which are monœcious plants, that is to say, the same plant produces
snake-like fertilizing spermatozoids and female germ cells. For example,
the Vaucheria sessilis consists of one long branched cell; on the same
side of it two swellings appear near to each other, one of which
elongates, curls round like a horn, and is soon filled with snake-shaped
filaments having long cilia at their thin end, with which they move
rapidly both within the horn, and after they come out of it into the
water. They are perfectly colourless, and correspond to the pollen of
flowering plants. The other protrusion which swells into a globose germ
cell, and which corresponds to the pistil of a flower, contains a mass
of green endochrome, which, after being fertilized by the snake-like
filaments, becomes a primordial cell which has no motion, but after
having secreted a strong coating of cellulose, it sinks to the bottom of
the water, becomes a winter or resting spore, and lays the foundation
for a new generation of plants. The resting spores produce new forms,
while the zoospores, like buds, only multiply the type of the individual
plant with all its peculiarities.
The marine genus Bryopsis grows in New Zealand, the Falkland Islands,
and the seas about Cape Horn. The species are mostly parasites on other
Algæ, and produce innumerable zoospores. The genus Codium is found in
high latitudes, and appears under four different forms on the British
coasts; one of these inhabits turfy banks exposed to the spray of the
sea, the others grow in deep water, or on rocks never uncovered but at
spring tides. Species of this genus are found as far south as
Kerguelen’s Land, and in most of the intervening latitudes. The
Caulerpas inhabit the warmer districts in the northern hemisphere, and
furnish five species in New Zealand. The numerous species afford almost
the whole food of turtles on many coasts, and other genera furnish
nutriment to a host of smaller animals.[39]
The Achlya prolifera is also a unicellular plant, much smaller than the
Vaucheria, but whether an Alga or a Fungus is not very clearly settled.
To the naked eye it appears as a cluster of colourless threads on dead
flies floating in water, on the gills of fishes, and sometimes on frogs.
With a microscope the tufts are seen to consist of tubes extending in
all directions, filled with a nearly colourless granular matter, the
particles of which are seen to move slowly in streams along the walls of
the tubes, the currents sometimes anastomosing with each other. When the
plant is about thirty-six hours old, the endochrome begins to accumulate
in the dilated ends of the tubes, and is cut off from the remainder by a
transverse division, the motion of the particles being still visible in
the part cut off. The endochrome breaks up into a number of long masses,
each of which acquires a cell wall and two cilia, and begins to move
about within the parent cell; when mature they are set free by the
rupture in its wall, and germinate, and produce a facsimile of the
parent. It appears that, in some species, the transverse dividing film
becomes convex as soon as the motile bodies are discharged, a new
fertile articulation is formed and new motile spores are set free, and
this process is continued till the vital powers of the plant are
exhausted. The Achlya has resting spores, which may remain long in the
water without change, but if a dead insect be put into it, they fix on
it and germinate immediately. It is supposed that these resting spores
are fertilized by filamental bodies. The Achlya prolifera goes through
all its changes in an hour and a half or two hours. It is found in the
thermal springs at Vichy, Nevis, and Vaux, where it contains an alkaline
iodide.
The whole of the plants which have been described in the preceding pages
belong to the group of green Algæ, although many are inhabitants of
fresh water. The structure of the marine Algæ is entirely cellular.
Deprived of vascular tubes, they can have no circulation of sap,
consequently they derive their nourishment by absorption throughout
their whole surface from the medium in which they live, for their root,
or rather fulcrum, only serves to fix them to the rocks and stones to
prevent them from being buffeted by the waves. Since solar light and
heat decrease rapidly with the depth, each family of Algæ has a zone
peculiar to itself. The first zone extends from high to low water mark,
and is inhabited by plants periodically exposed to the atmosphere, to
the direct light and heat of the sun, and occasionally to rain. Some of
the Algæ that are long left dry are believed to derive some nourishment
from the substances to which they are fixed. The second zone, which
extends from low water mark to a depth of fifteen fathoms, is the region
of the great marine forests which encircle the globe in both
hemispheres. Other two zones follow at greater and greater depths, but
all are divided into various minor regions, below the last of which the
Algæ decrease as the depth increases, till, as far as we know,
vegetation ceases altogether; that depth, however, must be very great,
as diatoms are sometimes found, and in great quantities, three hundred
fathoms deep.
The marine Confervas, like those growing in fresh water, are
slender-jointed filaments formed of one series of cells joined end to
end. The cells become more or less flattened on the surface of contact,
while the side walls retain their natural curvature, which may be
cylindrical or oval. The filament may, therefore, be cylindrical or
beaded. The cells are almost always longer than they are broad, and for
the most part equal and similar in the same plant, although there are
exceptions to uniformity of size. The cells contain a transparent liquid
through which minute solid particles of various shades of green are
pretty evenly scattered. The conversion of these particles into
zoospores has already been described. Since these Algæ have no roots,
and the cell wall no opening, each cell of a Conferva elaborates
independently the nutriment it absorbs from the water. Some species form
a fleecy layer over rocks, and on the bottoms of salt-water pools and
estuaries, others extend in bundles in salt-water ditches, and some are
found on rocks, between tide marks, rising in long, straight, stiff, and
wiry tufts, from three to eight or twelve inches high.
The genus Hormotrichum, which forms tufts several inches long, of bright
grass green, differs from the Confervas in being soft and gelatinous,
and even more by its mode of increase, which, however, is still by
zoospores. The H. collabens may be taken as the type of this genus. It
forms a long and large tuft of soft gelatinous and slippery filaments of
glossy green. The joints of the filaments are once, or once and a half,
longer than they are broad, and the green granular matter within them is
collected into a round sac or sporidium in the centre of each, and after
being converted into zoospores, the sac comes through a rupture in the
joint into the water, opens, and sets the zoospores free.
The genus Cladophora, which has twenty-five species in the British seas
alone, forms tufts of jointed filaments from four to eight, ten, or even
twenty inches high. In some species the filaments are rigid, bristly,
and wiry; in others they are soft and silky; but they are always richly,
variously, and sometimes densely, branched and rebranched. In some the
branches and branchlets are forked, in others tripartite; the Cladophora
pellucida, which is a rigid, wiry plant, combines both these forms.
The genus Bangia consists of purple filamentous jointed and unbranched
Algæ, which are distinguished from all others by the microscopic
arrangement of their endochrome, which is enclosed in little cells
placed according to a definite plan within the transparent and tubular
joints of the filaments. In the Bangia fuscopurpurea, whose blackish
purple tufts, several inches long, cling closely to the rocks near
high-water mark, the tubular joints contain rows of minute colour cells
radiating from a centre. In the narrow filaments there is but one colour
cell in a joint, but in the broader filaments there are from three to
five, forming a tesselated line across it. In this plant one spore is
produced in each joint. The Bangia ciliaris forms a scarcely perceptible
rosy pink fringe of hair-like jointed filaments on the Zostera marina,
and also on other Algæ. The filaments are not more than the tenth or
fifth of an inch long, consequently their joints are most minute, yet
the microscope shows that they contain from two to three colour cells
set as if radiating from a centre, and that the granular endochrome in
each cell is converted into two zoospores. The Bangia ceramicola, which
forms purplish pink tufts on small Algæ in rock pools, differs from both
of the preceding. The joints of its filaments are once or twice as long
as they are broad, and contain colour cells like long upright lines. By
aided vision zoospores are seen to be formed within the linear colour
cells, then the cells run together into a globular mass, which bursts
through the cell wall, leaving the joint empty. The whole genus is soft
and sometimes gelatinous.
