Part 8

They are organisms without chlorophyll, and in their vegetative condition are masses of protoplasm without cell-wall (plasmodia). They multiply by means of spores, which in the true Slime-Fungi are produced in sporangia, but in some others free. The spores are round cells (Fig. 1 a) which in all the true Slime-Fungi are surrounded by a cell-wall. The wall bursts on germination, and the contents float out in the water which is necessary for germination. They move about with swimming and hopping motions like swarmspores (e, f), having a cilia at the front end and provided with a cell-nucleus and a pulsating vacuole. Later on they become a little less active, and creep about more slowly, while they continue to alter their form, shooting out arms in various places and drawing them in again (g, h, i, k, l, m); in this stage they are called Myxamœbæ.

The Myxamœba grows whilst taking up nourishment from the material in which it lives, and multiplies by division. At a later stage a larger or smaller number of Myxamœbæ may be seen to coalesce and form large masses of protoplasm, plasmodia, which in the “Flowers of Tan” may attain the size of the palm of a hand, or even larger, but in most others are smaller. The plasmodia are independent, cream-like masses of protoplasm, often containing grains of carbonate of lime and colouring matter (the latter yellow in the Flowers of Tan). They creep about in the decaying matter in which they live, by means of amœboid movements, internal streamings of the protoplasm continually taking place; finally they creep out to the surface, and very often attach themselves to other objects, such as Mosses, and form sporangia (Fig. 2). These are stalked or sessile and are generally cylindrical (Fig. 3), spherical or pear-shaped (Fig. 4); they rarely attain a larger size than that of a pin’s head, and are red, brown, white, blue, yellow, etc., with a very delicate wall. In some genera may be found a “Capillitium” (Fig. 4 cp), or network of branched fine strands between the spores. Flowers of Tan (Fuligo septica) has a fruit-body composed of many sporangia (an Æthalium), which has the appearance of flat, irregular, brown cakes, inside the fragile external layer of which a loose powder, the spores, is found. It generally occurs on heaps of tanners’ bark, and appears sometimes in hot-beds in which that material is used, and is destructive by spreading itself over the young plants and choking them.

All the motile stages may pass into resting stages, the small forms only surrounding themselves with a wall, but the large ones at the same time divide in addition into polyhedral cells. When favourable conditions arise, the walls dissolve and the whole appears again as a naked (free-moving) mass of protoplasm.

To the genuine Slime-Fungi belong: Arcyria, Trichia, Didymium, Physarum, Stemonitis, Lycogala, Fuligo, Spumaria, Reticularia.

Some genera wanting a sporangium-wall belong to the Slime-Fungi: Ceratiomyxa, whose fruit-body consists of polygonal plates, each bearing stalked spores; Dictyostelium, in which the swarm-stage is wanting and which has stalked spores. Plasmodiophora brassicæ preys upon the roots of cabbages and other cruciferous plants, causing large swellings. Pl. alni causes coral-shaped outgrowths on the roots of the Alder (Alnus). Phytomyxa leguminosarum may be found in small knobs (tubercles) on the roots of leguminous plants. It is still uncertain whether it is this Fungus or Bacteria which is the cause of the formation of these tubercles.

Sub-Division II.--=ALGÆ=.

=Mode of Life.= The Algæ (except most of the Bacteria) are themselves able to form their organic material by the splitting up of the carbonic acid contained in the water, or air in some cases, and for this purpose need light. The majority live in water, fresh or salt, but many are present on damp soil, stones, bark of trees, etc.

With the exception of the Bacteria, no saprophytes have actually been determined to belong to this group, and only very few true parasites (for instance, Phyllosiphon arisari, Mycoidea, etc.), but a good many are found epiphytic or endophytic on other Algæ, or water plants, and on animals (for instance, certain Schizophyceæ and Protococcoideæ; Trichophilus welckeri in the hairs of Bradypus, the Sloth), and several species in symbiotic relation to various Fungi (species of Lichen), to Sponges (e.g. Trentepohlia spongiophila, Struvea delicatula), and to sundry Infusoria and other lower animals as Radiolarias, Hydra, etc. (the so-called Zoochlorella and Zooxantella, which are perhaps partly stages in development of various Green and Brown Algæ).

=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.

