Part 2

SYSTEMATIC DIVISION OF THE THALLOPHYTES. To the Thallophytes belong three sub-divisions--Slime-Fungi, Algæ, and Fungi. Formerly the Thallophytes were divided into Algæ, Fungi, and Lichens. But this last group must be placed among the Fungi, since they are really Fungi, which live symbiotically with Algæ. The _Slime-Fungi_ must be separated from the true Fungi as a distinct subdivision. The _Algæ_ possess a colouring substance, which is generally green, brown, or red, and by means of which they are able to build up organic compounds from carbonic acid and water. The Bacteria, especially, form an exception to the Algæ in this respect; like the Fungi and Slime-Fungi they have as a rule no such colouring material, but must have organic carbonaceous food; these plants form no starch, and need no light for their vegetation (most Fungi require light for fructification). The Myxomycetes, Bacteria, and Fungi derive their nourishment either as _saprophytes_ from dead animal or vegetable matter, or as _parasites_ from living animals or plants (hosts), in which they very often cause disease.

A remark, however, must be made with regard to this division. Among the higher plants so much stress is not laid upon the biological relations as to divide them into “green” and “non-green”; _Cuscuta_ (Dodder), a parasite, is placed among the Convolvulaceæ, _Neottia_ and _Corallorhiza_, saprophytes, belong to the Orchidacere, although they live like Fungi, yet their relations live as Algæ. In the same manner there are some colourless parasitic or saprophytic forms among the Algæ, and stress must be laid upon the fact that not only the Blue-green Algæ, but also the Bacteria, which cannot assimilate carbonic-acid, belong to the Algæ group, Schizophyceæ. The reason for this is that systematic classifications must be based upon the relationship of form, development, and reproduction, and from this point of view we must regard the Bacteria as being the nearer relatives of the Blue-green Algæ. All the Thallophytes, which are designated Fungi (when the entire group of Slime-Fungi is left out), form in some measure a connected series of development which only in the lower forms (Phycomycetes) is related to the Algæ, and probably through them has taken its origin from the Algæ; the higher Fungi have then developed independently from this beginning. The distinction of colour referred to is therefore not the only one which separates the Algæ from the Fungi, but it is almost the only characteristic mark by which we can at once distinguish the two great sub-divisions of the Thallophytes.

The first forms of life on earth were probably “Protistæ,” which had assimilating colour material, or in other words, they were Algæ because they could assimilate purely inorganic food substances, and there are some among these which belong to the simplest forms of all plants. Fungi and Slime-Fungi must have appeared later, because they are dependent on other plants which assimilate carbon.[2]

_Sub-Division I._--=MYXOMYCETES, SLIME-FUNGI.=

The Slime-Fungi occupy quite an isolated position in the Vegetable Kingdom, and are perhaps the most nearly related to the group of Rhizopods in the Animal Kingdom. They live in and on organic remains, especially rotten wood or leaves, etc., on the surface of which their sporangia may be found.

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[3] are produced in sporangia, but in some others[4] 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.

The _Syngeneticæ_ are closely related to certain forms in the animal kingdom, as the Flagellatæ.

Order 1. =Chrysomonadinaceæ.= Individuals, uni- or multicellular, swimming in free condition, naked or surrounded by a mucilaginous covering. The cells are generally oval or elongated, with 2 (rarely only 1) cilia, almost of the same length, and generally with a red “eye-spot” at their base, and with 2 (rarely 1 only) band-shaped chromatophores. Reproduction by the longitudinal division of the individual cells either during the swarming, or during a resting stage; in the multicellular forms also by the liberation of one or more cells, which in the latter case are connected together.

A. Unicellular: _Chromulina_, _Cryptoglena_, _Microglena_, _Nephroselmis_.

B. Multicellular: _Uroglena_, _Syncrypta_ (Fig. 11), _Synura_.

Among the unicellular Chrysomonadinaceæ are probably classed some forms which are only stages in the development of the multicellular, or of other _Syngeneticæ_.

Order 2. =Chrysopyxaceæ= are unicellular, and differ mainly from the preceding in being attached either on a slime-thread (_Stylochrysalis_), or enclosed in an envelope (_Chrysopyxis_, Fig. 12). They have two cilia, and multiply by longitudinal (_Chrysopyxis_) or transverse division, and the swarming of one of the daughter-individuals (zoospore). Division may also take place in a motionless stage (_palmella-stage_).

Order 3. =Dinobryinaceæ.= The individuals are originally attached, uni- or multicellular; each individual cell is distinctly contractile, and fixed at the bottom of a cup-shaped, open envelope. Cilia 2, but of unequal length. Asexual reproduction by zoospores, which are formed by straight or oblique longitudinal division of the mother-cell, during a palmella-stage which is produced in the winter aplanospores. _Epipyxis_, _Dinobryon_.