The Enteromorpha genus is characterized by a cylindrical and tubular
stem and branches. These plants form two groups, one whose filaments and
branches swell from a narrow base upwards and terminate in a blunt
extremity, while in the other group the tips of the branches are
pointed. The Enteromorpha intestinalis, which is an inhabitant of many
seas, has a thin membranous, tubular, cylindrical, and unbranched stem,
inflated upwards into a broad round head, being more or less wrinkled
and curled throughout. Downwards it tapers to a fine thread, and
although attached at first, at last it becomes floating. Several of
these plants rise from the same root, sometimes to the height of two
feet, at others not more than an inch, and they are of every width, from
the tenth of an inch to three inches, their colour being grass green.
The typical form of the other group is much branched, and all the
branchlets are finely pointed.
The three genera Codium, Bryopsis, and the marine Vaucherias are all
soft plants characterized by their filaments being tubular, however much
they may be branched. They agree also in being reproduced by zoospores
developed from the green matter within little sacs attached to the
exterior of their filaments. The species of the genus Codium differ
much, although formed of similar elements. In the C. tomentosum, which
is from three to twelve inches long, the dark green stem is thicker than
a crow’s quill and much branched; while Codium Bursa, on the contrary,
is a dark green round spongy lump of tubular filaments, densely
interwoven and matted together. These masses, which are from one to
eight inches in diameter, become hollow when old, and different sizes
and ages grow together in a group.
The Bryopsis is a yellowish green tubular plant, from two to four inches
high, plumed like a feather, and sometimes replumed. It is a rare plant
in England, and grows on the larger Algæ in deep water.
The Vaucheria marina forms soft limp tufts of hair-like filament filled
with bright green matter, which often runs partially out. It is from one
to three inches high, and has a few long upright branches, to which are
attached small stalked pear-shaped sacs containing zoospores. Both this
plant and the Vaucheria velutina, grow on muddy shores.
[Illustration: Fig. 22. Ulva latissima: _a_, portion of ordinary frond;
_b_, cells in which the endochrome is beginning to break up; _c_, cells
from the boundary between the coloured and colourless portion, some
containing zoospores; _d_, ciliated zoospores; _e_, development of
zoospores.]
The Ulvas, which are the grass green layers seen on all our coasts,
originate in the simple vegetable cell, whatever form their foliaceous
fronds may ultimately assume. When the cell is divided in one direction
only, a confervoid filament is the result; and if the filament should
increase in breadth as well as length, according to a determinate law, a
ribbon-shaped frond may be produced; but when the original cell is
divided into four cells, and each of these four and all their successors
undergo similar division, the increase being as the series 1, 4, 16, 64,
&c., a membranous expansion is formed, in which all the cells are firmly
attached to one another, and every portion is the exact counterpart of
another. The cells of the Ulvas frequently exhibit an imperfect
separation of the granular endochrome into four parts preparatory to
multiplication by double division, and the entire frond or leaf shows
the groups of cells arranged in clusters containing some multiple form
of four, as in fig. 3, page 171.
The frond membrane of the true Ulvas, as that of the Ulva lactuca, is
formed of but one layer of cells; the frond itself is thin as cambric
paper, almost transparent, and of a pretty light green. When young it is
a puckered inflated bag, which afterwards bursts and opens into a flat,
ribless, wavy, more or less rounded expansion, three to six inches long,
and as many broad. This plant, which is attached to the rocks between
the tide marks on our shores, is rare in the Mediterranean; nor is it so
common in Britain as the Ulva latissima (fig. 22), which is cosmopolite,
and abundant everywhere. It is found as a ribless irregular expansion of
a full bright green in deep water, and of a yellow apple green when in
shallow water, and exposed to the light. The base and stem are very
short, and the frond, which the microscope shows to be formed of two
layers of cell membrane, spreads so rapidly into crisp wide-lobed
foliations, that the parts often overlap each other in stiff bulging
folds. It is from six inches to a foot in height, and from three to
twelve inches wide. The frond of the Ulva Linza is also formed of two
layers of cells, but so small and so closely pressed together that the
two layers can only be detected by the microscope. This plant, which is
from six inches to two feet long, is a ribless, narrow, ribbon-shaped
expansion with curled wavy edges tapering to a base, and either blunt or
pointed at the top. Its colour is the same as that of the Ulva lactuca.
In the Ulvas, which are multilocular plants, some cells are selected to
bear fruit, and others not. The granular endochrome of these chosen
cells divides into several parts, which are at first in close contact
and at rest; then they become restless, acquire four or a greater number
of cilia, and pass through a fracture in the cell wall into the water,
in which they swim freely as zoospores. After a time they come to rest,
attach themselves to some object, and begin to grow. The walls of the
cells which have thus discharged their endochrome in the form of
zoospores, remain as colourless spots on the frond. The whole colouring
matter of a portion of the frond may escape as zoospores, leaving behind
it nothing but a white membrane. With a microscope, this process may
sometimes be observed in all the different stages of its progress.
Every full-grown Ulva has its own precise and definite form, but
whatever that may be, the young plants on their first appearance from
the shore are in all respects similar to Confervas; the top cells soon
divide, and a plane or sac-like frond is formed.
Certain Ulvas, which have a yellow tint, produce small zoospores with
only two cilia, but in the Ulva bullosa and the Ulva latissima four
zoospores are produced in the same cell, each having four cilia. The
same fructification prevails also in the purple Ulvas—Porphyra laciniata
and vulgaris. The latter is seen in winter and the early spring,
covering the rocks near high water mark, with its tiny bright purple
lanceolate leaves. Later in the season it grows into a flat narrow
ribless frond with a pointed end, and about two feet long, the margin of
the frond becoming waved and plaited as the plant increases in growth.
At a later period, it is seen mixed with the Porphyra laciniata, which
is a ribless flat frond of a dull purple; sometimes it is very thin,
divided or torn, and occasionally growing in a circle round its root.
Both forms are sold as laver.[40]
The RHODOSPERMEÆ, Florideæ, or Rosetangles, are the most beautiful of
the marine vegetation. No sea plant surpasses them in delicacy and grace
of form or richness of colouring, but the most beautiful are seldom
seen, because they grow below the line of ebb tides, or under the
shelter of other sea weeds in the rock pools left at low water, their
crimson tints being deepest when sheltered from strong light. The
Rhodosperms, which have representatives in every sea, are much more
numerous than the green Algæ both in genera and species. Thirteen
orders, comprising sixty-seven genera, inhabit the British coasts. Many
are exceedingly minute, forming patches and velvety cushions on rocks
and other Algæ; a vast number have jointed filamentous fronds, while
others consist of tubular filaments, and many exhibit a shrub-like
collection of firm branches; some are flat and foliaceous expansions
without a midrib, either thin and delicate, or thick and strong, while a
very brilliant group of both narrow and spreading fronds possess a
midrib as a distinguishing character. The structure of the frond varies
from a simple membranous to a cartilaginous or even horny substance,
caused by a greater development of the cellular tissue, which in the
higher kinds of Florideæ divides the epidermal layer or skin from the
parenchyme or spongy matter within.
The mode of reproduction by tetraspores, as well as by simple spores,
distinguishes the Rhodosperms from the other two great divisions of the
marine Algæ. These bodies are produced by the division of the red or
crimson endochrome into four parts, which remain in the cells till they
acquire an envelope; their form, which is much varied, depends upon that
of the endochrome. Some are produced by the breaking up of a globe of
endochrome from the centre into four pyramidal segments; or should the
endochrome be elliptical, by dividing it into four by three parallel
segments, or a mass may be divided into four by horizontal and vertical
sections. Some of these are represented, greatly magnified, in fig. 23.
