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.
In the Ulvas, which are multilocular plants, some cells are selected to bear fruit, and others not. The granular endochrome of these chosen cells divides into several parts, which are at first in close contact and at rest; then they become restless, acquire four or a greater number of cilia, and pass through a fracture in the cell wall into the water, in which they swim freely as zoospores. After a time they come to rest, attach themselves to some object, and begin to grow. The walls of the cells which have thus discharged their endochrome in the form of zoospores, remain as colourless spots on the frond. The whole colouring matter of a portion of the frond may escape as zoospores, leaving behind it nothing but a white membrane. With a microscope, this process may sometimes be observed in all the different stages of its progress.
Every full-grown Ulva has its own precise and definite form, but whatever that may be, the young plants on their first appearance from the shore are in all respects similar to Confervas; the top cells soon divide, and a plane or sac-like frond is formed.
Certain Ulvas, which have a yellow tint, produce small zoospores with only two cilia, but in the Ulva bullosa and the Ulva latissima four zoospores are produced in the same cell, each having four cilia. The same fructification prevails also in the purple Ulvas—Porphyra laciniata and vulgaris. The latter is seen in winter and the early spring, covering the rocks near high water mark, with its tiny bright purple lanceolate leaves. Later in the season it grows into a flat narrow ribless frond with a pointed end, and about two feet long, the margin of the frond becoming waved and plaited as the plant increases in growth. At a later period, it is seen mixed with the Porphyra laciniata, which is a ribless flat frond of a dull purple; sometimes it is very thin, divided or torn, and occasionally growing in a circle round its root. Both forms are sold as laver.[[40]]
The Rhodospermeæ, Florideæ, or Rosetangles, are the most beautiful of the marine vegetation. No sea plant surpasses them in delicacy and grace of form or richness of colouring, but the most beautiful are seldom seen, because they grow below the line of ebb tides, or under the shelter of other sea weeds in the rock pools left at low water, their crimson tints being deepest when sheltered from strong light. The Rhodosperms, which have representatives in every sea, are much more numerous than the green Algæ both in genera and species. Thirteen orders, comprising sixty-seven genera, inhabit the British coasts. Many are exceedingly minute, forming patches and velvety cushions on rocks and other Algæ; a vast number have jointed filamentous fronds, while others consist of tubular filaments, and many exhibit a shrub-like collection of firm branches; some are flat and foliaceous expansions without a midrib, either thin and delicate, or thick and strong, while a very brilliant group of both narrow and spreading fronds possess a midrib as a distinguishing character. The structure of the frond varies from a simple membranous to a cartilaginous or even horny substance, caused by a greater development of the cellular tissue, which in the higher kinds of Florideæ divides the epidermal layer or skin from the parenchyme or spongy matter within.
The mode of reproduction by tetraspores, as well as by simple spores, distinguishes the Rhodosperms from the other two great divisions of the marine Algæ. These bodies are produced by the division of the red or crimson endochrome into four parts, which remain in the cells till they acquire an envelope; their form, which is much varied, depends upon that of the endochrome. Some are produced by the breaking up of a globe of endochrome from the centre into four pyramidal segments; or should the endochrome be elliptical, by dividing it into four by three parallel segments, or a mass may be divided into four by horizontal and vertical sections. Some of these are represented, greatly magnified, in [fig. 23]. The tetraspores are lodged in wart-like excrescences, immersed either partially or wholly in some part of the frond.
Fig. 23. A, Polyides rotundus:—a, thin slice showing the wedge-shaped spores; b, tetraspores.
B, Furcellaria fastigiata:—c, thin slice showing a nucleus with the dividing spores; d, one of the large cells; e, a tetraspore.
The simple spores are produced within colourless tubercules called nuclei, variously situated upon the plant, as at [fig. 23] a, c. These nuclei contain many microscopic spores. Sometimes the nuclei are enclosed in conceptacles or ovate sacs, which are either perforate or not at the apex. These contain many microscopic strings of cells like jointed threads, and the endochrome in each joint of these threads is converted into a spore successively from the summit downward. Sometimes the endochrome in one or two joints only, becomes a spore whether terminal or central, and when the spores break through the joint wall and fall off from the threads, they are collected without any definite order into a mass within the nuclei. Sometimes new joints or cells are produced on the threads when the old ones have yielded their fruit. Occasionally a globose nucleus contains several secondary nucleoli full of spores. In every instance, the perfect spore is a dense grumous mass surrounded by a hyaline sub-gelatinous coat consisting of at least two membranes. The situation, mode of growth, and structure of the nuclei vary almost infinitely, and together with the structure of the frond afford the distinctive marks by which the genera are separated from each other.
The spores and tetraspores are equally capable, like buds, of reproducing their species; but the spores are believed to be in some cases fertilized by spindle-shaped particles, and consequently are considered to be the true fruit. Antheridia, or sacs containing these particles, have been discovered in various genera of Rhodosperms. Although, as a rule, the red Algæ have two modes of vegetative reproduction, yet there are various species in which tetraspores only have hitherto been met with.