Part 15

Family 3. Siphoneæ.

The thallus has apical growth, and in the vegetative condition consists generally of one single (in the Valoniaceæ most frequently of more) multinuclear cell, which may be much branched, and whose separate parts in the higher forms (e.g. Bryopsis, Fig. 57; Caulerpa, Fig. 59, etc.) may be differentiated to perform the various physiological functions (as root, stem and leaf). Vegetative multiplication by detached portions of the thallus (gemmæ); asexual reproduction by zoospores, akinetes, or aplanospores. Sexual reproduction by gamete-conjugation, rarely by oogamous fertilisation. The zygote or oospore germinates as a rule without any resting-stage.

Most of the Siphoneæ occur in salt water or on damp soil. Many (e.g. Dasycladaceæ) are very much incrusted with lime, and occur, in the fossilized condition, in the deposits from the Cretaceous period to the present time. The Siphoneæ are connected by their lowest forms (Botrydiaceæ or Valonia) with the Protococcaceæ, but show also, through the Valoniaceæ, points of relationship to the Cladophoraceæ.

Order 1. =Botrydiaceæ.= The thallus in the vegetative condition is unicellular, club-shaped, with a small single (Codiolum) or repeatedly dichotomously branched system of colourless rhizoids (Botrydium, Fig. 56 a), by which it is attached to objects immersed in salt water (Codiolum) or to damp clay soil (Botrydium). Asexual reproduction by zoospores with one (Botrydium) or two cilia, and by aplanospores. The sexual reproduction is only known in Botrydium, and takes place in the following manner: in the part of the thallus which is above ground and in an active vegetative condition, several round cells (Fig. 56 c) are formed, which may be green or red according as they grow under water, or exposed to the strong light of the sun. These cells must be considered as “gametangia” as they produce many gametes (d) provided with two cilia. The zygote (h, i) formed by the conjugation (e, f, g) may either germinate immediately, or become a thick-walled resting-cell of an irregular, angular form.

Order 2. =Bryopsidaceæ.= The thallus in the vegetative condition is unicellular, and consists at the lower extremity of branched rhizoids, while the upper portion is prolonged into a stem-like structure of unlimited growth, producing, acropetally, branches and leaf-like structures. The latter have limited growth, and are separated by a cross wall from the stem, and become gametangia, or drop off. The gametes have two cilia, and are of two kinds: the female, which are green and large and the male, which are of brownish colour and smaller. Zoospores or any other method of asexual reproduction are unknown. Only one genus, Bryopsis, living in salt water.

Order 3. =Derbesiaceæ.= Only one genus, Derbesia, living in saltwater. The zoospores, which are formed in a few lateral, swollen zoosporangia, possess one nucleus which has arisen through the coalescence of several, and they resemble the zoospores of Œdogonium by having a circle of cilia attached at the base of the colourless spot.

Order 4. =Vaucheriaceæ.= The thallus consists, in the vegetative condition, of a single irregularly or dichotomously branched cell, without differentiation into stem or leaf; root-like organs of attachment may however occur. Asexual reproduction by zoospores, which are formed singly in the extremity of a branch cut off by a transverse wall. They contain many nuclei, and bear small cilia situated in pairs, which give the appearance of a fine “pile” covering the whole or a great part of the surface. Akinetes, aplanospores, and phytoamœbæ (naked masses of protoplasm, without cilia, which creep like an amœba on a substratum) may occur under certain conditions.

The sexual reproductive organs are formed on short lateral branches, and are separated from the vegetative cell (Fig. 58 A) by cell-walls. Numerous spermatozoids, each with two cilia, are developed in the coiled antheridium (A, b). The oogonium is a thick, egg-shaped, often oblique cell, with its protoplasm rounded into an oosphere, which has a hyaline “receptive-spot” (A, a) immediately beneath the aperture formed in the wall of the oogonium. A slimy mass, which serves to receive the spermatozoids, is formed in some species in this aperture. The spermatozoids when liberated swim towards and enter the oosphere, which then immediately surrounds itself with a thick cell-wall. The mature oospore (B) contains a large quantity of oil. At germination the outer cell-wall bursts and a new plant is formed. There is only one genus, Vaucheria, with species living in salt as well as in fresh water and on damp soil.

Order 5. =Phyllosiphonaceæ= are parasites in the leaves and stalks of Flowering-plants.

Order 6. =Caulerpaceæ.= The thallus has distinct differentiation into root, stem and leaf-like members (Fig. 59); it is unicellular. Within the cell, strong, branched threads of cellulose extend from one side to the other serving as stays to support the thallus. Reproduction takes place by detached portions of the thallus; no other modes of reproduction are known. This order may most approximately be classed with the Bryopsidaceæ. The genus Caulerpa consists of more than seventy species which inhabit the tropical seas.