The tetraspores are lodged in wart-like excrescences, immersed either
partially or wholly in some part of the frond.
[Illustration:
Fig. 23. A, Polyides rotundus:—_a_, thin slice showing the
wedge-shaped spores; _b_, tetraspores.
B, Furcellaria fastigiata:—_c_, thin slice showing a nucleus with the
dividing spores; _d_, one of the large cells; _e_, a tetraspore.
]
The simple spores are produced within colourless tubercules called
nuclei, variously situated upon the plant, as at fig. 23 _a_, _c_. These
nuclei contain many microscopic spores. Sometimes the nuclei are
enclosed in conceptacles or ovate sacs, which are either perforate or
not at the apex. These contain many microscopic strings of cells like
jointed threads, and the endochrome in each joint of these threads is
converted into a spore successively from the summit downward. Sometimes
the endochrome in one or two joints only, becomes a spore whether
terminal or central, and when the spores break through the joint wall
and fall off from the threads, they are collected without any definite
order into a mass within the nuclei. Sometimes new joints or cells are
produced on the threads when the old ones have yielded their fruit.
Occasionally a globose nucleus contains several secondary nucleoli full
of spores. In every instance, the perfect spore is a dense grumous mass
surrounded by a hyaline sub-gelatinous coat consisting of at least two
membranes. The situation, mode of growth, and structure of the nuclei
vary almost infinitely, and together with the structure of the frond
afford the distinctive marks by which the genera are separated from each
other.
The spores and tetraspores are equally capable, like buds, of
reproducing their species; but the spores are believed to be in some
cases fertilized by spindle-shaped particles, and consequently are
considered to be the true fruit. Antheridia, or sacs containing these
particles, have been discovered in various genera of Rhodosperms.
Although, as a rule, the red Algæ have two modes of vegetative
reproduction, yet there are various species in which tetraspores only
have hitherto been met with.
[Illustration: Fig. 24. Vertical sections of conceptacles:—_a_,
Gracilaria armata; _b_, Grinnelia americana; _c_, Corallina officinalis,
the membrane of which, more highly magnified, is shown at _d_.]
A large proportion of the higher Rhodosperms is distinguished from those
possessing the preceding mode of fructification by the internal
structure of their reproductive nuclei. In some of these Algæ the nuclei
are divided into two equal chambers by a fibro-cellular substance to
which the spores are attached; in others, pear-shaped spores radiate
from a fibro-cellular substance at the base of the nucleus. There are,
moreover, Algæ which have nuclei containing conical spores whose broad
bases radiate from the centre, and other arrangements occur.
The Rhodosperms are comparatively small plants. Some which form velvety
cushions on stones, or minute tufts on small Algæ, are only the fraction
of an inch high, but the larger kinds range from one to four, six, ten,
or twenty inches; probably none exceed two feet. In thickness, some
fronds are fine, like jointed and branched hairs, while others are
thick, like hog’s bristles or crow quills. Numerous as the forms are,
the simple jointed filamentous frond is connected by a series of forms
with the highest order of the class.
A great portion of the Rhodosperms on the British coasts is composed of
the exquisitely beautiful order of the Ceramiaceæ. They abound in every
rocky pool, on every piece of wood that has been long exposed to the
waves, on rocks and stones, and, above all, they fringe the Zostera
marina, or sea wrack, as well as the firmer Algæ, with every shade of
red from bright crimson to purple. They are articulated filiform plants,
approaching in simplicity of form to the Confervas. The genus
Callithamnion, which has thirty species in the British seas, consists of
cylindrical jointed threads more or less profusely branched, and
distinguished by having the divisions between the joints opaque and of
various shades of red and purple, while the joints themselves are
transparent and colourless, so that the stem and branches appear to be
striped across by alternately white and coloured bands which are often
visible to the naked eye, notwithstanding the smallness of the plants
and the delicacy of their filaments, as the C. sparsum,—which is a soft
purple tuft of jointed threads scarcely one-tenth of an inch high.
The Callithamnion corymbosum has a soft jointed filamentous stem,
hair-like below, fine as a cobweb above, and excessively branched, with
dichotomous branches. In fig. 25 _a_ represents a thread of this plant
with tetraspores, much magnified; _b_, a portion of the same, more
highly magnified; _c_, a thread with naked nuclei, gongylospermous, that
is, filled with a mass of spores, magnified; and _d_, a spore, magnified
more highly. The nuclei are naked in all the Ceramiaceæ.
[Illustration: Fig. 25. Callithamnion corymbosum.]
The genus Ceramium, some species of which have spinulose branchlets, is
characterized by the tips of the forks of its terminal branchlets being
hooked inwards, and by the stems and branches being striped by alternate
hyaline and coloured bands as in the preceding genus, though the
arrangement of the colours is somewhat different. The Ceramium ciliatum,
which is a dense tuft of capillary jointed filaments, from two to six
inches long, repeatedly and regularly forked, has the tips of the last
forks so much hooked inwards, that the extremities of the branchlets
look as if they were heart-shaped. It has minute spores in globular
nuclei, sessile on the branches with two or three branch-like hairs
beneath them, and tetraspores set in the coloured parts of the joints
with a thorn between each, for in this plant the centre of the joint is
hyaline, the rest coloured.
The genus Griffithsia contains various species of bright rose-coloured
plants, which become bleached when put into freshwater, and form a
circle when spread out. Soft, tender, and gelatinous, they form dense
tufts of jointed and branched filaments on rocks at low water mark. The
filaments are slender below, capillary and forked above, and the joints
contain one linear upright rose-coloured tube, which is seen throughout
their transparent walls: a distinguished mark of the genus. Tetraspores
are borne on the hair-like jointed ramuli, and spores are amassed in
coated roundish sessile nuclei, surrounded by minute hair-like fibres.
Several species once called Griffithsia differ so much from the others,
that they are by some referred to Halurus, which has the stems and
branches thickened by overlapping whorls of tiny forked jointed and
curved ramuli. They are propagated by spores, enclosed in clusters of
nuclei borne on the tips of short branches, with a mass of curved ramuli
folding over them, and by tetraspores attached to the inside of another
set of curved ramuli. Antheridia have been discovered in several genera
of the group Ceramiaceæ, especially in Ceramium, Callithamnion,
Griffithsia, and Halurus. They consist of little clusters of cells
variously arranged, in which the active particles known as spermatozoids
are generated.
The Polysiphonia are Algæ, seen in tufts from ten to twelve inches long,
of usually much branched jointed filaments, on rocks, corallines, and
the smaller Algæ at low water mark. The joints of the filaments contain
upright tubes, full of purple or reddish brown matter, which is seen
through their transparent walls. The number of these colour tubes vary
from four to ten, eighteen, or even twenty, and form the characteristic
of the genus. Thus there is a similarity of structure between the
Polysiphonia, a genus of the highest order amongst Rhodosperms, and the
Griffithsia, which is one of the lowest. The Polysiphonia elongata,
which is from six to twelve inches high, has four primary and several
secondary colour tubes in the transparent joints of its filaments. Like
many of its congeners, this plant does not come to perfection or bear
fruit till the second spring. In its youth, it resembles the full grown
plant but is smaller, and the colour tubes are not formed in the
capillary threads of the tufts, which with many of its branchlets are
deciduous, leaving the plant in its naked winter state. With returning
warmth, it assumes its perfect form, and in March and April bears fruit,
which consists of nuclei in conceptacles, sessile on the branches,
either clustered or scattered. The spores are at the top of jointed
threads rising from a substance at the base of the nuclei. In some
species of this genus, tetraspores only have been found.
