The thallus in the simplest forms is unicellular; in the majority,
however, it is built up of many cells, which in a few instances are
exactly similar; but generally there is a division of labour, so
that certain cells undertake certain functions and are constructed
accordingly, while others have different work and corresponding
structure. Vessels or similar high anatomical structures are seldom
formed, and the markings on the cell-wall are with few exceptions very
simple. The Myxomycetes occupy quite an isolated position; their organs
of nourishment are naked masses of protoplasm (plasmodia).
As regards the external form, the thallus may be entirely without
special prominences (such as branches, members), but when such are
present they are all essentially alike in their origin and growth,
that is, disregarding the hair-structures which may be developed. A
shoot of a Seaweed or of a Lichen, etc., is essentially the same as
any other part of the plant; only among the highest Algæ (Characeæ,
certain Siphoneæ, _Sargassum_, and certain Red Seaweeds) do we find the
same differences between the various external organs of the plant body
as between stem and leaf, so that they must be distinguished by these
names.
_Roots_ of the same structure and development as in the Seed-plants
are not found, but _organs of attachment_ (rhizoids and haptera) serve
partly the biological functions of the root.
SYSTEMATIC DIVISION OF THE THALLOPHYTES. To the Thallophytes belong
three sub-divisions--Slime-Fungi, Algæ, and Fungi. Formerly the
Thallophytes were divided into Algæ, Fungi, and Lichens. But this
last group must be placed among the Fungi, since they are really
Fungi, which live symbiotically with Algæ. The _Slime-Fungi_ must be
separated from the true Fungi as a distinct subdivision. The _Algæ_
possess a colouring substance, which is generally green, brown, or
red, and by means of which they are able to build up organic compounds
from carbonic acid and water. The Bacteria, especially, form an
exception to the Algæ in this respect; like the Fungi and Slime-Fungi
they have as a rule no such colouring material, but must have organic
carbonaceous food; these plants form no starch, and need no light for
their vegetation (most Fungi require light for fructification). The
Myxomycetes, Bacteria, and Fungi derive their nourishment either as
_saprophytes_ from dead animal or vegetable matter, or as _parasites_
from living animals or plants (hosts), in which they very often cause
disease.
A remark, however, must be made with regard to this division.
Among the higher plants so much stress is not laid upon the
biological relations as to divide them into “green” and
“non-green”; _Cuscuta_ (Dodder), a parasite, is placed among
the Convolvulaceæ, _Neottia_ and _Corallorhiza_, saprophytes,
belong to the Orchidacere, although they live like Fungi,
yet their relations live as Algæ. In the same manner there
are some colourless parasitic or saprophytic forms among the
Algæ, and stress must be laid upon the fact that not only the
Blue-green Algæ, but also the Bacteria, which cannot assimilate
carbonic-acid, belong to the Algæ group, Schizophyceæ. The
reason for this is that systematic classifications must be based
upon the relationship of form, development, and reproduction,
and from this point of view we must regard the Bacteria as
being the nearer relatives of the Blue-green Algæ. All the
Thallophytes, which are designated Fungi (when the entire
group of Slime-Fungi is left out), form in some measure a
connected series of development which only in the lower forms
(Phycomycetes) is related to the Algæ, and probably through them
has taken its origin from the Algæ; the higher Fungi have then
developed independently from this beginning. The distinction
of colour referred to is therefore not the only one which
separates the Algæ from the Fungi, but it is almost the only
characteristic mark by which we can at once distinguish the two
great sub-divisions of the Thallophytes.
The first forms of life on earth were probably “Protistæ,”
which had assimilating colour material, or in other words, they
were Algæ because they could assimilate purely inorganic food
substances, and there are some among these which belong to the
simplest forms of all plants. Fungi and Slime-Fungi must have
appeared later, because they are dependent on other plants which
assimilate carbon.[2]
_Sub-Division I._--=MYXOMYCETES, SLIME-FUNGI.=
The Slime-Fungi occupy quite an isolated position in the Vegetable
Kingdom, and are perhaps the most nearly related to the group of
Rhizopods in the Animal Kingdom. They live in and on organic remains,
especially rotten wood or leaves, etc., on the surface of which their
sporangia may be found.
They are organisms without chlorophyll, and in their vegetative
condition are masses of protoplasm without cell-wall (_plasmodia_).
They multiply by means of _spores_, which in the true Slime-Fungi[3]
are produced in sporangia, but in some others[4] free. The spores
are round cells (Fig. 1 _a_) which in all the true Slime-Fungi are
surrounded by a cell-wall. The wall bursts on germination, and the
contents float out in the water which is necessary for germination.
They move about with swimming and hopping motions like swarmspores
(_e_, _f_), having a cilia at the front end and provided with a
cell-nucleus and a pulsating vacuole. Later on they become a little
less active, and creep about more slowly, while they continue to alter
their form, shooting out arms in various places and drawing them in
again (_g_, _h_, _i_, _k_, _l_, _m_); in this stage they are called
_Myxamœbæ_.
