Part 8

Order 1. =Fucaceæ.= The following species are common on our coasts: _Fucus vesiculosus_ (Fig. 70) has a thallus with an entire margin, and with bladders arranged in pairs; _F. serratus_ (Fig. 71) without bladders, but with serrated margin; _Ascophyllum nodosum_ has strap-like shoots, which here and there are swollen to form bladders; _Halidrys siliquosa_ has its swimming bladders divided by transverse walls; _Himanthalia lorea_, which is found on the west coast of Norway, and the south coast of England, has a small perennial, button-shaped part, from the centre of which proceeds the long and sparsely branched, strap-like, annual shoot, which bears the conceptacles. The Gulf-weed (_Sargassum bacciferum_, Fig. 72) is well known historically from the voyage of Columbus; it is met with in large, floating, detached masses in all oceans, and is found most abundantly in the Atlantic, off the Canary Islands and the Azores, and towards the Bermudas. The stalked, spherical air-bladders are the characteristic feature of this genus. The thallus is more highly developed than in _Fucus_, and there is a contrast between the stem and leaf-like parts. The portions which are found floating are always barren, only those attached are fertile.

USES. The Fucaceæ, like the Laminariaceæ, are used as manure (the best kinds being _Fucus vesiculosus_ and _Ascophyllum nodosum_), for burning to produce kelp, and as food for domestic animals (_Ascophyllum nodosum_ is especially used for this purpose).

Class 9. =Dictyotales.=

The plants in this class are multicellular, and brown, with apical growth, new cells being derived either from a flat apical cell, or from a border of apical cells. The thallus is flat, leaf- or strap-shaped, attached by haptera, which are either found only at the base, or on the whole of the lower expansion of the thallus. The cells are differentiated into the following systems of tissues: an external, small-celled layer of assimilating cells, generally one cell in thickness, and an internal, large-celled layer of one or only a few cells in thickness, forming the mechanical and conducting tissues. All the reproductive cells are motionless. Asexual reproduction by naked, motionless spores (tetraspores) which are formed 1–4 in each tetrasporangium, the latter being outgrowths from the surface cells of special, sexless individuals. Zoospores are wanting. The sexual organs are of two kinds, oogonia and antheridia, which are formed from the surface cells, either on the same or different individuals. The oogonia are spherical or oval, and are generally placed close together; each contains one oosphere, which on maturity is ejected into the surrounding water, and is then naked and motionless. The antheridia are formed of longitudinal cells, united in groups, whose contents by repeated divisions--transverse and longitudinal--are divided into a large number of small, colourless, motionless spermatia--round or elongated--which are set free by the dissolution of the wall of the antheridium. The process of fertilisation has not yet been observed.

The Dictyotales, in having tetraspores and spermatia, deviate considerably from the Phæophyceæ, but may be classed near to the Tilopteridæ, in which there are asexual spores with 4 cell-nuclei, which may be considered as an indication of the formation of tetraspores.

Order 1. =Dictyotaceæ.= _Dictyota dichotoma_ which has a thin, regularly dichotomously divided thallus, occurs on the coasts of the British Isles. _Padina_ is found on the south coast.

Class 10. =Rhodophyceæ (Red Seaweeds).=

The plants comprised in this class are multicellular; they are simple or branched filaments, or expansions consisting of 1 to several layers of cells; the thallus may be differentiated (as in many _Florideæ_), to resemble stem, root, and leaf. The cells contain a distinctly differentiated nucleus (sometimes several), and distinct chromatophores, coloured by rhodophyll. The chlorophyll of the chromatophores is generally masked by a red colouring matter (phycoerythrin), which may be extracted in cold, fresh water; or rarely by phycocyan. Pyrenoids occur in some. Starch is never formed in the chromatophores themselves, but a modification--Florideæ starch--may be found in the colourless protoplasm. Asexual reproduction by motile or motionless spores (tetraspores) which are devoid of cilia and of cell-wall. Swarmspores are never found.

Sexual reproduction is wanting, or takes place by the coalescence of a spermatium and a more or less developed female cell. The spermatia are naked masses of protoplasm, devoid of cilia and chromatophores. The female cell (carpogonium) is enclosed by a cell-wall, and after fertilisation forms a number of spores, either with or without cell-walls (carpospores), which grow into new individuals.

