246 
THE SEAWEEDS 
g. Fronds erect or decumbent at the 
base, terete or compressed, branch- 
ing in all directions (with 4 to 20 
pericentral siphons) or corticated 
(with 4 or 6 to 10, not five 
siphons). Tetrasporangia in a 
spiral line 
gg. Fronds erect or decumbent at the 
base, terete or compressed, dis- 
tichously branched. Siphons 4 to 
12. Sporangia in a straight line . . 
ee. Frond adorned with persistent lateral 
mono-siphonous filaments. 
Pericentral siphons 4 to 8, naked or corti- 
cated. Sporangia verticillate in each 
segment, or single or twinned . . 
dd. Fronds creeping or erect, endowed with 
dorso-ventral structure. 
h. Tetrasporangia single in each seg- 
ment. 
i. Frond terete or compressed, 
branched with pairs of 
marginal alternating branches, 
one of each pair short, the 
other long 
ii. Frond not with paired branches 
hh. Tetrasporangia two in each seg- 
ment, thus forming two longi- 
tudinal series. 
Frond various, caespituli of fila- 
ments (when present) and fruits 
arranged in a longitudinal series 
in the dorsal region of the 
frond 
ce. Pericentral cells alternately divided by trans- 
verse septa (sometimes also longitudinal) so 
that the central axis is quickly obscured. 
Sporangia spirally disposed, single in each seg- 
ment. Pericentral cells at first four . . 
aa. The whole plant a sympodium. 
Frond usually terete, laterally or sub-dichoto- 
mously branched. Axis with conspicuous 
siphons, naked or corticated. Sporangia arranged 
in definitely constituted proper stichidia, often 
verticillately grouped 
Polysiphonieae. 
Pterosiphonieae. 
Lophothalieae. 
Polyzonieae. 
Herposiphonieae. 
Rytiphloeae. 
Heterocladieae. 
Dasyeae. 