[Illustration: FIG. 1.--_a-l_ Development of “_Fuligo_” from
spore to Myxamœba; _a-m_ are magnified 300 times; _m_ is a
Myxamœba of _Lycogala epidendron_; _l´_ three Myxamœbæ of
_Physarum album_ about to unite; _o_, a small portion of
plasmodium, magnified 90 times.]
[Illustration: FIG. 2.--The plasmodium (_a_) of _Stemonitis
fusca_, commencing to form into sporangia (_b_); drawn on July
9. The dark-brown sporangia were completely formed by the next
morning; _c-e_ shows the development of their external form.]
[Illustration: FIG. 3.--Four sporangia of _Stemonitis fusca_,
fixed on a branch. _a_ The plasmodium.]
[Illustration: FIG. 4.--Sporangium of _Arcyria incarnata_. _B_
closed; _C_ open; _p_ wall of sporangium; _cp_ capilitium.]
The Myxamœba grows whilst taking up nourishment from the material in
which it lives, and multiplies by division. At a later stage a larger
or smaller number of Myxamœbæ may be seen to coalesce and form large
masses of protoplasm, _plasmodia_, which in the “Flowers of Tan” may
attain the size of the palm of a hand, or even larger, but in most
others are smaller. The plasmodia are independent, cream-like masses of
protoplasm, often containing grains of carbonate of lime and colouring
matter (the latter yellow in the Flowers of Tan). They creep about in
the decaying matter in which they live, by means of amœboid movements,
internal streamings of the protoplasm continually taking place; finally
they creep out to the surface, and very often attach themselves to
other objects, such as Mosses, and form sporangia (Fig. 2). These are
stalked or sessile and are generally cylindrical (Fig. 3), spherical
or pear-shaped (Fig. 4); they rarely attain a larger size than that of
a pin’s head, and are red, brown, white, blue, yellow, etc., with a
very delicate wall. In some genera may be found a “Capillitium” (Fig. 4
_cp_), or network of branched fine strands between the spores. Flowers
of Tan (_Fuligo septica_) has a fruit-body composed of many sporangia
(an Æthalium), which has the appearance of flat, irregular, brown
cakes, inside the fragile external layer of which a loose powder, the
spores, is found. It generally occurs on heaps of tanners’ bark, and
appears sometimes in hot-beds in which that material is used, and is
destructive by spreading itself over the young plants and choking them.
All the motile stages may pass into _resting stages_, the small forms
only surrounding themselves with a wall, but the large ones at the
same time divide in addition into polyhedral cells. When favourable
conditions arise, the walls dissolve and the whole appears again as a
naked (free-moving) mass of protoplasm.
To the genuine Slime-Fungi belong: _Arcyria_, _Trichia_, _Didymium_,
_Physarum_, _Stemonitis_, _Lycogala_, _Fuligo_, _Spumaria_,
_Reticularia_.
Some genera wanting a sporangium-wall belong to the Slime-Fungi:
_Ceratiomyxa_, whose fruit-body consists of polygonal plates, each
bearing stalked spores; _Dictyostelium_, in which the swarm-stage is
wanting and which has stalked spores. _Plasmodiophora brassicæ_ preys
upon the roots of cabbages and other cruciferous plants, causing large
swellings. _Pl. alni_ causes coral-shaped outgrowths on the roots of
the Alder (_Alnus_). _Phytomyxa leguminosarum_ may be found in small
knobs (tubercles) on the roots of leguminous plants. It is still
uncertain whether it is this Fungus or Bacteria which is the cause of
the formation of these tubercles.
_Sub-Division_ II.--=ALGÆ=.
=Mode of Life.= The Algæ (except most of the Bacteria) are themselves
able to form their organic material by the splitting up of the carbonic
acid contained in the water, or air in some cases, and for this purpose
need light. The majority live in water, fresh or salt, but many are
present on damp soil, stones, bark of trees, etc.
With the exception of the Bacteria, no saprophytes have actually been
determined to belong to this group, and only very few true parasites
(for instance, _Phyllosiphon arisari_, _Mycoidea_, etc.), but a
good many are found epiphytic or endophytic on other Algæ, or water
plants, and on animals (for instance, certain _Schizophyceæ_ and
_Protococcoideæ_; _Trichophilus welckeri_ in the hairs of _Bradypus_,
the Sloth), and several species in symbiotic relation to various Fungi
(species of Lichen), to Sponges (_e.g. Trentepohlia spongiophila_,
_Struvea delicatula_), and to sundry Infusoria and other lower
animals as Radiolarias, _Hydra_, etc. (the so-called _Zoochlorella_
and _Zooxantella_, which are perhaps partly stages in development of
various Green and Brown Algæ).