The Rhodophyceæ may be divided into two families:

1. BANGIOIDEÆ. 2. FLORIDEÆ.

Family 1. =Bangioideæ.=

The thallus consists of a branched or unbranched cell-filament, formed of a single row or of many rows of cells, or of an expansion, one or two layers of cells in thickness, but without conspicuous pores for the intercommunication of the cells. The growth of the thallus is chiefly intercalary. The star-like chromatophores contain chlorophyll and are coloured blue-green with phycocyan, or reddish with phycoerythrin; all these colouring matters are occasionally found in the same cell (_Bangia_-species). Asexual reproduction by tetraspores, without cilia, but capable of amœboid movements.

Sexual reproduction is wanting, or takes place by the coalescence of a spermatium with a carpogonium, which is only slightly differentiated from the vegetative cells, and is devoid of a trichogyne. The carpospores are destitute of cell-wall and arise directly by the division of the fertilised oosphere. The Bangioideæ occur chiefly in salt water.

Order 1. =Goniotrichaceæ.=--The thallus consists of a branched cell-filament without rhizoids. Tetraspores are formed directly from the entire contents of the mother-cell, without any preceding division. Fertilisation unknown. _Asterocystis_, _Goniotrichum_.

The _Goniotrichaceæ_, through the blue-green _Asterocystis_, are allied to the Myxophyceæ, and through _Goniotrichum_ to the _Porphyraceæ_.

Order 2. =Porphyraceæ.=--The thallus is formed of an expansion consisting of a layer of 1–2 cells, which, at the base, are attached to the substratum by means of a special form of haptera (_Porphyra_, _Diploderma_); or of unbranched (very rarely slightly branched) filaments, attached at the base by haptera (_Bangia_): or it extends from a prostrate cell-disc (various species of _Erythrotrichia_). Tetraspores are formed after one or more divisions of the mother-cell, either from the whole or only a part of its contents; they possess amœboid movements, or have a jerky, sliding-forward motion. The antheridia have the same appearance as the vegetative cells, but divide several times, and several spermatia are formed, either simultaneously from the whole contents (_Porphyra_, _Bangia_), or the spermatia are successively formed from a part of the contents of the antheridium (_Erythrotrichia_). The carpogonium is without a trichogyne, but the oosphere has a colourless spot which may sometimes rise a little above the surface of the thallus, and may be considered as an early stage in the development of the trichogyne. The spermatia form a canal through the membrane of the carpogonium, and their contents coalesce with the oosphere at its colourless spot. The fertilised oosphere divides on germination into a number of carpospores, which are set free as naked, motionless masses of protoplasm, which grow and give rise to new individuals (alternation of generations).

Family 2. =Florideæ.=

The thallus has one or more apical cells, grows principally by apical growth, and may be differentiated into root, stem, and leaf. The chromatophores vary in form, but have a red or brownish colour, due to chlorophyll and phycoerythrin. Asexual reproduction by motionless tetraspores, which generally arise by the division into four of the contents of the tetrasporangium. The carpogonium has a trichogyne, and the carpospores, which are formed indirectly from the fertilised oosphere, possess a cell-wall.

The thallus may assume very different forms. In the simplest species it is filamentous and formed of single, branched rows of cells (_Callithamnion_, etc., Fig. 73). _Ceramium_ has a filamentous thallus, generally dichotomously forked (Fig. 75), or sometimes pinnately branched, which, at the nodes, or throughout its entire length, is covered by a layer of small cortical cells. _Polysiphonia_ (Fig. 74) has a filamentous, much branched thallus, made up of a central cylindrical cell, surrounded by a layer of other cells, cortical cells, which in length and position correspond to the central ones. In many of the Red Algæ the vegetative organs are differentiated into stems and leaves, the former having, as in _Chara_, unlimited growth in length, whilst the latter soon attain their full development. _Chondrus_ has a fleshy, gelatinous thallus, without nodes; it is repeatedly forked into flat branches of varying thickness. _Furcellaria_ has a forked thallus with thick branches and without nodes. The thallus of _Delesseria_ (Fig. 76) consists of branches, often bearing leaf-like structures, with a midrib and lateral ribs springing from it. These ribs persist through the winter, and at the commencement of the succeeding period of vegetation the lateral ribs become the starting points for new leaves. In _Corallina_ the thallus is pinnately branched, and divided into nodes and internodes. The name has been given to this genus from the fact that the thallus is incrusted with carbonate of lime to such a degree that it becomes very hard, and the whole plant adopts a coral-like appearance. Other genera which are similarly incrusted, and have a leaf-like or even crustaceous thallus (such as _Melobesia_, _Lithothamnion_), are included in this family.

