1920] Sctchell-aardncr: Chlorophyceae 153 



As to the stalk (or stipe) it is very distinct in some specimens 

 while absolutely wantino: in most of the others, but the shape of 

 the cell is, in general, sufficiently distinctive to permit of the ready 

 separation of tliese specimens from those usually referred to Chloro- 

 chiifriuni. 



Order 2. SIPIIONALES (grev.) oltmanns 



Fronds filamentous, either simple or variously entan<2:led or inter- 

 woven, sometimes producing- complex individuals, devoid of septa (or 

 very nearly so) in the activelj^ vegetative portions, but septa appear- 

 ing in the reproductive portions, multinucleate and with many small 

 chromatophores ; multiplication vegetative, by non-sexual spores, and 

 b}' zygotes; vegetative by abscission of proliferous shoots or frag- 

 mentation ; non-sexual spores, either aplanospores or zoospores, 

 produced usuall.y in specialized zoosporangia ; zygotes from either 

 isogametes or heterogametes. 



Siphoiialfs Oltmanns, Morph. und Biol. d. Algen, vol. 1, 1904, 

 pp. 134, 291 ; Blackman and Tansley, Revis. Class. Green Algae, 1902, 

 p. 114; Collins, Green Alg. N. A., 1909, p. 385; West, Algae, vol. 1, 

 1916, p. 222. Siphoneae Greville, Alg. Brit., 1830, p. 183. 



There is a very considerable variety both in the structure of the 

 frond and in the methods of multiplication to be found among the 

 Siphonales. From the simple globular but i)edicellate Halicystts or 

 the dichotomously filamentous Vaucheria or Derhesia, through the 

 more or sometimes less specialized species of Bryopsis, to the elab- 

 orately constructed fronds of the Codiaceae, which are, however, made 

 up of interwoven filaments with or without calcareous incrustation, 

 there is a series of increasing complexities. In sexual multiplication 

 there is also a series of increasing complexities from the isoplano- 

 gametes of Bryopsis through the heteroplanogametes of Cod {urn to the 

 condition in Vaucheria, where the female gamete is large and motion- 

 less and the male gamete is small and motile. Complexity of form 

 and differentiation of gametes do not proceed along the same lines, 

 e.g., Vaucheria has a frond of simple structure, while its male and 

 female gametes are most widely different from one another. 



Key to the Families 



1. Thallus filamentous 2 



1. Thallus not filamentous 3. Protosiphonaceae (p. 154) 



2. Filaments free 3 



2. Filaments densely interwoven to form a complex thalliip 



6. Codiaceae (p. 166) 



