Vegetative Organs. The cells in all the Algæ (excepting certain reproductive cells) are surrounded by a membrane which (with the exception of the Bacteria) consists of pure or altered cellulose, sometimes forming a gelatinous covering, at other times a harder one, with deposits of chalk or silica formed in it. The cell-nucleus, which in the Schizophyta is less differentiated, may be one or more (e.g. Hydrodictyon, Siphoneæ) in each cell. Excepting in the majority of the Bacteria, colour materials (of which chlorophyll, or modifications of it, always seems to be found) occur, which either permeate the whole cytoplasm surrounding the cell-nucleus, as in most of the coloured Schizophyta, or are contained in certain specially formed small portions of protoplasm (chromatophores).
The individual at a certain stage of development consists nearly always of only one cell; by its division multicellular individuals may arise, or, if the daughter-cells separate immediately after the division, as in many of the simplest forms, the individual will, during the whole course of its existence, consist of only a single cell (unicellular Algæ). In multicellular individuals the cells may be more or less firmly connected, and all the cells of the individual may be exactly alike, or a division of labour may take place, so that certain cells undertake certain functions, and are constructed accordingly; this may also occur in parts of the cell in the large unicellular and multinuclear Algæ (Siphoneæ, p. [62]).
The cells in most of the Algæ belong to the parenchymatous form; these, however, in the course of their growth, may very often become somewhat oblong; in many Algæ (particularly Fucoideæ and Florideæ) occur, moreover, hyphæ-like threads, which are very long, often branched, and are either formed of a single cell, or, more frequently, of a row of cells, having a well-pronounced apical growth. The parenchymatous as well as the hyphæ-like cells may, in the higher Algæ (especially in certain Fucoideæ and Florideæ), be further differentiated, so that they form well-defined anatomico-physiological systems of tissue, i.e. assimilating, conducting, storing, and mechanical.
With regard to the external form, the thallus may present no differentiation, as in many unicellular Algæ, or in multicellular Algæ of the lower order, which are then either equally developed in all directions (e.g. Pleurococcus, Fig. [47]), or form flat cell-plates (Merismopedium) or threads (Oscillaria, Fig. [21]). The first step in the way of differentiation appears as a difference between apex and base (Rivularia, Porphyra); but the division of labour may proceed so that differences may arise between vegetative and reproductive cells (Œdogonium, Fig. [54]); hairs and organs of attachment (rhizoids and haptera), which biologically serve as roots, are developed, and even leaves in certain forms of high order, belonging to different classes (e.g. Caulerpa, Fig. [59]; Characeæ, Fig. [61]; Sargassum, Fig. [72]; and many Florideæ).
The non-sexual reproduction takes place vegetatively, in many instances, simply by division into two, and more or less complete separation of the divisional products (Diatomaceæ, Desmidiaceæ (Fig. [36]), many Fission-plants, etc.), or by detached portions of the thallus (e.g. Caulerpa, Ulva lactuca, etc.; among many Schizophyceæ, small filaments known as hormogonia are set free), or asexually by special reproductive cells (spores) set free from the thallus; these may be either stationary or motile. The stationary reproductive cells (spores) may either be devoid of cell-wall (tetraspores of the Florideæ), or may possess a cell-wall; in the latter case they may be formed directly from the vegetative cells, generally by the thickening of the walls (akinetes), or only after a process of re-juvenescence (aplanospores). Aplanospores, as well as akinetes, may either germinate immediately or may become resting-cells, which germinate only after a period of rest.
The motile asexual reproductive cells are spherical, egg- or pear-shaped, naked, swarmspores (zoospores), which have arisen in other cells (zoosporangia), and propel themselves through the water by means of cilia; or they are Phyto-Amœbæ, which have no cilia and creep on a substratum by means of pseudopodia. The cilia, which are formed from the protoplasm (in the Bacteria, however, from the membrane), are mostly situated at the pointed and colourless end, which is directed forwards when in motion, and are 1, 2 (Fig. [5] B), 4 or more. Both the cilia in the Brown Algæ are attached to one side (Fig. [65]); they are occasionally situated in a circle round the front end (Œdogonium, Fig. [6] a, and Derbesia), or are very numerous and situated in pairs distributed over a large part or nearly the whole of the zoospore (Vaucheria). Besides being provided with one or more nuclei (Vaucheria), they may also have a red “eye spot” and vacuoles, which are sometimes pulsating, i.e. they appear and reappear at certain intervals. The swarmspores move about in the water in irregular paths, and apparently quite voluntarily, revolving round their longer axes; but they come to the surface of the water in great numbers either because of their dependence on light, or driven by warm currents in the water, or attracted by some passing mass of food material. The swarmspores germinate, each forming a new plant, as their movement ceases they surround themselves with a cell-wall, grow, and then divide; in Fig. [6] b, two may be seen in the condition of germination, and about to attach themselves by means of the front end, which has been developed into haptera (see also Fig. [5] B, lowest figure).
The sexual reproduction here, probably in all cases, consists in the coalescence of two masses of protoplasm, that is, in the fusion of their nuclei.
Fig. 5.—Cladophora glomerata. A The lower cells are full of swarmspores, whilst from the upper one the greater part have escaped through the aperture m. B Free and germinating swarmspores.