The Cryptonemiaceæ are the most numerous and diversified of all the
orders of the Rhodosperms. Thirty-five genera are widely dispersed
throughout the world, chiefly in the northern hemisphere; twenty-four
genera at least occur on the east coast of North America; and fifteen
genera have representatives in the British seas. This multitude of
generic forms is divided into two groups of gelatinous structure, the
one having inarticulate fronds composed of articulate threads closely
incorporated, the other membranaceous, formed of cells closely
incorporated into a foliaceous expansion. Most of these plants have a
stratum of cellular tissue, interposed between a spongy matter in the
interior of the frond, and the epiderm or external skin, which for the
most part consists of a simple layer of minute cells firmly united by
their sides, generally forming a mere film; but it may be thin and
flexible, thick, tough, or leathery, according to circumstances.
The Furcellaria fastigiata (fig. 23 B) has an intermediate layer of
cellular tissue between its skin, and a pulpy interior. The frond is
cylindrical, smooth, strong, and opaque, repeatedly forked with long
narrow forkings. The root is fibrous, and the stem short and tapering.
Masses of spores nestle under the skin and swell out the upper forkings,
and oblong tetraspores are deeply imbedded in the same.
In the Dumontia filiformis the simple undivided stem and branches are
filled with a watery jelly.
The stem of the Chylocladia kaliformis is a cylindrical tube, from four
to eighteen inches high, constricted at intervals of half an inch or
more into long hollow joints; branches of the very same construction but
smaller spring from each constriction either opposite to one another or
in whorls; these again have lesser branches, all tapering more or less
to each end. The plant, which is of a pink colour fading to greenish
yellow, is propagated by tetraspores imbedded in the branches, and by
transparent conceptacles sessile on the branchlets, enclosing nuclei
containing pyramidal spores. We neither possess the Constantinea rosa
marina, nor the C. sitchensis, some of the largest and finest plants of
the group, both being inhabitants of high latitudes, but there are some
very pretty species on the British coasts. They are supposed to be
annuals.
The red dulses belong to the foliaceous and gelatinous part of this
order. The Chondrus crispus, or Irish moss, sold as Carrigeen, is very
common on rocky coasts in the northern seas. It is from three to eight
inches high, and exceedingly varied in form. The frond is thickish,
firm, and elastic, with a stratum of cellular tissue under the skin,
which is probably much developed, as the plant becomes horny when dried.
It is reproduced by tetraspores, in large oval groups scattered all over
the surface, often prominent on one side only, and, in some rare
instances, spores in prominent oval conceptacles are immersed in the
lesser frond divisions. Besides these are warts composed of radiating
threads, possibly antheridia, but not made out.
[Illustration: Fig. 26.—A, Rhabdonia Coulteri, portion of nucleus. B,
Sphærococcus coronopifolius; portion of nucleus and single spore. C,
Wrangelia penicillata, spore threads. D, Cruoria pellita, tetraspores.]
The Rhodymeniaceæ are sometimes filiform, but for the most part they are
compressed flat cellular fronds, spreading widely from a short delicate
stem. They are usually of a blood red, but Rhodymenia palmata, or common
Scotch dulse, is of a dark purple. The tetraspores are variously
disposed, and simple or compound globular conceptacles containing nuclei
are either attached externally to the filiform fronds, or partly
immersed in those that are foliaceous. The spores are produced in the
joints of moniliform threads within the nuclei, which are sometimes
divided into two chambers by threads running from wall to wall.
Rhabdonia (fig. 26 A) belongs to this group.
The Wrangeliaceæ are filiform, many species consisting of a central
thread coated more or less with smaller ones, sometimes so disposed as
to form a most elegant lacework. Each joint of the stem, branches, and
branchlets is beset with whorls of short slender forked and jointed
ramuli. They have clusters of spores in stalked capsules. The spore
threads of Wrangelia penicillata (fig. 26 C) are surrounded by a whorl
of ramuli composed of radiating pyriform spores arising from the
endochrome of terminal cells.
The Squamariæ resemble lichens in spreading themselves in a red crust
over stones and rocks. They have roots below, and warts, on their upper
surface, in which there are tufts of moniliform spore-bearing threads.
The tetraspores of Cruoria pellita are shown in fig. 26 D; its
repeatedly forked filaments taper upwards, and the tetraspores are
formed in the swollen centre cell of the filaments. The Peyssonnelia
grows on shells and other marine objects, and extends from the
Mediterranean to Ireland, and the east coast of North America.
The Polyides rotundus (fig. 23 A), representing the only genus of the
Spongiocarpeæ, has a dark purple solid gristly cylindrical stem,
repeatedly and regularly forked, all being of the same thickness. The
tips of the last forkings, which are small and equal, give the top of
the plant a rounded form. The microscope shows that the stem and
branches are composed of a central column of interlaced threads and
radiating cells; it shows, moreover, that hyaline nuclei containing a
cluster of conical spores whose broad bases radiate in all directions
from a centre, as in fig. 23 _a_, are scattered among the articulated
threads of oblong irregular spongy warts which clasp or embrace the stem
and branches. The tetraspores are buried in the ends of the last forks.
This plant is so like the Furcellaria fastigiata (fig. 23 B), that it
affords a remarkable instance of similarity of form and total diversity
of fructification, not only in the spores and their arrangements, but in
the form of the tetraspores; for in the Polyides they are formed by two
sections, one vertical and the other horizontal, while in the
Furcellaria the endochrome is divided by three annular sections, as in
fig. 23 E.
The Gelidium corneum, common in Britain and almost everywhere,
representing the group Gelidiaceæ, is opaque, firm, and of a dark
purple. The axis with its alternate and repeated branches lying all in
one plane, is composed of confervoid threads. This plant is
distinguished by having its spore-cases or nuclei divided into two
chambers by a fibro-cellular substance; the spores are either attached
to this or to a network of threads; these bodies and the tetraspores are
lodged in the tips of the branchlets. It is one of the most variable of
all Algæ.
The Sphærococcoideæ comprise some of the most common and beautiful Algæ,
remarkable for their brilliant rose and purple tints. This section
consists of those red Algæ which have their nuclei lodged in an external
subglobose conceptacle, the spores being formed at the tips of jointed
threads rising from a substance at the base of the nucleus. A portion of
a nucleus of Sphærococcus coronopifolius, and a single spore magnified,
is shown at fig. 26 _b_. The tetraspores are variously disposed. The
fronds in this family are either gristly or membranaceous, and totally
different from those which follow. They often assume a leafy aspect from
the regularity of the nerves, which sometimes perform the functions of a
stem when the membraneous border has decayed, and then they give rise in
turn to new fronds. That happens in some species of Nitophyllum: a very
short stem rises from a minute disc, and spreads widely into a flat
ribless expansion, more or less deeply slit into broad rounded
divisions. Wavy nerves from the top of the stem spread through the
fronds, which are left bare in winter, and give rise to new fronds in
spring. The leaves of the Delesseria sanguinea, from two to eight inches
long and from one to six inches broad, are of the richest colour and
most delicate structure, with evenly curled edges, and a firm solid
stem, with prominent midrib and nerves. In winter globose stalked spore
conceptacles are borne on the skeleton midribs of the summer’s leaves
from which the margin has decayed, which thus become the stems of the
next year’s plant. In this plant tetraspores in small special stalked
leaflets fringe the skeleton midribs; in the Nitophyllums they are
either scattered in dots over the frond, confined to the centre, or in
lines round the margin. As regards the internal structure of this order,
nothing can be more various, but they never acquire a truly articulate
form. The genera and species of this group are widely distributed; they
have many representatives in the Mediterranean.[41] The genus
Sphærococcus is confined to Europe, while numerous genera are
exclusively tenants of the southern hemisphere. The Gracilaria
lichenoides, the Ceylon moss, is celebrated for its gelatinous
qualities; and the Gracilaria compressa on our own shores is excellent
as a pickle or preserve, and very ornamental. One of the most beautiful
Algæ known is the Grinnelia americana, which abounds on the eastern
coast of North America; fig. 24 _b_, is a vertical section of its
conceptacle, showing the rudimentary placenta and spore threads. It
differs singularly from the Delesseria sanguinea, of which it is an
exact analogue, in the capsules being scattered over the surface of the
frond instead of being situated on the midrib.[42] The Delesseria
sanguinea is now known as the Wormskioldia sanguinea.
