100 
DICTYOTACEiE. 
IY. 
thread, accompanied by the expansion of the walls into thin membranes. Next, in 
Punctaria , the bag becomes flattened into a nerveless leaf. In higher groups the 
tubular or flattened frond is divided into a branching stem, which, however, does not 
develope any separate leafy organs. In one case (HaUseris) this stem is winged 
throughout with membrane, or may be described as a midribbed branching frond. 
Among the most highly developed genera (Zonaria and Paclina) the frond shows a 
tendency to assume a fan-shaped outline, having a definite, subcircular margin at 
the summit, and gradually widening from the base upwards. Such fronds are 
usually marked at regular intervals with concentric lines, and are formed of longi- 
tudinal rows of cells collaterally united in membranes ; the rows diverging as they 
grow, and new rows of cells being introduced in the interspaces. In many, and 
perhaps in all, the growing frond is clothed with exceedingly slender, jointed, and 
often colourless hairs, which sometimes, whilst expanded under water, decompose 
the rays of light, and cause the frond to display brilliant prismatic colours. These 
hairs are prolongations of the surface- cells, or issue from their sides, and are 
probably organs of the same kind as the pencilled fibres already noticed in the 
Sporochnaceoe. 
The fructification exhibits considerable diversity of aspect in the various genera, 
but the characters are of minor value, chiefly relating to the form and position of 
the masses of fruit. In all, the spores are developed externally, either being formed 
from the surface-cells, which, when fertilized, stand out prominently from the 
ordinary cells ; or from those cells immediately beneath the epidermis, in which 
case the spore-cell bursts through the external coat, carrying it outwards as a 
separated membrane. Usually each perispore contains but a single sporular mass, 
but in Padina, four spores are found at maturity in each perispore ; and in 
Cutleria , eight spores. In some genera the spores are scattered singly over the sur- 
face of the frond but in by far the greater number they are collected into definite 
spots, or sori, which are round, oblong, or linear, and are either dispersed irregularly 
over the whole surface, or confined to a certain part of it ; or else ranged in 
transverse, horizontal, or concentric bands. In some, both scattered and aggregated 
spores are found on the same individual, or on different individuals of the same 
species. In such cases, the scattered spores are usually of larger size and paler 
colour than the aggregated ones, and their contents appear to be different. They 
have sometimes been supposed to be anther idia, but have not, as yet, been examined 
with sufficient care. The spores in most cases are accompanied by barren, jointed 
hairs, or paranemata , which appear to be formed from the same parts as the fertile 
spores, but to have developed into numerous cells. In Stilophora these paranemata 
compose the greater part of the warts of fructification. In some of the more 
perfect forms, as in Cutleria and Padina, antheridia have been noticed ; these are 
sometimes found on the same individuals as the spores, and sometimes on different 
individuals. 
This Order is of decidedly rare occurrence on the American coast, and scarcely 
attracts much notice, from the amount of individuals representing the species, 
