GILBERT M. SMITH S^? 



zoogametes of both sexes of the green alga Tetraspora lubrica excrete 

 sexual substances. He placed gametes of one sex in water from which 

 gametes of opposite sex had been removed by centrifuging. This was 

 soon followed by a clumping of the introduced gametes, but not by 

 their fusion in pairs. 



The most striking of all results with algae immediately after being 

 taken from field to laboratory are those reported by Moewus (13) for 

 Mo?iostroma. This heterothallic marine green alga has a blade-like thallus 

 one cell in thickness and one in which each fertile cell produces a number 

 of biflagellate gametes. A zygote formed by union of two biflagellate 

 gametes enlarges greatly and then has a division of its protoplast into 

 32 haploid quadriflagellate zoospores. Half of the zoospores develop 

 into female thalli and half develop into male thalli. Moewus placed 

 fertile portions of a thallus in a small amount of water, added quartz 

 sand, ground in a mortar, and then filtered. The effect of the sexual 

 substances in the filtrate upon germinating zygotes was studied and 

 found to be the same with filtrates from either plus or minus thalli, 

 but to vary according to dilution of the filtrate. Mature zygotes placed 

 in an undiluted filtrate formed 64 (instead of 32) small biflagellate 

 swarmers when the zygote germinated. These swarmers fused in pairs 

 shortly after liberation. Mature zygotes placed in filtrates diluted 1:2 

 or 1:4 produced 32 large biflagellate swarmers. When liberated these 

 neither fused in pairs nor reacted to plus or to minus gametes. Mature 

 zygotes placed in a filtrate diluted 1:10 produced 32 large quadriflagellate 

 swarmers which also showed no sexuality. 



In another series of experiments mature zygotes were placed in various 

 dilutions of the filtrate until the beginning of cleavage of their contents 

 and then transferred to undiluted filtrate. Zygotes first placed in dilu- 

 tions of 1 :2 or 1 :4 and then in undiluted filtrate produced 32 biflagellate 

 swarmers that fused in pairs when liberated. Those first placed in a 

 dilution of 1:10 produced 32 quadriflagellate swarmers that also fused 

 in pairs when liberated. These experiments with extracts obtained from 

 fertile portions of Monostroma show that asexual swarmers can absorb 

 sexual substances from the surrounding water and function as gametes. 

 However, there are certain unanswered questions as to the manner in 

 which the sexual substances affect the swarmers. One question is the 

 manner in which a filtrate containing sexual substances extracted from 

 a female plant, or those extracted from a male plant, causes half the 