In some instances the cells of the thallus may be found _differentiated_ into more or less well defined tissues, so that it is possible to find special assimilating, mechanical, and conducting tissues, the last named in some cases having the double function of conducting and of serving as a reservoir in which starch is found as a reserve material. The cells of the Florideæ, which are formed by the division of a mother-cell into two daughter-cells of unequal size, have always larger or smaller pits in the cell-walls, and the thin cell-wall separating two pits from each other is perforated by a number of small holes. These pits are particularly developed in the conducting tissues, but sieve-tubes are very rarely to be found.

_Tetraspores_ may be wanting (_e.g. Lemanea_) or may often arise on special, non-sexual individuals. In some (_e.g. Batrachospermum_) only one tetraspore is formed in each tetrasporangium, but the number is generally four, which may be formed tetrahedrally (Fig. 73) or by divisional walls perpendicular to each other, or even in a single row. The tetrasporangia in some species are free (Fig. 73), but in the majority they are embedded in the thallus.

The sexual reproduction (discovered by Thuret and Bornet, 1867) differs in the essential points from that of all other plants, and approaches most nearly to the sexual reproduction of the _Bangioideæ_. The sexual cells are developed from the terminal cells (never nodal cells) of the branched cell-filaments, which constitute the thallus. The mother-cells of the spermatia (_spermatangia_) are generally arranged in a group, in the so-called _antheridia_ (Figs. 74, 77 _A_, _a_). On becoming ripe the membrane of the spermatangium ruptures and the _spermatia_ emerge as spherical or ovoid, naked (a little later they may possess a cell-wall) masses of protoplasm which are not endowed with the power of motion, and hence are carried passively by the current of the water in which they may happen to be, to the female cell. This latter is analogous with the oogonium of the Green Algæ. The female reproductive organ is termed the _procarpium_, and consists of two parts, a lower swollen portion--the _carpogonium_ (Fig. 77 _b_ in _A_ and _B_)--which contains the cell-nucleus, and an upper filamentous prolongation--the _trichogyne_ (Fig. 77 _B_)--which is homologous with the colourless receptive spot of the oosphere of the Green Algæ, and the _Porphyraceæ_. In the sexual reproduction of the majority of the Florideæ, a very important part is played by certain special cells, rich in cell-contents--the _auxiliary cells_. These are either dispersed in the interior of the thallus, or are arranged together in pairs with the cell-filament which bears the carpogonium, and are generally united with this to form an independent multicellular _procarpium_. The spermatia attach themselves firmly to the trichogyne and surround themselves with a cell-wall. The dividing wall at the point of contact is perforated, and the nucleus of the spermatium probably travels through the trichogyne to the swollen part of the procarpium--the _carpogonium_--and fuses with its nucleus. After fertilisation the trichogyne withers (Fig. 77 _C_), but the lower portion of the procarpium, constituting the _fertilised oosphere_, grows out and forms in various ways, first a tuft of spore-forming filaments known as _gonimoblasts_, and finally the _carpospores_. These latter form a new asexual generation (compare the germination of the oospore of _Œdogonium_ and _Coleochæte_).

The gonimoblasts may arise in three ways:--

1. In the _Nemalionales_, branched filaments grow out from the oosphere and form an upright, compressed or expanded tuft of spore-forming filaments.

2. In the _Cryptonemiales_, several branched or unbranched filaments (_ooblastema-filaments_) grow out from the oosphere, and conjugate in various ways with the auxiliary cells. The gonimoblasts are then formed from the single cells produced by the conjugation.

