THE ORIGIN OF LIFE 



Figure 33. Cldaini/domonas. (Re- 

 drawn from Weatherwax, "Plant 

 Biolociy," 2nd Ed., W. B. Saunders 

 Co., 1947.) 



EYE SPOT 

 NUCLEUS 



CELL WALL 

 CHLOROPLAST 



actively swimming cells, like an animal). These zoospores are then re- 

 leased by the dissolution of the cell wall, and each swims away, an inde- 

 pendent plant, like the parent in every respect except size. This difference 

 is soon bridged by growth. However, sexual reproduction may also occur, 

 for the parent plant may divide to form eight, sixteen, or thirty-two gam- 

 etes, cells which resemble the zoospores and the adults, except that they 

 are much smaller. Like the zoospores, these gametes are released into the 

 water, where those from different parent cells unite in pairs to form zy- 

 gotes. The zygote forms a thick wall about itself and remains quiescent 

 for a time. It is in this highly resistant encysted condition that the plant 

 survives unfavorable conditions such as the drying of ponds. In time, the 

 zygote again becomes active. It then undergoes two divisions, the meiotic 

 or reduction divisions, with the production of four zoospores, which are 

 released to form adult algae. 



Chlamijdomonas appears to be very close to the origin of sex, and may 

 afford some insight into that problem. It may be noted that most of the 

 Ufe cycle is passed with only the haploid number of chromosomes, for the 

 reduction divisions proceed immediately when the zygote becomes active. 

 Haploidy, that is single representation of each type of chromosome or 

 genetic factor, was undoubtedly the normal situation for all organisms 

 before the origin of sex, and it is still the normal thing for organisms 

 which do not reproduce sexually. Diploidy is a necessary consequence of 

 sexual reproduction, for the union of two gametes can have no other 

 result. Gametes could not be reduced below the haploid condition without 

 qualitative loss of genetic material. It appears that in these organisms, 

 so close to exclusively haploid ancestors, meiosis serves primarily to re- 

 store the ordinary, physiological, haploid chromosome number; while dip- 

 loidy is introduced as a temporary concomitant of a "new" method of 

 reproduction. Why sexual reproduction should ever have been developed 



105 