The Corallines are florid Algæ, which absorb such a quantity of
carbonate of lime from the surrounding water, that they become rigid,
hard, and often stony. They are purple or pink when fresh, white and
sometimes brittle when dry, and are propagated by strings of spore
threads rising from the base of the nuclei which are enclosed in
conceptacles or spore cases, open at the top. Some are articulate,
composed of closely compacted threads, as the Corallina officinalis, a
pretty little branched and bushy plant, most luxuriant in deep water,
and particularly abundant in the rocky pools. Its urn-shaped spore sacs
are attached to the tips or sides of the branches; fig. 24 _c_ is a
vertical section of one of them magnified, and _d_ is a membrane of the
same, more highly magnified, with impressions of the external cells. The
joints of the articulate corallines, which are flexible and vary much in
length, are either free from carbonate of lime, or ornamented with
calcareous plates; it is through these open spaces that the plant is
believed to obtain nourishment. The forms of the corallines are varied
beyond description; many are mere amorphous crusts on stones and sea
weeds, increasing from the centre outwards as in the lichens, others are
lobed and branched like real corals. Corallines ascend to very high
latitudes, but abound most in warm and tropical seas: either free, or
coating pebbles at vast depths, they form the last zone of vegetable
life.
The Laurenciaceæ have fronds which are soft and thread-like, or solid,
fleshy, and inarticulate; both are repeatedly branched. The colour of
these plants is purple or a dullish red, but they are extremely
sensitive to the influence of light and air, changing through every
shade of orange, yellow, or green, according to the exposure, and like
many other florid Algæ they lose their colour in fresh water. They are
amongst our commonest sea weeds. The Laurencia pinnatifida is the pepper
dulse of Scotland, and is also native on the eastern and western coasts
of North America. Species have been found at the Cape of Good Hope,
Australia, and New Zealand. The fructification in this section is quite
peculiar. They have tetraspores lodged in the branchlets; and egg-shaped
conceptacles with a terminal pore, enclosing nuclei with pear-shaped
spores radiating from a fibro-cellular mass at their base. The
antheridia, which differ in the different species, attain a greater
degree of complication than in other tribes. In Laurencia tenuissima
they form curious lateral twisted plates of a greyish tint, bordered
with large cells. The plate is occupied by the productive cells of a
much smaller size, evidently springing from a cellular branched axis. In
Laurencia pinnatifida instead of a plate there is a somewhat hollow
cup-shaped disc, formed of dart-like vertical groups of pale cells
surmounted by two or three larger oily-looking sacs filled with yellow
pigment. These bodies are sometimes forked, and appear to shoot out from
the mass. L. dasyphylla presents a third modification, the antheridium
being a sac, and the dart-like groups of cells being ejected from the
minute terminal orifice. The moving particles produced in the cells of
these three forms, differ a little in shape, and as they do not
germinate they are believed to be spermatozoids, though no cilia have
been found on them.
The Rhodomelaceæ, the last and highest family of florid Algæ, are, as
the name implies, of a rich red brown colour. None of the other
Rhodosperms can vie with them in peculiarity or variety of structure.
The fronds may be areolate or reticulate, filiform or variously leafy,
articulate or inarticulate.
[Illustration: Fig. 27. Dictyurus purpurascens.]
Some genera, as for example Dasya, have slender, often elegantly
branched threads, while such genera as Amansia and Odonthalia have
instead a flat and pinnatifid frond. The latter, which has a very
conspicuous cellular reticulation, is a genus of high latitudes, but is
common on some parts of the Scotch and North American coasts. The
British seas are rich in many genera of this order, and analogous forms
occur in the southern hemisphere, where there are at least twenty-three
genera. Many are remarkable for their singularity of structure: the
Claudea for example, which is one of the most elegant of the Algæ, has a
cancellated frond and is the ornament of warm seas; the Amansia and
Leveillea which are distinguished by the beautiful reticulation of their
fronds caused by large hexagonal cells; and the Dictyurus, in which the
net forms a spiral web round the principal stem. Fig. 27 shows a portion
of the network of Dictyurus purpurascens magnified. All the genera of
this order possess free areolate hollow conceptacles perforated above,
and containing nuclei, from the base of which short tufts of threads
arise, each bearing a large obovate spore at its apex. The tetraspores
are arranged in series either within the frond, or in distinct pod-like
receptacles called stichidia. Fig. 28 shows the Polyzonia cuneifolia
with its tetraspores arranged in rows in their pod-like stichidia,
together with the areolated conceptacle and spores, all highly
magnified. The antheridia differ in form in the different genera. In the
Dasya they assume that of pods full of cells, in which the motile
particles are generated; in the Rytiphlæa tinctoria the antheridia
resemble those of the Dasya except in being elliptical, and in the
Rytiphlæa pinastroides they are cellular bodies, without any investing
membranes, clothed with delicate hairs.
The form of the Rhodosperms, as well as the limits of the species, like
those of other Algæ, are affected by many circumstances known and
unknown, such as the depth, temperature, saltness, and currents in the
water. The Gelidium corneum varies to such an extent that its forms may
not only be considered as distinct species, but even as belonging to
different genera. The Delesseria alata is sometimes destitute of its
margin, and then its midribs alone being left, it has the form of the
Delesseria angustissima. Several species of the florid Algæ, which in
their natural state have the tips of their fronds even and straight,
occasionally produce hooked and clasping tips.
[Illustration: Fig. 28. Polyzonia cuneifolia.]
Brackish water is often a cause of change. The Irish moss, Chondrus
crispus, when exposed to the fresh water of an estuary acquires great
breadth and thickness, while at low water mark it is thin and has narrow
forked branches, and there are many intermediate forms. The fruit rarely
varies with these changes; its disposition and intimate structure, as
well as that of the frond, are the points of prime importance for the
determination of genera and species in the Algæ.
The MELANOSPERMEÆ, or Melanosperms, are olive-green Algæ, sometimes
inclining to brown. They have fewer species than the Rhodosperms, but
the individuals exceed in abundance and in magnitude all the other Algæ.
These large Melanospermous Algæ, which form marine forests in both
hemispheres, are excessively strong and tough on the exterior but of a
looser texture within, so that the cells of their tissue are of
different sizes and forms, according to the degree of pressure. The
stems and branches are more dense than the leaves. This highest order,
however, has small and delicate Algæ united to the largest by many
intermediate forms. The Melanosperms are either monœcious or diœcious,
and bear their olive-green spores in cases, that is cysts, variously
disposed on the plants. Many have two kinds of zoospores differing in
nothing but size; they are produced in different organs; in some species
both are fertile, in others only one, and, in these cases, the other is
therefore supposed to be a fertilizing body, but however that may be,
there are certainly antherozoids in this group of Algæ, especially in
the order Fucaceæ.