3. In the _Gigartinales_ and _Rhodymeniales_ the oosphere conjugates with an auxiliary cell by means of a short ooblastema-filament, and from this auxiliary cell a gonimoblast is produced.

The motionless _carpospores_, which sometimes in the early stages are naked, and afterwards invested with a cell-wall, are developed from the terminal cells (and perhaps also from some of the other cells) of the branches of the gonimoblast. The gonimoblasts constitute sharply defined parts of the plant in which the carpospores arise. These parts are called _cystocarps_ and are either naked (Fig. 77 _E_), or surrounded by a covering (pericarp or involucre, Fig. 77 _A_) formed in different ways. On this account the Florideæ were formerly divided into GYMNOSPOREÆ (_Batrachospermum_, _Nemalion_, _Ceramium_, etc.) and ANGIOSPOREÆ (_Farcellaria_, _Lejolisia_, _Delesseria_, _Melobesia_, etc.).

The Florideæ are divided into four sub-families:--

Sub-Family 1. =Nemalionales.= The fertilised oosphere produces directly the gonimoblast.

Order 1. =Lemaneaceæ.= Algæ of brownish colour and living in fresh water. They lack tetraspores, and the very sparingly branched fertile filaments, composed of many rows of cells, grow out from a proembryo, which consists of a single row of cells bearing branches. _Lemanea fluviatilis_, often found on rocks and stones in quickly flowing streams.

Order 2. =Helminthocladiaceæ.= Tetraspores are generally wanting (_e.g._ in _Nemalion_) or arise one in each tetrasporangium (_e.g. Batrachospermum_) and it is only in _Liagora_ that four cruciate tetraspores are formed. _Chantransia corymbifera_ consists of simple, branched cell-rows, and is an independent species. Several other _Chantransia-forms_, living in fresh water, are “proembryos” of species of the genus _Batrachospermum_. The germinating carpospore grows out into filaments and forms a so-called proembryo which, if not shaded, attains only a small size, but when growing in shady situations presents a much greater development. These highly developed proembryos have been described as species of _Chantransia_. The proembryo can reproduce by division, or by tetraspores which are developed singly in the sporangia; in _B. vagum_ and _B. sporulans_ which do not possess fully developed female reproductive organs, the proembryos serve almost entirely to reproduce the species. The young _Batrachospermum_-plant arises from the end of an upright filament of the proembryo. The proembryo is generally persistent, and continually produces new _Batrachospermums_. These latter bear the sexual reproductive organs and also whorls of branches: the central row of cells is enclosed by cells growing from the base of the whorls of branches, and from these cortical cells secondary proembryos are developed. In this alternation of shoots there is really no alternation of generations, since the proembryo and the shoots with the sexual reproductive organs are parts of the same thallus.

Several species of _Batrachospermum_ have a bluish green or verdigris colour. _Nemalion multifidum_ has a brown-red thallus, slightly branched, which is attached to rocks near the water’s edge.

Order 3. =Chætangiaceæ.= _Galaxaura_ has a thallus thickly incrusted with lime.

Order 4. =Gelidiaceæ.= _Naccaria, Gelidium._

Sub-Family 2. =Gigartinales.= The fertilised auxiliary cell grows towards the thallus, to produce the gonimoblasts. Procarpia generally present.

Order 5. =Acrotylaceæ.= _Acrotylus._

Order 6. =Gigartinaceæ.= _Gigartina_, _Phyllophora_, _Ahnfeltia_; _Chondrus crispus_, with dark red, dichotomously branched thallus, is common on the coasts of Scandinavia and Great Britain.

Order 7. =Rhodophyllidaceæ.= _Rhodophyllis_, _Euthora_; _Cystoclonium purpurascens_ is common, and sometimes the ends of its branches may be modified into tendril-like haptera.

Sub-Family 3. =Rhodymeniales.= The fertilised auxiliary cell forms the gonimoblast on the side away from the thallus. Procarpia are abundantly produced.

Order 8. =Sphærococcaceæ.= _Gracilaria._

Order 9. =Rhodymeniaceæ.= _Rhodymenia palmata_ is a common species. _Lomentaria_, _Chylocladia_, _Plocamium_.

Order 10. =Delesseriaceæ.= _Delesseria sanguinea_; _D. alata_ and _D. sinuosa_ are handsome forms which are not uncommon.

