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188<br />
THE MARINE ALGÆ OF NEW ENGLAND.<br />
Sporangia at the base of filaments, which form scattered external tufts Stilophora.<br />
38. Fronds minute, ending in a hyaline hair, monosiphonous below, densely beset above<br />
with very short branches, between which are the sporangia Myriotrichia.<br />
Fronds ending in a large, single cell, the cells of the lower part giving off descending<br />
filaments, which become interwoven and form a false cortex . 39<br />
39. Rhizoidal filaments few and limited to the base of the plant, branching, irregularly<br />
pinnate ...................................................................... Sphacelaria.<br />
Rhizoidal filaments numerous ........................................................ 40<br />
40. Branches distichously pinnate ................................. Chætopteris.<br />
Branches whorled ......................................................... Cladostephus.<br />
41. Fronds simple or occasionally proliferous ................................ 42<br />
Fronds branching ............................................................................. 47<br />
42. Midrib present ........................................................................... 43<br />
Midrib wanting ................................................................................ 44<br />
43. Fronds stipitate, perforated with numerous holes ........ Agarum.<br />
Fronds entire, with a few separate leaflets on the stipe below the lamina Alaria.<br />
44. Fronds thin, subsessile .............................................................. 45<br />
Fronds thick and coriaceous, distinctly stipitate ........................... 46<br />
45. Sporangia densely covering the surface of frond ........... Phyllitis.<br />
Sporangia external in scattered spots ........................... Punctaria.<br />
46. Cryptostomata present, fronds attached by short, nearly simple rhizoids<br />
Saccorhiza.<br />
Cryptostomata wanting, fronds attached by prominent, branching rhizoids<br />
Laminaria.<br />
47. Fronds without distinction of midrib and lamina, fruit borne on short lateral<br />
branches ................................................................... Ascophyllum.<br />
Blade distinct from the midrib, bladders borne in the laminae, fruit terminal Fucus.<br />
Bladders and fruit borne on special stalks ..................... Sargassum.<br />
48. Fronds calcareous ...................................................................... 49<br />
Fronds not calcareous ...................................................................... 50<br />
49. Fronds erect, filiform, articulated ................................ Corallina.<br />
Fronds thin, horizontally expanded .................................. Melobesia.<br />
Fronds thick, horizontally expanded, but rising at intervals in irregular protuberances<br />
Lithothamnion.<br />
50. Fronds horizontally expanded, crustaceous or membranaceous51<br />
Fronds erect or umbilicate .............................................................. 52<br />
51. Fronds parenchymatous, spores in external warts Peyssonnelia.<br />
Fronds parenchymatous, spores in cavities sunk in the<br />
frond .......................................................................... Hildenbrandtia.