IV. 
DICTY OTACEtE. 
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 Dictyotaceie 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 
which 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 mark, 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. 
Dictyota 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 Haliseris , which 
is scarcely represented on the North American coasts, ten species are known, all of 
them tropical or sub-tropical ; although one ( H . polypodioides ) 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 Dictyotaceie are used in the arts. 
SYNOPSIS OF THE NORTH AMERICAN GENERA. 
* Frond flat, dichotomous , traversed by a midr ib. 
I. Haliseris. 
** Frond fiat , without 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. 