The Ectocarpeæ have many representatives on our coasts, all of which are
tufts of articulated threads from one to eighteen inches long, branched
or simple. They are generally soft, some so flaccid that they cling
together, but sometimes they are firm and stiff. The cysts which are
attached to these threads have various forms; they are spherical,
siliquose (that is, like long pods), or of other shapes, according to
the species; but whatever form they may assume, they are filled with a
dense endochrome. Besides these they have active granules contained in
other distinct organs. M. Thuret has decided beyond a doubt that the
latter are small zoospores, and it is presumed that the endochrome in
the cysts is resolved into zoospores, but of a different order, as in
the Ulvas. These two organs are for the most part situated on different
individuals; in Ectocarpus pusillus (fig. 29 _b_) they are on the same.
The different forms of fruit carpels are represented magnified in fig.
29.
[Illustration: Fig. 29. Fruit of Ectocarpus:—_a_, E. sphærosporus; _b_,
E. pusillus; _c_, E. fenestratus; _d_, E. fasciculatus.]
The Ectocarpeæ contain little or no gelatine, whereas the genera of the
group Chordariæ have soft gelatinous fronds of many forms, either
incrustations, convex lumps, or tubers, like the Leathesia so common on
our coasts; small plants as the Mesogloias, which have soft slippery
filiform stems beset with myriads of moniliferous worm-like branches; or
lastly the Chorda filum, a simple unbranched slimy cylindrical cord,
varying from a quarter of an inch to the thickness of a pencil, and from
one to twenty or even forty feet in length in deep water. The cord is
tubular, divided into chambers by transverse partitions, formed of
interlaced vertical and horizontal articulated threads. It tapers at
each extremity, and the exterior, which is brown, is clothed with
pellucid hairs. Vertical spores are immersed throughout the whole
surface of the cord, and Dr. Harvey says that, mixed with these, there
are numerous narrow, elliptical, transversely striated cells, which
according to M. Thuret produce zoospores. Each plant rises solitary from
its own little disc, but as the Chorda filum is a social plant, vast
assemblies of it cover extensive areas of sand and mud, and form dense
thickets in our northern seas. There are bands of it in the North Sea 15
to 20 miles long, and more than 600 feet wide; there is a submarine
forest of it in Skapta Bay, Orkney; and in passing through the sounds of
the western islands, as between Kerrera and the mainland, there are
others. The long cords always lean in the direction of the tide, and
must oscillate between two zones of rest, one at the turn of the flood,
and another at the turn of the ebb. When dried the people use them for
fishing lines. In the Chordaria divaricata both kinds of spore cysts are
external, and give rise to zoospores.
In the preceding divisions of the Melanosperms the fronds consist of
articulated threads; in the succeeding divisions the fronds are
inarticulate. The latter comprise four very remarkable groups, of which
the Dictyoteæ are distinguished by a leathery or membranous frond,
sometimes cylindrical, but mostly flat, the surface of which is
reticulated and sprinkled with groups or little patches of naked spores
or cysts. The endochrome in the cysts is sometimes quadripartite, or
even divided into eight parts. In one of the genera only, anything like
antheridia have been found. The zoospores produced from the
quadripartite endochrome are large, of a dark colour, and have two
lateral cilia, while the bodies in the filiform much divided antheridia
seated variously in the tufted threads are far more minute and pale, but
with similar cilia. This order obtains its maximum of development in the
tropical and subtropical regions; several species are found in the
Mediterranean, while a few occur on our coasts, and on those of North
America.[43]
The genus Dictyota begins the zonarioid group, whose structure is very
curious. Every band (lacinia) of the frond terminates in a single cell,
by the constant division of which at the lower side, the other cells of
the frond are formed, the terminal cell of the frond being thus
continually pushed onwards. Hence it results that the longitudinal lines
of superficial cells converge, thus affording a ready method of
ascertaining the genus in default of fructification. When a new centre
of growth is to be made, that is, when the frond is to become forked,
the terminal cell divides longitudinally and then each half-cell grows
according to its own law. Fig. 30 shows the tip of the frond of the
Dictyota dichotoma magnified; the cells on its surface are square, and
the interior of each has a spiral structure.
[Illustration: Fig. 30. Dictyota dichotoma:—_a_, tip of young frond;
_b_, cell showing spiral structure; _c_, threads from marginal cells;
_d_, sorus of spores; _e_, _f_, terminal cells dividing into new centres
of growth.]
The Padina Pavonia, or Peacock’s-tail laver of our southern coast, and
those of North America and the Mediterranean, is sometimes included in
the genus Zonaria. The species is remarkable for its wedge-shaped
fronds, which are olive green shaded with rust colour, and, when in
fruit, they are striped across with dark concentric zones, which are
merely lines of spores immersed in the frond and seen through its
transparent superficial membrane. Each zone is ornamented with a fringe
of orange-coloured hairs. Parallel to, or rather concentric with, the
spores, is a row of articulated threads, which bear so strong a
resemblance to the antheridia of the Cutleria that a similarity of
function is suspected by Mr. Berkeley. Species of Zonaria, Padina, and
Haliseris, which is the most highly developed of the Dictyoteæ, are most
abundant in tropical and low latitudes.
The Cutleria multifida is a small plant not exceeding eight inches in
length, of an olive green varied with rusty tints. The frond is a flat
ribless expansion many times variously slit in the upper part. It is
beautifully marked by prominent dot-like tufts of fructification
scattered over both sides of the frond, and grows on rocks and shells in
from four to fifteen fathoms water.[44]
The great Laminariæ form the principal part of those vast submarine
forests which encircle the globe in the arctic and antarctic oceans.
None of these gigantic Algæ are to be met with in low latitudes, but
there are several smaller species. The Laminaria debilis of the
Mediterranean is not more than five inches high, and we have some
ribbon-shaped species also of small size. Besides, many small
individuals of the large species grow on our coasts at low water mark or
below it; but the largest individuals are only found at depths suited to
their size, so that the great Laminaria, or tangle forests, extend from
low water mark to a depth of fifteen fathoms.
The fronds of these Algæ are for the most part leathery and of a
fibro-cellular consistence. The Laminaria bulbosa is the largest of our
sea weeds. Mr. Berkeley says that individuals are sometimes found which
are a sufficient load for a man to carry. A flat stem, often more than a
foot long, rises with a twist from a round hollow bulb a foot in
diameter, throwing out numerous stout fibrous roots below; the stem is
bordered by a thin wavy membrane, whence these plants are commonly
called sea furbelows. At the top of the stalk there is a broad leafy
expansion cut into straps or segments, twelve or more feet long, and
from one to two feet wide.
The Laminaria digitata, commonly called the great tangle, oar weed, or
sea girdle, has a fibrous root, a stem six or more feet long, with a
wide expansion at its top cut into very long narrow segments. The fronds
of some Laminariæ are deciduous; the stem increases in size year by
year, a new frond springing from the apex and replacing the old one,
which at last separates from the point of junction with the new frond,
to which it is attached till the latter has attained its natural form
and dimensions.
The Laminaria saccharina, called the devil’s apron on our northern
coasts, is of a greenish olive when young, brownish when old. It has a
fibrous root, a stem several feet long, ending in a flat ribless
ribbon-like expansion, always very much longer than the stem, and
terminating in a point. The margin of the frond is even, but wavy or
puckered.
‘The fruit of these three great Laminariæ is imbedded here and there in
the surface of the frond, thickening it and forming cloudy patches.’[45]
It consists of thick club-shaped perpendicular cells in which the
endochrome is ultimately divided into four parts. This is certainly the
case in the Laminaria bulbosa, and also in the Alaria Pylaii, a species
of which latter genus, the Alaria esculenta of our own coasts, is a much
esteemed British dulse.
