476 SMITH 



Polytoma. Within each of these three unicellular genera the type of gametic 

 union ranges from isogamy to oogamy. Progression from isogamy to oogamy 

 has arisen independently more than once among the algae. In the Chloro- 

 phyceae this is found in the Volvocales, the Ulothricales, and the Siphonales, 

 three orders not closely related to one another. The same progressive change 

 from isogamy to oogamy is also found in the Phaeophyceae, algae wholly 

 unrelated to the Chlorophyceae. 



Algae have also been of service in development of concepts concerning 

 the difference between maleness and femaleness. Are maleness and femaleness 

 two distinct categories, or are they different degrees of the same category? 

 The latter idea is to be found in Hartmann's theory of relative sexuality, a 

 theory which he first clearly enunciated in 1925. For heterothallic (dioecious) 

 species Hartmann holds that the intensity of sexuality in either female or 

 male gametes may not be the same for all individual plants of a species. 

 Female gametes from one individual of a species may be of a strong intensity 

 of sexuality, whereas those from another individual of the species may be 

 of a weak intensity. The same holds for male gametes. Female gametes of 

 either intensity unite with male gametes of either intensity. On the other 

 hand, when in the presence of female gametes of strong sexual intensity, female 

 gametes of low sexuality behave as male gametes and there is thus a union 

 of two gametes of the same sex. Male gametes of low and high sexual intensity 

 behave in a similar fashion. Thus maleness and femaleness are to be con- 

 sidered different degrees of the same category. Hartmann's theory of relative 

 sexuality was based upon study of sexual reproduction in certain isogamous 

 and anisogamous algae. 



Relative sexuality has been found in Enteromorpha (Hartmann, 1929) and 

 in Dasycladus (Hartmann, 1943), isogamous algae in which it is impossible 

 to differentiate between male and female gametes. It has also been recorded 

 for Ectocarpus (Hartmann, 1925, 1934), an isogamous alga but one in which 

 it is possible to differentiate between male and female gametes by behavior of 

 the two during gametic union. Union of male gamete with male gamete, and 

 female gamete with female gamete, has also been found in Bryopsis (Hart- 

 mann, 1955), an anisogamous species in which gametes of the two sexes are 

 microscopically different. Moewus (1939b) has reported relative sexuality 

 for Chlamydomonas eugametos. Hartmann (1955) questions this because 

 Forster and Wiese, working in his laboratory, have been unable to confirm it 

 in cultures supplied by Moewus. 



Two sets of substances of a hormone-like nature are involved in gametic 

 union. These are the gamones involved in the coming together of gametes 

 and the termones determining the sexuality of gametes. A secretion of gamones 

 into surrounding water was first observed by Geitler (1931) when working 

 with Tetraspora, whose sexual reproduction is isogamous. He filtered off the 

 water in which gametes of one sex were swimming and then added gametes 



