; Ducker, 1965; 

 Meinesz, 1972b). 



In the laboratory, after the gametes have been released, the white 

 thallus disintegrates, with segment after segment being shed until, 

 within two or three weeks, little more than a heap of segments remains 

 (Fig. 62). Merten (1971) observed that in the field the thallus collapsed 

 in a day or two after the gametes were released. Occasionally only a few 

 segments become fertile in laboratory cultures, and then only one or so 

 branches are shed, but this pattern is uncommon. Usually most of the 

 alga is involved. 



