IV. DICTYOTACE^. 101 



until we proceed as far south as Florida, where, on the Keys, several kinds occur in 

 such abundance as to be conspicuous among the ordinary shore plants. This 

 increase in numbers to the southward is characteristic of the Dictyotaceaj in general. 

 Very few are found in high latitudes, and they gradually become more numerous, 

 and of higher type of structure, the nearer we approach the torrid zone. Those 

 Avhich occur in temperate waters show their propensity for warmth by growing in 

 shallow tide-pools near high-water mark, where they can enjoy a warm bath for 

 many hours of a summer's day. Thus Padina Pavonia, which, on the American 

 shore, is not found farther north than the Florida Keys, in lat. 25°, where it inhabits 

 a region extending below low-water mai'k, reaches the latitude of 51° on the south 

 coast of England, its farthest observed northern limit ; but there it is found only in 

 warm pools near high-water mark, and in sheltered situations. This plant has a 

 very wide distribution, being a native of all the warmer parts of the Atlantic, 

 Pacific, and Indian Oceans, as well as one of the most abundant shore-plants in the 

 Mediterranean. It is possible that more than one species may be confounded under 

 this name, but no satisfactory diagnostic characters have yet been pointed out. 

 Didyota dichotoma is equally cosmopolitan, and has been noticed in the cold waters 

 of the Antarctic Ocean, as well as on the shores of New Zealand, the Cape of Good 

 Hope, and on the western coast of South America. Of the genus ffaliseris, which 

 is scarcely represented on the North American coasts, ten species are known, all of 

 them tropical or sub-tropical ; although one (H. polyj^odioides) extends far to the 

 north, and has been traced from the Canary Islands (lat. 28°) along the Atlantic 

 shores of Europe, as far as lat. 53° 45' on the west of Ireland : and if the Tasmanian 

 specimens and those reported from the Brazilian shores really belong to the same 

 species, it has a nearly equal dispersion in the Southern Ocean, 



None of the Dictyotaceas are used in the arts. 



SYNOPSIS OF THE NORTH AMERICAN GENERA. 



Frond flat^ dichotomous, traversed by a midrib. 



I. Haliseris. 



** Frond flat, ivithout midrib. 

 f Frond fan-shaped, vertically cleft. 



II. Padina. Sori linear, concentric, bursting through the epidermis. 



III. ZoNARiA. Sori roundish, scattered. 



IV. Taonia. Sori linear, concentric, superficial, alternating with scattered solitary 



spores. 