Order 7. =Codiaceæ.= The thallus has various forms, but without distinct differentiation in stem- or leaf-structures, sometimes (e.g. Halimeda) it is very much incrusted with lime. In the early stages it is unicellular (later, often multicellular), very much branched, with the branches, at any rate partly, so united or grown in amongst one another (Fig. 60) that an apparently parenchymatous cellular body is formed. Akinetes or aplanospores are wanting; zoospores (or gametes?) may be developed in some species, however, in special swollen sporangia. Fertilisation similar to that in Bryopsis occurs perhaps in Codium. They are all salt water forms.

Order 8. =Valoniaceæ.= The thallus is generally multicellular, without differentation into stem- or leaf-structures, but the cells are sometimes united together and form a leaf-like reticulate expansion (e.g. Anadyomene). Zoospores are known in some, and they are then formed directly in the vegetative cells. In others (e.g. Valonia), a mass of protoplasm, which maybe separated through the damaging of a cell, can surround itself with a cell-wall, and grow into a new plant. No other modes of reproduction are known. The most important genera are: Valonia, Siphonocladus, Chamædoris, Struvea, Microdictyon, Anadyomene. They are all salt water forms.

As already pointed out, the Valoniaceæ occupy a somewhat central position among the Siphoneæ, and present points of similarity and contrast with the Botrydiaceæ and the Bryopsidaceæ through Valonia, with the Dasycladaceæ through Chamædoris, and also with the Cladophoraceæ through Siphonocladus, and Struvea.

Order 9. =Dasycladaceæ.= The thallus consists of an axile longitudinal cell, destitute of transverse walls, attached at the base by root-like organs of attachment, and producing acropetally whorls of united, single or branched, leaf-like structures with limited growth. Asexual reproduction is wanting. Sexual reproduction by conjugation of gametes which arise in separate, fertile leaves, either directly or from aplanospores, which develope into gametangia. The principal genera are: Acetabularia, Dasycladus, Neomeris, Cymopolia. All marine.

The curiously shaped Acetabularia mediterranea grows gregariously on limestone rocks, and shells of mussels in the Mediterranean; it resembles a minute umbrella with a small stem, sometimes as much as nine centimetres in height, and a shade which may be more than one centimetre in diameter. The cell-membrane is thick, and incrusted with carbonate and oxalate of lime. Only the lower, root-like part of the thallus, which penetrates the calcareous substratum survives the winter, and may grow up into a new plant. The sterile leaves, which drop off early, are dichotomously branched and formed of cylindrical cells separated from each other by cross-walls, but they are not grown together. The shade is formed by a circle of 70–100 club-shaped rays (fertile leaves) grown together, in each ray 40–80 aplanospores are formed, which become liberated at the breaking of the shade, and later on are changed to gametangia (compare Botrydium) which open by a lid and allow a large number of egg-shaped gametes with two cilia to escape. Gametes from various gametangia conjugate with one another; the product of the conjugation swarms about for some time, rounds off, and then surrounds itself with a cell-wall. The zygote germinates after a period of rest and then produces a sexual plant. The aplanospores (gametangia) thus represent the sexual generation.

Class 7. =Characeæ.=

The thallus has a stem with nodes and internodes; and whorls of leaves, on which may be developed the antheridia and oogonia, are borne at the nodes. Vegetative reproduction by bulbils and accessory shoots. Zoospores are wanting. The antheridia are spherical, and contain a number of filaments in which the spirally coiled spermatozoids, each with two cilia, are formed. The oogonium is situated terminally, and is at first naked, but becomes later on surrounded by an investment, and forms after fertilisation the so-called “fruit.” The oospore, after a period of rest, germinates by producing a “proembryo,” from which the young sexual plant arises as a lateral branch. The Characeæ are distinguished by the structure of their vegetative system as well as by the spirally-coiled spermatozoids, and stand as an isolated group among the Thallophytes, of which, however, the Siphoneæ appear to be their nearest relations. They were formerly, but wrongly, placed near the Mosses. The class contains only one order, the Characeæ.

