72 PLANT MORPHOLOGY 



clusters on both sides of the thallus, intermixed with sterile hairs called 

 paraphijses. Each has a short stalk and a number of gamete-producing 

 cells. The male gametangium (antheridium) is a club-shaped organ 

 consisting of over 200 small cells arranged in many tiers (Fig. 57 B). 

 Each cell produces a single sperm. The female gametangium (oogonium) 

 has fewer cells, about 20 to 60, each giving rise to an egg (Fig. 57C). The 

 eggs are considerably larger than the sperms but both are laterally 

 biciliate and free-swimming (Fig. 57 D). The eggs are less active than 

 the sperms, however, and usually come to rest first. The zygote germi- 

 nates at once, giving rise to a sporophyte. 



Zanardinia. This genus differs from Cutleria in several ways. The 

 gametophyte and sporophyte are ahke vegetatively, both being disk- 

 like, several layers of cells thick, and about 5 cm. or more in diameter. 

 Each sporangium produces four large bicihate zoospores, the reduction of 

 chromosomes occurring when they are formed. The gametophytes are 

 monoecious, the two kinds of gametangia being intermixed. The male 

 gametangium produces about 250 sperms, the female gametangium about 

 12 to 36 eggs. The gametes resemble those of Cutleria, both being 

 laterally biciliate. 



Summary. The Cutleriales have a flat plate-like thallus that may be 

 either erect or prostrate. Its growth is entirely or partially intercalary. 

 The zoospores are borne in unicellular sporangia, the gametes in multi- 

 cellular gametangia. The group has well-marked heterogamy, but both 

 the sperms and eggs are ciliated. A distinct alternation of generations is 

 present, the gametophyte and sporophyte being either vegetatively 

 similar (Zanardinia) or dissimilar {Cutleria). 



4. Dictyotales 



The Dictyotales are a distinct group of brown algae occupying a some- 

 what intermediate position with respect to the other groups. They are 

 found in both tropical and temperate seas but always grow in warm 

 waters. There are 18 genera and about 100 species. Dictijota, Padina, 

 and Zonaria are well-known members. 



Dictyota. Although found along both the Atlantic and Pacific coasts 

 of North America, this genus does not occur north of about 35° latitude. 

 The plants grow attached to rocks in tidepools and are always submersed. 

 The vegetative body consists of a thin, flat, dichotomously branched 

 thallus with a basal holdfast (Fig. 58). It is composed of three layers of 

 cells: an upper and a lower layer of small photosynthetic cells with a layer 

 of large colorless cells between them. The thallus grows by means of a 

 large apical cell, one of which lies at the tip of each branch (Fig. 59) . The 

 sporophyte and gametophyte are alike vegetatively, and so alternation of 

 generations is isomorphic. 



