THE BROWN ALGÆ (_Phæophyceæ_).
[Illustration: FIG. 24.--Forms of diatoms. _A_, _Pinnularia_. i, seen
from above; ii, from the side. _B_, _Fragillaria_ (?). _C_,
_Navicula_. _D_, _F_, _Eunotia_. _E_, _Gomphonema_. _G_, _Cocconeis_.
_H_, _Diatoma_. All × 300.]
These plants are all characterized by the presence of a brown pigment,
in addition to the chlorophyll, which almost entirely conceals the
latter, giving the plants a brownish color, ranging from a light
yellowish brown to nearly black. One order of plants that possibly
belongs here (_Diatomaceæ_) are single celled, but the others are for
the most part large seaweeds. The diatoms, which are placed in this
class simply on account of the color, are probably not closely related
to the other brown algæ, but just where they should be placed is
difficult to say. In some respects they approach quite closely the
desmids, and are not infrequently regarded as related to them. They
are among the commonest of organisms occurring everywhere in stagnant
and running water, both fresh and salt, forming usually, slimy,
yellowish coatings on stones, mud, aquatic plants, etc. Like the
desmids they may be single or united into filaments, and not
infrequently are attached by means of a delicate gelatinous stalk
(Fig. 25).
[Illustration: FIG. 25.--Diatoms attached by a gelatinous stalk.
× 150]
They are at once distinguished from the desmids by their color,
which is always some shade of yellowish or reddish brown. The
commonest forms, _e.g._ _Navicula_ (Fig. 24, _C_), are boat-shaped
when seen from above, but there is great variety in this respect.
The cell wall is always impregnated with large amounts of flint, so
that after the cell dies its shape is perfectly preserved, the flint
making a perfect cast of it, looking like glass. These flinty shells
exhibit wonderfully beautiful and delicate markings which are
sometimes so fine as to test the best lenses to make them out.
This shell is composed of two parts, one shutting over the other
like a pill box and its cover. This arrangement is best seen in such
large forms as _Pinnularia_ (Fig. 24, _A_ ii).
Most of the diatoms show movements, swimming slowly or gliding over
solid substances; but like the movements of _Oscillaria_ and the
desmids, the movements are not satisfactorily understood, although
several explanations have been offered.
They resemble somewhat the desmids in their reproduction.
THE TRUE BROWN ALGÆ.
These are all marine forms, many of great size, reaching a length in
some cases of a hundred metres or more, and showing a good deal of
differentiation in their tissues and organs.
[Illustration: FIG. 26.--_A_, a branch of common rock weed (_Fucus_),
one-half natural size. _x_, end of a branch bearing conceptacles. _B_,
section through a conceptacle containing oögonia (_og._), × 25. _C_,
_E_, successive stages in the development of the oögonium, × 150. _F_,
_G_, antheridia. In _G_, one of the antheridia has discharged the mass
of spermatozoids (_an._), × 150.]
One of the commonest forms is the ordinary rock weed (_Fucus_), which
covers the rocks of our northeastern coast with a heavy drapery for
several feet above low-water mark, so that the plants are completely
exposed as the tide recedes. The commonest species, _F. vesiculosus_
(Fig. 26, _A_), is distinguished by the air sacs with which the stems
are provided. The plant is attached to the rock by means of a sort of
disc or root from which springs a stem of tough, leathery texture, and
forking regularly at intervals, so that the ultimate branches are very
numerous, and the plant may reach a length of a metre or more. The
branches are flattened and leaf-like, the centre traversed by a
thickened midrib. The end of the growing branches is occupied by a
transversely elongated pit or depression. The growing point is at the
bottom of this pit, and by a regular forking of the growing point the
symmetrical branching of the plant is brought about. Scattered over
the surface are little circular pits through whose openings protrude
bunches of fine hairs. When wet the plant is flexible and leathery,
but it may become quite dry and hard without suffering, as may be seen
when the plants are exposed to the sun at low tide.
The air bladders are placed in pairs, for the most part, and buoy up
the plant, bringing it up to the surface when covered with water.
The interior of the plant is very soft and gelatinous, while the outer
part forms a sort of tough rind of much firmer consistence. The ends
of some of the branches (Fig. 26, _A_, _x_) are usually much swollen,
and the surface covered with little elevations from which may often be
seen protruding clusters of hairs like those arising from the other
parts of the plant. A section through one of these enlarged ends shows
that each elevation corresponds to a cavity situated below it. On some
of the plants these cavities are filled with an orange-yellow mass; in
others there are a number of roundish olive-brown bodies large enough
to be easily seen. The yellow masses are masses of antheridia; the
round bodies, the oögonia.
If the plants are gathered while wet, and packed so as to prevent
evaporation of the water, they will keep perfectly for several days,
and may readily be shipped for long distances. If they are to be
studied away from the seashore, sections for microscopic examination
should be mounted in salt water (about 3 parts in weight of common
salt to 100 of water). If fresh material is not to be had, dried
specimens or alcoholic material will answer pretty well.
To study the minute structure of the plant, make a thin
cross-section, and mount in salt water. The inner part or pith is
composed of loosely arranged, elongated cells, placed end to end,
and forming an irregular network, the large spaces between filled
with the mucilaginous substance derived from the altered outer walls
of these cells. This mucilage is hard when dry, but swells up
enormously in water, especially fresh water. The cells grow smaller
and more compact toward the outside of the section, until there are
no spaces of any size between those of the outside or rind. The
cells contain small chloroplasts like those of the higher plants,
but owing to the presence of the brown pigment found in all of the
class, in addition to the chlorophyll, they appear golden brown
instead of green.