Abundance of colossal Algæ are found in the North Pacific, about the
Kurile and Aleutian Islands, and along the deeply indented and
channel-furrowed northwestern coast of America. The Nereocystis Lutkeana
forms dense forests in Norfolk Bay, and all about Sitka. Its stem
resembles whipcord, and is sometimes 300 feet long. It is exceedingly
slender at the top, where it terminates in an enormous air-bladder six
or seven feet long, and about four feet and a half in diameter at its
widest part, the lower extremity passing into the stem. This huge
air-vessel, which is the usual seat of the sea otter, is crowned with a
tuft of twin leaves mostly rising on five stalks. These leaves, which
are membranous and lanceolate when young, and from one to two feet long
and two inches broad at the centre, are only marked with a few faint
nerves, but they ultimately split lengthwise, cover a large space, and
attain a length of twenty-seven or thirty feet, or even more. The growth
of the Nereocystis must be enormously rapid, since it is an annual, and
must therefore develop its whole gigantic proportions in one summer.[46]
Boats cannot pass through the floating masses of this plant, whose stem
is used for fishing lines, and whose cylindrical air-vessel serves as a
siphon for pumping water out of boats.
The Thalassiophyllum Clathrus is also an inhabitant of the Russian coast
of North America. It is about six feet high, very bushy and branched,
each branch bearing a broad leaf at its extremity which unfolds
spirally, and by this gradual development produces the stem with its
branches and lateral divisions. A spiral border wound round the stem
indicates the growth of the frond, which presents a large convex bent
lamina without nerves, or a leaf of which one-half is wanting. Numerous
long narrow perforations, arranged in a radiating form, give it the
appearance of a cut fan.
The Macrocystis pyrifera and the Laminaria radiata are the most
remarkable of marine plants, for their gigantic size and the extent of
their range. They are met with on the antarctic coasts two degrees
nearer the pole than any other vegetable, except the Diatomaceæ. The
stem of the Macrocystis is slender, smooth, round, and slimy, rising
from a fibrous root, like other Laminariæ, and bearing at its tip a
lanceolate or oblong lanceolate frond. This frond divides at the base;
the fissures extend upwards so as to form two petioles, each of which
swells into an oblong or pyriform air-vessel. Another fissure is formed
in a similar way a little above, and so on, till a single frond may at
the same time have eight or ten fissures, each of which will ultimately
gain the common apex. The margins of the fissures are at first perfectly
smooth, but they soon become ciliated like the outer edge. The
continuity with the fibrous base is at last broken, and the divisions of
the leaves going on indefinitely, the whole reaches the length of some
hundred feet, forming enormous floating masses which are wafted by the
waves hundreds of miles from their origin. Fructification only takes
place in young plants; consequently in such as are still attached to
their native rocks. Even in that youthful state, Mr. Darwin mentions
that such is the buoyancy of this powerful weed, that there is scarcely
a loose block of stone on the coasts of Cape Horn that is not buoyed up
by it.[47] The Macrocystis is native on the shores of the Atlantic, from
Cape Horn to 43° S. latitude; but on the Pacific coast, according to Dr.
Hooker, it extends to the river San Francisco in California, and perhaps
to Kamschatka. The plant is reproduced by pyriform cells, full of
endochrome, in nearly parallel rows imbedded in the fronds.
The rocky coasts of the Falkland Islands are covered with a vast growth
of the gigantic Macrocystis mixed with forests of the arborescent
Lessonia, which forms large dichotomous trees with a stem from eight to
ten feet high and a foot in diameter. The leaves are two or three feet
long, drooping from the forked branches like weeping willows. In the
Lessonia nigrescens the quadripartite endochrome, ultimately resolved
into spores, is contained in thickened club-shaped cells springing
vertically between the surfaces of the frond.
A transverse section of the stem of many of the larger sea weeds
presents zones, formed period by period, corresponding with the
development of the laminæ, roots, and branches. The stem of the Lessonia
bears a strong analogy to that of dicotyledons in having rings of
growth, though there is a great difference. As increase in Lessonia
takes place by the constant division of a flat leaf, the basilar portion
of which becomes the petiole and ultimately swells into a branch, the
stems have always a more or less elliptical form, and their section
exhibits an elliptical core. This form of the core is not however
peculiar, but exists in other Algæ. It is probable that the Lessoniæ,
although attaining so large a size, are really of rapid growth.[48]
The Ecklonia is essentially a southern genus, though one species ascends
to Spain and the Canaries. The frond is pinnatifid, the segments arising
from the evolution of marginal teeth. The stem of the Ecklonia
buccinalis, which is three or four inches thick and strongly inflated
above, exhibits rings of growth with an orbicular central pith.
The group of the Fucaceæ exhibits the highest structure of all the
olive-green Algæ, and forms a large portion of the sea weeds on our
coasts, but they abound more in individuals than in the number of genera
and species. A few have cylindrical stems and branches swelling out at
intervals into large oblong inflated air-vessels, which gives them
buoyancy in the water. The rest have a flat, ribbon-like stem, and for
the most part dichotomous branches with a decided midrib, but no
air-vessels, because they chiefly grow at half-tide level, and are
exposed twice every twenty-four hours. The most common of our fuci, the
Fucus vesiculosus, or bladder-wrack, has a midrib with air-vessels,
generally in pairs on each side of it, formed by the inflation of the
frond; these vessels, however, are frequently wanting, for it is the
most variable in form and most widely spread of the Fuci. The
fructification of this group is contained in large clavate receptacles
or expansions of an orange or greenish yellow colour situated at the
extremities or borders of the branches.
MM. Thuret and Decaisne discovered, by microscopic investigation, that
the fuci have a truly sexual fructification, consisting of male and
female cells inclosed in these receptacles. In the common Fucus
vesiculosus it was found that the male and female cells are either in
different individuals, or in different conceptacles on the same
individual; whilst in the Fucus platycarpus, both the male and female
cells were found to be contained in a globular cavity enclosed in the
flattened receptacles which grow at the extremities of the branches. The
cavity is lined with jointed hair-like filaments formed of cells, some
of which are so long as to project through a pore on the surface of the
receptacle in a spreading brush (see fig. 31, where the whole is highly
magnified). Towards maturity, the cells of some of these filaments
assume an ovoid form; the white viscous, granular matter in their
interior acquires an orange hue, and is divided into a multitude of
hyaline particles, each having an orange spot and two cilia of unequal
lengths, which enable these spermatozoids to swim with great vivacity in
the water as soon as they are set free by the rupture of the cell in
which they are inclosed. Besides these, dark olive-green female cells,
of a large pyriform shape, are fixed to the walls of the same cavity by
very short stems; their contents spontaneously divide into eight spore
cells, never more; each contains a colourless viscous liquid, which is
mixed with protein and yellow-green matter, and is inclosed in a double
coat. ‘The coats are united at the base, and when the spores are ready
for dispersion, the inner coat bursts through the apex of the outer one,
dragging with it a portion of the latter in the form of a little
peduncle. The immediate covering of the spores at length bursts, and
they are set free.’[49] In Fucus serratus, vesiculatus, and nodosus,
swarms of spermatozoids are produced, but M. Thuret has proved by
experiment that they never come to anything of themselves, and the
unfertilized spores perish.
[Illustration: Fig. 31. Vertical section of receptacle of Fucus
platycarpus.]
When a fertilized spore begins to grow, it assumes a pear shape, and
sends out from its narrow end filaments or footstalks containing solid
yellow grains at their extremities, where a hook or claw is formed by
which it fixes itself to rocks or stones. The spore then divides itself
into four equal cells of a brown colour, and by the continued
subdivision of these into four, the plant increases in size, and assumes
a form corresponding to the genus and species of the spore. Dr.
