In the preceding divisions of the Melanosperms the fronds consist of articulated threads; in the succeeding divisions the fronds are inarticulate. The latter comprise four very remarkable groups, of which the Dictyoteæ are distinguished by a leathery or membranous frond, sometimes cylindrical, but mostly flat, the surface of which is reticulated and sprinkled with groups or little patches of naked spores or cysts. The endochrome in the cysts is sometimes quadripartite, or even divided into eight parts. In one of the genera only, anything like antheridia have been found. The zoospores produced from the quadripartite endochrome are large, of a dark colour, and have two lateral cilia, while the bodies in the filiform much divided antheridia seated variously in the tufted threads are far more minute and pale, but with similar cilia. This order obtains its maximum of development in the tropical and subtropical regions; several species are found in the Mediterranean, while a few occur on our coasts, and on those of North America.[43] The genus Dictyota begins the zonarioid group, whose structure is very curious. Every band (lacinia) of the frond terminates in a single cell, by the constant division of which at the lower side, the other cells of the frond are formed, the terminal cell of the frond being thus continually pushed onwards. Hence it results that the longitudinal lines of superficial cells converge, thus affording a ready method of ascertaining the genus in default of fructification. When a new centre of growth is to be made, that is, when the frond is to become forked, the terminal cell divides longitudinally and then each half-cell grows according to its own law. Fig. 30 shows the tip of the frond of the Dictyota dichotoma magnified; the cells on its surface are square, and the interior of each has a spiral structure. [Illustration: Fig. 30. Dictyota dichotoma:—_a_, tip of young frond; _b_, cell showing spiral structure; _c_, threads from marginal cells; _d_, sorus of spores; _e_, _f_, terminal cells dividing into new centres of growth.] The Padina Pavonia, or Peacock’s-tail laver of our southern coast, and those of North America and the Mediterranean, is sometimes included in the genus Zonaria. The species is remarkable for its wedge-shaped fronds, which are olive green shaded with rust colour, and, when in fruit, they are striped across with dark concentric zones, which are merely lines of spores immersed in the frond and seen through its transparent superficial membrane. Each zone is ornamented with a fringe of orange-coloured hairs. Parallel to, or rather concentric with, the spores, is a row of articulated threads, which bear so strong a resemblance to the antheridia of the Cutleria that a similarity of function is suspected by Mr. Berkeley. Species of Zonaria, Padina, and Haliseris, which is the most highly developed of the Dictyoteæ, are most abundant in tropical and low latitudes.