No non-sexual reproductive bodies are known in the rock weeds,
beyond small branches that occur in clusters on the margins of the
main branches, and probably become detached, forming new plants. In
some of the lower forms, however, _e.g._ _Ectocarpus_ and
_Laminaria_ (Fig. 28, _A_, _C_), zoöspores are formed.
The sexual organs of the rock weed, as we have already seen, are
borne in special cavities (conceptacles) in the enlarged ends of
some of the branches. In the species here figured, _F. vesiculosus_,
the antheridia and oögonia are borne on separate plants; but in
others, _e.g._ _F. platycarpus_, they are both in the same
conceptacle.
The walls of the conceptacle (Fig. 26, _B_) are composed of closely
interwoven filaments, from which grow inward numerous hairs, filling
up the space within, and often extending out through the opening at
the top.
The reproductive bodies arise from the base of these hairs. The
oögonia (Fig. 26, _C_, _E_) arise as nearly colorless cells, that
early become divided into two cells, a short basal cell or stalk and
a larger terminal one, the oögonium proper. The latter enlarges
rapidly, and its contents divide into eight parts. The division is
at first indicated by a division of the central portion, which
includes the nucleus, and is colored brown, into two, four, and
finally eight parts, after which walls are formed between these. The
brown color spreads until the whole oögonium is of a nearly uniform
olive-brown tint.
When ripe, the upper part of the oögonium dissolves, allowing the
eight cells, still enclosed in a delicate membrane, to escape
(Fig. 27, _H_). Finally, the walls separating the inner cells of the
oögonium become also absorbed, as well as the surrounding membrane,
and the eight egg cells escape into the water (Fig. 27, _I_) as
naked balls of protoplasm, in which a central nucleus may be dimly
seen.
The antheridia (Fig. 26, _F_, _G_) are small oblong cells, at first
colorless, but when ripe containing numerous glistening, reddish
brown dots, each of which is part of a spermatozoid. When ripe, the
contents of the antheridium are forced out into the water (_G_),
leaving the empty outer wall behind, but still surrounded by a thin
membrane. After a few minutes this membrane is dissolved, and the
spermatozoids are set free. These (Fig. 27, _K_) are oval in form,
with two long cilia attached to the side where the brown speck, seen
while still within the antheridium, is conspicuous.
The act of fertilization may be easily observed by laying fresh
antheridia into a drop of water containing recently discharged egg
cells. To obtain these, all that is necessary is to allow freshly
gathered plants to remain in the air until they are somewhat dry,
when the ripe sexual cells will be discharged from the openings of
the conceptacles, exuding as little drops, those with antheridia
being orange-yellow; the masses of oögonia, olive. Within a few
minutes after putting the oögonia into water, the egg cells may be
seen to escape into the water, when some of the antheridia may be
added. The spermatozoids will be quickly discharged, and collect
immediately in great numbers about the egg cells, to which they
apply themselves closely, often setting them in rotation by the
movements of their cilia, and presenting a most extraordinary
spectacle (_J_). Owing to the small size of the spermatozoids, and
the opacity of the eggs, it is impossible to see whether more than
one spermatozoid penetrates it; but from what is known in other
cases it is not likely. The egg now secretes a wall about itself,
and within a short time begins to grow. It becomes pear-shaped, the
narrow portion becoming attached to the parent plant or to some
other object by means of rootlets, and the upper part grows into the
body of the young plant (Fig. 27, _M_).
[Illustration: FIG. 27.--_H_, the eight egg cells still surrounded by
the inner membrane of the oögonium. _I_, the egg cells escaping into
the water. _J_, a single egg cell surrounded by spermatozoids. _K_,
mass of spermatozoids surrounded by the inner membrane of the
antheridium. _L_, spermatozoids. _M_, young plant. _r_, the roots.
_K_, × 300; _L_, × 600; the others, × 150.]
The simpler brown seaweeds, so far as known, multiply only by means of
zoöspores, which may grow directly into new plants, or, as has been
observed in some species, two zoöspores will first unite. A few, like
_Ectocarpus_ (Fig. 28, _A_), are simple, branched filaments, but most
are large plants with complex tissues. Of the latter, a familiar
example is the common kelp, "devil's apron" (_Laminaria_), often three
to four metres in length, with a stout stalk, provided with root-like
organs, by which it is firmly fastened. Above, it expands into a
broad, leaf-like frond, which in some species is divided into strips.
Related to the kelps is the giant kelp of the Pacific (_Macrocystis_),
which is said sometimes to reach a length of three hundred metres.
[Illustration: FIG. 28.--Forms of brown seaweeds. _A_, _Ectocarpus_,
× 50. Sporangia (_sp._). _B_, a single sporangium, × 150. _C_, kelp
(_Laminaria_), × ⅛. _D_, _E_, gulf weed (_Sargassum_). _D_, one-half
natural size. _E_, natural size. _v_, air bladders. _x_, conceptacle
bearing branches.]
The highest of the class are the gulf weeds (_Sargassum_), plants of
the warmer seas, but one species of which is found from Cape Cod
southward (Fig. 28, _D_, _E_). These plants possess distinct stems and
leaves, and there are stalked air bladders, looking like berries,
giving the plant a striking resemblance to the higher land plants.