THALLOPHYTA: ALGAE 



73 



Numerous unicellular sporangia are scattered over both surfaces of the 

 sporophyte (Fig. 60). Each sporangium, borne on a one-celled stalk, 

 produces four nonmotile spores (apian ospores). In connection with the 

 formation of four free nuclei from the single nucleus of the young spo- 

 rangium, the number of chromosomes is reduced one-half. Two of the 



Fig. 58. Dictyota binghamiae. Portion of plant showing dichotomous branching, three- 

 fourths natural size. 



Fig. 59. Longitudinal section of a bifurcating thallus of Dictyota dichotoma, cut parallel to 

 its flat surface. The branch tip on the left shows a large undivided apical cell, while the one 

 on the right has just undergone a second dichotomy. 



spores from each sporangium give rise to male plants and two to female 

 plants. 



Like other members of the order, Dictyota displays well-developed 

 heterogamy. The antheridia are borne in clusters of about 100 to 300 on 

 both surfaces of the male plants (Fig. 61 A). The clusters are surrounded 

 by several rings of sterile cells. Each antheridium is composed of a stalk 



