The Ulvas, which are the grass green layers seen on all our coasts, originate in the simple vegetable cell, whatever form their foliaceous fronds may ultimately assume. When the cell is divided in one direction only, a confervoid filament is the result; and if the filament should increase in breadth as well as length, according to a determinate law, a ribbon-shaped frond may be produced; but when the original cell is divided into four cells, and each of these four and all their successors undergo similar division, the increase being as the series 1, 4, 16, 64, &c., a membranous expansion is formed, in which all the cells are firmly attached to one another, and every portion is the exact counterpart of another. The cells of the Ulvas frequently exhibit an imperfect separation of the granular endochrome into four parts preparatory to multiplication by double division, and the entire frond or leaf shows the groups of cells arranged in clusters containing some multiple form of four, as in fig. 3, page 171. The frond membrane of the true Ulvas, as that of the Ulva lactuca, is formed of but one layer of cells; the frond itself is thin as cambric paper, almost transparent, and of a pretty light green. When young it is a puckered inflated bag, which afterwards bursts and opens into a flat, ribless, wavy, more or less rounded expansion, three to six inches long, and as many broad. This plant, which is attached to the rocks between the tide marks on our shores, is rare in the Mediterranean; nor is it so common in Britain as the Ulva latissima (fig. 22), which is cosmopolite, and abundant everywhere. It is found as a ribless irregular expansion of a full bright green in deep water, and of a yellow apple green when in shallow water, and exposed to the light. The base and stem are very short, and the frond, which the microscope shows to be formed of two layers of cell membrane, spreads so rapidly into crisp wide-lobed foliations, that the parts often overlap each other in stiff bulging folds. It is from six inches to a foot in height, and from three to twelve inches wide. The frond of the Ulva Linza is also formed of two layers of cells, but so small and so closely pressed together that the two layers can only be detected by the microscope. This plant, which is from six inches to two feet long, is a ribless, narrow, ribbon-shaped expansion with curled wavy edges tapering to a base, and either blunt or pointed at the top. Its colour is the same as that of the Ulva lactuca.