Order 1. =Characeæ.= Algæ with a peculiar odour, often incrusted with lime, and of a brittle nature. They generally grow gregariously in large masses at the bottom of fresh and brackish water, and are from a few inches to more than a foot in height. The stem has long internodes which in Nitella are formed of one cylindrical cell; in Chara of a similar cell, but closely surrounded by a cortical layer of smaller ones. The protoplasm in contact with the cell-wall exhibits in a well-marked degree the movement of rotation (cyclosis), carrying the chlorophyll corpuscles along with it. The internodes are separated from each other by a layer of small cells (nodal cells) from which the leaves are produced. The leaves are borne in whorls of from 5–12 which regularly alternate with one another as in the higher verticillate plants; a branch is borne in the axil of the first formed leaf of each whorl (Fig. 61 A, n).

The leaves are constructed in the same manner as the stem; they are divided into a series of joints, but have only a limited power of growth; their terminal cell, too, is not enclosed by a cortex. Leaflets are borne at their nodes. The growth of the stem is unlimited, and proceeds by means of an apical cell (Fig. 62 s). The apical cell divides into a segment-cell and a new apical cell. The segment-cell then divides by a transverse wall into two cells, one lying above the other; the lower one, without any further division, becomes one of the long, cylindrical, internodal cells (Fig. 62 in), and the upper one (Fig. 62 n) divides by vertical walls to form the nodal cells. The cortical cells (Fig. 62 r) which surround the long internodal cells of Chara, are derived from the divisions of the nodal cells; the cells covering the upper portion of an internodal cell being derived from the node immediately above it, and those in the lower part of the internode from the node below it.

The organs of reproduction are very conspicuous by their colour and form. They are always situated on the leaves, the plants being very frequently monœcious. The antheridia (Fig. 61 B, a) are modified leaflets or the terminal cell of a leaf; they are spherical and become red when mature. Their wall consists of 8 “shields,” i.e. of plate-like cells, 4 of which cover the upper half, and are triangular; the 4 round the lower half, to which the stalk of the antheridia is attached, being quadrilateral, with sides of unequal length. The shields (Fig. 61 C) have dentated edges, with the teeth fitting into one another, and their faces ornamented with ridges. From the centre of the internal face of each shield (C) a cylindrical cell, the manubrium, projects nearly as far as the centre of the antheridium; at the inner end of each of the manubria a spherical cell, the capitulum, is situated. Each capitulum bears six secondary capitula, from each of which four long coiled filaments (C, D) project into the cavity of the antheridium. These filaments are divided by transverse walls into from 100–200 discoid cells, in each of which a biciliated, coiled spermatozoid is developed (D, E) from the nucleus. The spermatozoids escape from their mother-cell and are set free by the shields separating from one other.

The female organ of reproduction (Fig. 61 B, 63) is a small modified shoot, whose apical cell functions as an oogonium, its protoplasm forming the oosphere, which has a colourless receptive-spot at the summit (Fig. 63 u). The oogonium is situated on a nodal cell, from which 5 cells grow out in a circle and coil round the oogonium, covering it with a close investment. These cells divide once or twice at the top, so that 5 or 10 small cells are cut off, which project above the oogonium and form the so-called “crown” (Fig. 63 k). The crown either drops off at fertilisation, or its cells separate to form a central canal for the passage of the spermatozoids. The wall of the oosphere above the receptive spot becomes mucilaginous, and allows the spermatozoid to fuse with the oosphere. The oospore, on germination (Fig. 64 sp), becomes a small filamentous plant of limited growth (Fig. 64 i, d, q, pl)--the proembryo--and from this, as a lateral outgrowth, the sexual generation is produced.

The order is divided into two sub-orders:--

A. NITELLEÆ. The crown consists of 10 cells; cortex absent: Nitella, Tolypella.

B. CHAREÆ. The crown consists of 5 cells; cortex present: Tolypellopsis, Lamprothamnus, Lychnothamnus, Chara.

Chara crinita is parthenogenetic; in large districts of Europe only female plants are found, yet oospheres are formed capable of germination.

About 40 species of fossilized Chara, determined by their carpogonia, are known in the geological formations from the Trias up to the present day.

Class 8. =Phæophyceæ (Olive-Brown Seaweeds).=

The Phæophyceæ are Algæ, with chromatophores in which the chlorophyll is masked by a brown colour (phycophæin). The product of assimilation is a carbohydrate (fucosan), never true starch. In the highest forms (Fucaceæ), the thallus presents differentiation into stem, leaf, and root-like structures. The asexual reproduction takes place by means of zoospores. The sexual reproduction is effected by the coalescence of motile gametes, or by oogamous fertilisation. The swarm-cells are monosymmetric, each moved by two cilia which are true protoplasmic structures, and generally attached laterally (Fig. 65). The Phæophyceæ are almost entirely saltwater forms; a few species of Lithoderma live in fresh water.