Order 11. =Bonnemaisoniaceæ.= _Bonnemaisonia._

Order 12. =Rhodomelaceæ.= _Rhodomela_, _Odonthalia_; _Polysiphonia_, of which many species are to be found on the coasts of Great Britain, has a filamentous, richly branched thallus consisting of a central row of cells surrounded by a varying number of cortical cells of similar size--the so-called “siphons.”

Order 13. =Ceramiaceæ.= Pretty Algæ, often branched dichotomously, or unilaterally pinnate. _Spermothamnion, Griffithsia, Callithamnion, Ceramium, Ptilota._

Sub-Family 4. =Cryptonemiales.= The cells formed by the coalescence of the auxiliary cells and the ooblastema-filaments, produce the gonimoblasts. The _carpogonium-filaments_ and the auxiliary cells are scattered singly in the thallus.

Order 14. =Gloiosiphoniaceæ.= _Gloiopeltis._

Order 15. =Grateloupiaceæ.= _Halymenia, Cryptonemia._

Order 16. =Dumontiaceæ.= _Dumontia, Dudresnaya._

Order 17. =Nemastomaceæ.= _Furcellaria_, which has dichotomously branched, round shoots, is common on the coasts of Great Britain.

Order 18. =Rhizophyllidaceæ.= _Polyides, Rhizophyllis._

Order 19. =Squamariaceæ.= The Algæ belonging to this order form crust-like coverings on stones, mussel-shells, and on other Algæ, but are not themselves incrustated: _Petrocelis_, _Cruoria_, _Peyssonellia_.

Order 20. =Corallinaceæ.= Partly crustaceous, partly erect, branched Algæ, thickly incrusted with lime, so that a few species (_Lithothamnia_, also called _Nullipora_) occur in fossilized condition from Jurassic to Tertiary periods. _Melobesia, Lithophyllum, Lithothamnion, Corallina._

USES. “Carragen” is the thallus of _Chondrus crispus_ (Irish Moss) and _Gigartina mamillosa_. It is a common article of food on the coasts of Ireland, and swells to a jelly when cooked. It is officinal. _Rhodymenia palmata_ is generally eaten as food in Ireland and in some places on the west coast of Norway; it is also used as food for sheep and hence is termed “Sheep-seaweed.” Agar-Agar is the jelly obtained from species of _Gelidium_ and _Gigartina_ growing in China and Japan.

Sub-Division III. =FUNGI.=

=Mode of Life.= The Fungi have no chlorophyll, and are thus unable in any stage of their existence to assimilate carbon; they must therefore live as _saprophytes_ or _parasites_. There is, however, no strong line of demarcation between these; many Fungi commence as true parasites, but only attain their full development upon or in dead plants or animals (_Rhytisma_, _Empusa_). Many saprophytes may occasionally appear as parasites, and are then designated “_facultative parasites_” (_Nectria cinnabarina_, _Lophodermium pinastri_), in contradistinction to those which only appear as parasites, “_obligate parasites_” (Mildew, Brand-and Rust-Fungi, _Cordyceps_).

The parasites which live on the surface of the host-plant are termed _epiphytic_ (Mildew, _Fusicladium_); and those living in its tissues are termed _endophytic_ (_Ustilago_, _Peronospora_). _Epizoic_ (_Oidium tonsurans_, _Laboulbenia_) and _endozoic_ Fungi (_Cordyceps_, _Entomophthora_), are distinguished, in the same manner, as those which live on the surface or in the interior of animals. The Fungi designated _pathogenic_ are especially those which produce disease in human beings and in animals.

Most of the diseases of plants are attributed to the parasitic Fungi. These force their way into the host-plant by piercing the outer wall of the epidermis, as in the Potato-disease; or by growing in through the stomata, _e.g._ the summer generations of the Rust of Wheat; or they can only penetrate through a wound, _e.g. Nectria_. Some effect an entrance into the host-plant by the secretion of a poisonous matter or ferment, which softens and destroys the cell-walls (_Sclerotinia_). Some Yeast and Mould Fungi secrete ferments (enzymes), which, for example, convert cane-sugar into a sugar capable of fermentation.