THALLOPHYTA: ALGAE 69 



cally by zoospores borne in unicellular (and multicellular) sporangia and 

 by motile isogametes borne in multicellular gametangia. There is an 

 alternation of generations, the haploid plants being either similar to the 

 diploid plants in size and vegetative structure, or much smaller and 

 simpler. 



A few heterogamous forms with unicellular sex organs borne on minute 

 gametophytes, but otherwise resembling the Ectocarpales, are now segre- 

 gated into two small orders: the Sporochnales and Desmarestiales. Some 

 authors also segregate into the Chordariales, Punctariales, and Dictyo- 

 siphonales isogamous forms with multicellular gametangia but with dis- 

 similar haploid and diploid plants. 



2. Sphacelariales 



The Sphacelariales are a small but distinct order related to the Ecto- 

 carpales. They are all littoral algae numbering 10 genera and 60 species, 

 chiefly tropical but also occurring in temperate regions. The two chief 

 genera are Sphacelaria and Stypocaulon. 



Sphacelaria. This alga grows in small tufts attached to rocks and 

 other algae. It occurs along both coasts of North America but is rather 

 uncommon. The vegetative body is differentiated into a flat, plate-like, 

 prostrate portion and a filamentous erect portion that is freely branched, 

 the branches increasing in length by means of a large apical cell (Fig. 56). 

 This cuts off a series of transverse segments that then divide both longi- 

 tudinally and transversely to form a polysiphonous thallus. In most 

 algae, growth is intercalary, which means that it occurs by division of all 

 or many of its cells. Where there is an apical cell, all the cells of the body 

 are descendants of it, even though some may later divide independently. 



The sporangia and gametangia of Sphacelaria are similar to those of 

 Edocarpus, the sporangia being unicellular and the gametangia multi- 

 cellular. Both are short-stalked and borne on the axes. The zoospores 

 and gametes are laterally biciliate and, in some species, one of the pairing 

 gametes is slightly larger than the other. As in Edocarpus, there is an 

 alternation of vegetatively similar generations and the number of chromo- 

 somes is reduced one-half in the young unicellular sporangium. A form 

 of vegetative reproduction common in Sphacelaria involves the production 

 of propagules. These are short, flattened, modified branches that become 

 detached and give rise to new plants. 



Summary. The thallus of the Sphacelariales is filamentous, being 

 monosiphonous near the tips and polysiphonous below. Growth takes 

 place by means of an apical cell. Reproduction occurs by zoospores 

 borne in unicellular sporangia and motile isogametes borne in multi- 

 cellular gametangia. The order displays an isomorphic alternation of 

 generations. 



