The Sexual Substances of Algae 



GILBERT M. SMITH 



UNION of two gametes to form a zygote is a widespread phenomenon 

 among algae, especially those belonging to the grass green algae 

 (Chlorophyta) and the brown algae (Phaeophyta). Depending upon 

 the alga, there is a union of two motile flagellated gametes, or a union 

 of a small motile gamete and a large immobile nonflagellated gamete, 

 or a union of two nonflagellated gametes one or both of which move 

 in an amoeboid manner. 



Sexuality among algae was first recognized by Nathan Pringsheim 

 (17) in 1858 in connection with his study of reproduction in Oedogonium, 

 a green alga in which a small motile male gamete (the antherozoid) 

 swims to and unites with a relatively large immobile egg lying within 

 the female sex organ (the oogonium). Within a few years it was suggested 

 that the swimming of antherozoids towards an egg is not a matter of 

 chance but is a chemotactic response to substances secreted by the egg. 

 This assumption was confirmed in the classical studies of Pfeffer (16) 

 where he showed that antherozoids of pteridophytes exhibit a positive 

 chemotactic response to a difl'usion gradient of malic acid and certain 

 other organic acids. In this connection it is interesting to note that he 

 reports negative results with gametes of the green algae Ulothrix and 

 Chlamydomonas. 



Subsequent studies on sexual substances of algae have, in the main, 

 been upon heterothallic (dioecious) species where each thallus produces 

 but one kind of gamete. Some of these studies have been upon collections 

 taken from field to laboratory; other studies have been upon clones 

 grown in vitro in the laboratory. Irrespective of the source of material, 

 the most satisfactory algae for study of sexuality are those in which both 

 male and female gametes are motile and biflagellate. When one mixes 