The simplest and lowest form is termed =conjugation=, or =isogamous= fertilisation, and is characterized by the fact that the two coalescing cells (termed gametes) are equal, or almost equal, in shape and size (the female gamete in the Cutleriaceæ, e.g. Zanardinia collaris, Fig. 7, is considerably larger than the male gamete). The cell in which the gametes are developed is called a gametaugium, and the reproductive cell formed by their union--which generally has a thick wall and only germinates after a short period of rest--is termed a zygote or zygospore. The conjugation takes place in two ways:--

(a) In the one way the gametes are motile cells (planogametes, zoogametes, Fig. 8), which unite in pairs during their swarming hither and thither in the water; during this process they lie side by side (Fig. 8 d), generally at first touching at the clear anterior end, and after a time they coalesce and become a motionless zygote, which surrounds itself with a cell-wall (Fig. 8 e). This form of conjugation is found in Ulothrix (Fig. 8 d), Acetabularia, and other Algæ (Figs. 45, 56, 66).

(b) Among other Algæ (e.g. Diatomaceæ and Conjugatæ), the conjugating cells continue to be surrounded by the cell-wall of the mother-cell (aplanogametes in an aplanogametangium); the aplanogametangia generally grow out into short branches, which lie close together and touch one another, the wall at the point of contact is then dissolved (Fig. 39). Through the aperture thus formed, the aplanogametes unite, as in the first instance, and form a rounded zygote, which immediately surrounds itself with a cell-wall. Various modifications occur; compare Figs. 37, 39, 41, 43.

The highest form of the sexual reproduction is the =Egg- or Oogamous= fertilisation. The two coalescing cells are in the main unlike each other in form as well as size. The one which is considered as the male, and is known as the spermatozoid (antherozoid), developes as a rule in large numbers in each mother-cell (antheridium); they are often self-motile (except in the Florideæ, where they are named spermatia), and are many times smaller than the other kind, the female, which is known as the egg-cell, (oosphere). The egg-cell is always a motionless, spherical, primordial cell which can either float about freely in the water, as in the Fucaceæ (Fig. 9), or is surrounded by a cell-wall (oogonium); generally only one oosphere is to be found in each oogonium, but several occur in Sphæroplea (Fig. 10). The result of the spermatozoid coalescing with the egg-cell is, as in this case, the formation of a oospore, which generally undergoes a period of rest before germination (the Florideæ are an exception, a fruit-body, cystocarp, being produced as the result of coalescence).

An example of fertilisation is afforded by the Alga, Sphæroplea annulina (Fig. 10). The filamentous thallus is formed of cylindrical cells with many vacuoles (r in A); some cells develope egg-cells (B), others spermatozoids (C), the latter in a particularly large number. The egg-cells are spherical, the spermatozoids of a club- or elongated pear-shape with two cilia at the front end (G; E is however a swarmspore). The spermatozoids escape from their cells through apertures in the wall (o in C) and enter through similar apertures (o in B) to the egg-cells. The colourless front end of the spermatozoid is united at first with the “receptive spot” of the egg-cell (see F), and afterwards completely coalesces with it. The result is the formation of a oospore with wart-like excrescences (D).

The female (parthenogenesis) or male (androgenesis) sexual cell may, sometimes without any preceding fertilisation, form a new individual (e.g. Ulothrix zonata, Cylindrocapsa, etc.).

=Systematic division of the Algæ.= The Algæ are divided into the following ten classes:

1. SYNGENETICÆ; 2. DINOFLAGELLATA, or PERIDINEA; 3. DIATOMACEÆ; 4. SCHIZOPHYTA, FISSION-ALGÆ; 5. CONJUGATÆ; 6. CHLOROPHYCEÆ, GREEN-ALGÆ; 7. CHARACEÆ, STONE-WORTS; 8. PHÆOPHYCEÆ; 9. DICTYOTALES; 10. RHODOPHYCEÆ.

Among the lowest forms of the Algæ, the Syngeneticæ, the Dinoflagellata, and the unicellular Volvocaceæ (Chlamydomoneæ), distinct transitional forms are found approaching the animal kingdom, which can be grouped as animals or plants according to their method of taking food or other characteristics. Only an artificial boundary can therefore be drawn between the animal and vegetable kingdoms. In the following pages only those forms which possess chromatophores, and have no mouth, will be considered as Algæ.

Class 1. =Syngeneticæ.=

The individuals are uni- or multicellular, free-swimming or motionless. The cells (which in the multicellular forms are loosely connected together, often only by mucilaginous envelopes) are naked or surrounded by a mucilaginous cell-wall, in which silica is never embedded. They contain one cell-nucleus, one or more pulsating vacuoles, and one to two band- or plate-like chromatophores with a brown or yellow colour, and sometimes a pyrenoid.

Reproduction takes place by vegetative division, or asexually by zoospores, akinetes (or aplanospores?). Sexual reproduction is unknown. They are all fresh water forms.

To this class may perhaps be assigned the recently arranged and very little known orders of Calcocytaceæ, Murracytaceæ, Xanthellaceæ, and Dictyochaceæ, which partly occur in the free condition in the sea, in the so-called “Plankton,” and partly symbiotic in various lower marine animals.