Carpenter mentions that in the Fucaceæ there is also a multiplication by
zoospores. These bodies are produced within certain of the cells that
form the superficial layer of the frond, and swim about freely for a
time in the water after their emission, until they fix themselves and
begin to grow; but these are merely gemmæ.
All the Fucaceæ are tough leathery plants. This is even characteristic
of the genus Cystoseira, various species of which may be seen on our
coasts at low water mark, or in the tide pools. They are little
shrub-like and somewhat thorny plants, not more than three feet high,
with a cylindrical stem and many branches, near the extremities of which
there are inflated air-vessels, sometimes two or three together; in some
species they are lower down. Long spiny conceptacles are situated at the
tips of the branches, but the endochrome does not divide in the germ
cells as it does in the Fuci, so that each cell produces but one spore.
‘Throughout all latitudes the two divisions of Fucaceæ—Fucoideæ and
Cystoseireæ, form the prevailing marine vegetation to which the name of
sea-weed is commonly applied, and the different genera so arrange
themselves as to present, with a few exceptions, a most harmonious
assemblage.’ ‘None of these approach the tropics; the Fucoideæ abound
towards the poles, and there attain their greatest bulk, diminishing
rapidly towards the equator, and ceasing some degrees from the line
itself; while the immense genus Sargassum finds its maximum in lower
latitudes and under the equator itself. In the opposite cold and frigid
zones the waters are inhabited by certain genera of Fucoideæ, which are
in a great measure representatives of one another.’[50] The huge
D’Urvillæa and the Sarcophycus in the Antarctic Ocean represent the
Himanthalia and Fucus proper in the north, and the Cystoseireæ and
Halidrys of the northern seas are represented by the Blossevillea and
Scytothalia in the southern.
The frond of the Himanthalia lorea is a knob about an inch high,
somewhat like a small mushroom; by degrees the top of the knob sinks in,
and the frond becomes cup-shaped. In the second year of growth it throws
out from its centre strap-shaped receptacles from two to three feet long
and the sixth of an inch wide; they are slimy, forked, and entirely
covered with fruit. The true frond sometimes becomes hollow and swells
into a bladder. This singular plant, which grows on our coasts, extends
from Norway to Spain. In the D’Urvillæa, its representative in the
southern hemisphere, the frond and receptacle are united, for the plant,
which is of large dimensions, has dichotomous fronds ten feet long, and
an inch or more in breadth. Their surface is ornamented with large
cavities like a honeycomb, and the fruit imbedded within them consists
of antheridia and club-shaped germ cells with four spores in each. These
plants form a large portion of the wrack and also of the living Algæ
which surround the Falkland Islands and Cape Horn; and they extend to
Western Chili, where the poorer class make a sweet mucilaginous soup of
them. The Sarcophycus potatorum, the only species of its order, is
nearly allied to the D’Urvillæa by the structure of its fruit, and is so
named from pieces of its frond being used to carry water. Many other
olive-green Algæ are peculiar to the southern hemisphere—among them the
Hormoseira, in which the frond, at first even and filiform, becomes
inflated so as to produce moniliform chains of vesicles, parts of which
are at length rough with the apertures of the conceptacles; this plant
has bladder-like air-vessels formed by swollen parts of the frond, like
many of our Fucaceæ.
Those genera which have distinct organs containing air, as the
Sargassum, of which there are numerous species, are either of low
latitudes or tropical, but are sometimes drifted by currents to the
extra-tropical shores. The Sargassum vulgare, however, grows on the
rocks in the Mediterranean. The whole plant is of a translucent reddish
brown; the stem has alternate branches, bearing lanceolate serrated
leaves with a midrib, and generally dotted with dark pores. The
air-vessels are small translucent round balls about the size of a
currant, borne on flat stalks in the axils of the branches, and the
spores are in conceptacles borne on the branchlets just above the
air-vessel. In one variety of this most variable plant, the Uva di mare,
the main stem ends in a loose bunch of these little air-balls.
The Sargassum bacciferum is often found in the Mediterranean, but only
as a wanderer drifted in from the Atlantic, where masses of it, like
floating meadows, occupy an area west of the Azores equal in extent to
that of France, which has never changed its position since the time of
Columbus, on account of the surrounding currents. Fields of it cover the
seas near the Bahama Islands, and another permanent area of Sargassum of
great extent occurs in the South Pacific. The Sargassum bacciferum is of
a pale translucent olive colour, having branched stems, with lanceolate,
midribbed, and serrated leaves, destitute of pores, and little stalked
air-balls in the axils of the branches. The same individual continually
produces new branches and leaves, and thus multiplies its species, but
it never produces fruit; consequently its habits exactly resemble those
of the Macrocystis, and as that plant becomes detached and floats after
fructification, it is supposed that the Sargassum bacciferum may grow on
rocks at the bottom of the Atlantic, between the parallels of forty
degrees north and south of the equator, and when detached after
fructification that it is uniformly drifted to particular spots which
never vary. ‘Multiplication is so rapid in the floating beds of the
Sargassum and Macrocystis, as to render fruit needless; and even the
common Fucus vesiculosus occurs in the Mediterranean under a peculiar
form consisting entirely of specimens derived from sea borne weed
carried in by the current which sets in towards the Mediterranean from
the Atlantic.’[51]
Kelp, the ashes of sea weeds, is the commercial source of iodine. Algæ
growing in deep water contain most of that substance; consequently the
kelp made at Guernsey, consisting chiefly of the ashes of Laminaria
digitata, is richer in iodine than that made elsewhere.
Marine vegetation varies both horizontally and vertically with the
depth, and it seems to be a general law throughout the ocean, that the
light of the sun and vegetation cease together. It consequently depends
upon the power of the sun, and the transparency of the water; so that
different kinds of sea weeds affect different depths, where the weight
of the water, and the quantity of light and heat, suit them best. One
great marine zone lies between high and low water marks, and varies in
species with the nature of the coasts, but exhibits similar phænomena
throughout the northern hemisphere. In the British seas this zone does
not extend deeper than thirty fathoms, but it is divided into two
distinct provinces, one to the south and another to the north. The
former includes the southern and eastern coasts of England, the southern
and western coasts of Ireland, and both the Channels; while the northern
flora is confined to the Scottish seas, and the adjacent coasts of
England and Ireland. The second British zone begins at low water mark,
and extends below it to a depth of from seven to fifteen fathoms. It
contains the great tangle sea weeds or marine forests mixed with fuci,
and is the abode of a host of animals. A coral-like sea weed is the last
plant of this zone and the lowest in these seas, where it does not
extend below the depth of sixty fathoms; but in the Mediterranean it is
found at seventy or eighty fathoms, and is the lowest plant in that sea.
The same law prevails in the Bay of Biscay, where one set of sea weeds
is never found lower than twenty feet below the surface, another only in
the zone between five and thirty feet, another between fifteen and
thirty-five feet. In these two last zones they are most numerous; at a
greater depth the kinds continue to vary, but their numbers decrease.
The distribution in the Ægean sea was found by Professor E. Forbes to be
perfectly similar, only that the vegetation is different and extends to
a greater depth in the Mediterranean than in more northern seas. He also
observed that sea weeds growing near the surface are more limited in
their distribution than those that grow lower down, and that with regard
to vegetation, depth corresponds with latitude, as height does on land.
Thus the flora at great depths in warm seas is represented by kindred
forms in higher latitudes. There is every reason to believe that the
same laws of distribution prevail not only throughout the ocean, but in
every sea.