=Vegetative Organs.= The cells in all the Algæ (excepting certain
reproductive cells) are surrounded by a membrane which (with the
exception of the Bacteria) consists of pure or altered cellulose,
sometimes forming a gelatinous covering, at other times a harder one,
with deposits of chalk or silica formed in it. The cell-nucleus,
which in the Schizophyta is less differentiated, may be one or more
(_e.g. Hydrodictyon_, _Siphoneæ_) in each cell. Excepting in the
majority of the Bacteria, _colour materials_ (of which _chlorophyll_,
or modifications of it, always seems to be found) occur, which either
permeate the whole cytoplasm surrounding the cell-nucleus, as in most
of the coloured Schizophyta, or are contained in certain specially
formed small portions of protoplasm (chromatophores).
The individual at a certain stage of development consists nearly always
of only one cell; by its division multicellular individuals may arise,
or, if the daughter-cells separate immediately after the division, as
in many of the simplest forms, the individual will, during the whole
course of its existence, consist of only a single cell (unicellular
Algæ). In multicellular individuals the cells may be more or less
firmly connected, and all the cells of the individual may be exactly
alike, or a division of labour may take place, so that certain cells
undertake certain functions, and are constructed accordingly; this
may also occur in parts of the cell in the large unicellular and
multinuclear Algæ (Siphoneæ, p. 62).
The cells in most of the Algæ belong to the _parenchymatous_ form;
these, however, in the course of their growth, may very often become
somewhat oblong; in many Algæ (particularly Fucoideæ and Florideæ)
occur, moreover, _hyphæ-like threads_, which are very long, often
branched, and are either formed of a single cell, or, more frequently,
of a row of cells, having a well-pronounced apical growth. The
parenchymatous as well as the hyphæ-like cells may, in the higher
Algæ (especially in certain Fucoideæ and Florideæ), be further
differentiated, so that they form well-defined anatomico-physiological
systems of tissue, _i.e._ assimilating, conducting, storing, and
mechanical.
With regard to _the external form_, the thallus may present no
differentiation, as in many unicellular Algæ, or in multicellular Algæ
of the lower order, which are then either equally developed in all
directions (_e.g. Pleurococcus_, Fig. 47), or form flat cell-plates
(_Merismopedium_) or threads (_Oscillaria_, Fig. 21). The first step
in the way of differentiation appears as a difference between apex and
base (_Rivularia_, _Porphyra_); but the division of labour may proceed
so that differences may arise between vegetative and reproductive cells
(_Œdogonium_, Fig. 54); hairs and organs of attachment (rhizoids and
haptera), which biologically serve as roots, are developed, and even
leaves in certain forms of high order, belonging to different classes
(_e.g. Caulerpa_, Fig. 59; _Characeæ_, Fig. 61; _Sargassum_, Fig. 72;
and many Florideæ).
=The non-sexual reproduction= takes place _vegetatively_, in many
instances, simply by division into two, and more or less complete
separation of the divisional products (Diatomaceæ, Desmidiaceæ (Fig.
36), many Fission-plants, etc.), or by detached portions of the thallus
(_e.g. Caulerpa_, _Ulva lactuca_, etc.; among many Schizophyceæ,
small filaments known as _hormogonia_ are set free), or _asexually_ by
special reproductive cells (_spores_) set free from the thallus; these
may be either _stationary_ or _motile_. The stationary reproductive
cells (spores) may either be devoid of cell-wall (tetraspores of the
Florideæ), or may possess a cell-wall; in the latter case they may be
formed directly from the vegetative cells, generally by the thickening
of the walls (_akinetes_), or only after a process of re-juvenescence
(_aplanospores_). Aplanospores, as well as akinetes, may either
germinate immediately or may become resting-cells, which germinate only
after a period of rest.
THE MOTILE ASEXUAL REPRODUCTIVE CELLS are spherical, egg- or
pear-shaped, naked, _swarmspores_ (_zoospores_), which have arisen
in other cells (_zoosporangia_), and propel themselves through the
water by means of cilia; or they are _Phyto-Amœbæ_, which have no
cilia and creep on a substratum by means of pseudopodia. The cilia,
which are formed from the protoplasm (in the Bacteria, however, from
the membrane), are mostly situated at the pointed and colourless end,
which is directed forwards when in motion, and are 1, 2 (Fig. 5 _B_),
4 or more. Both the cilia in the Brown Algæ are attached to one side
(Fig. 65); they are occasionally situated in a circle round the front
end (_Œdogonium_, Fig. 6 _a_, and _Derbesia_), or are very numerous
and situated in pairs distributed over a large part or nearly the
whole of the zoospore (_Vaucheria_). Besides being provided with one
or more nuclei (_Vaucheria_), they may also have a red “eye spot”
and vacuoles, which are sometimes pulsating, _i.e._ they appear and
reappear at certain intervals. The swarmspores move about in the
water in irregular paths, and apparently quite voluntarily, revolving
round their longer axes; but they come to the surface of the water in
great numbers either because of their dependence on light, or driven
by warm currents in the water, or attracted by some passing mass of
food material. The swarmspores germinate, each forming a new plant,
as their movement ceases they surround themselves with a cell-wall,
grow, and then divide; in Fig. 6 _b_, two may be seen in the condition
of germination, and about to attach themselves by means of the front
end, which has been developed into haptera (see also Fig. 5 _B_, lowest
figure).
=The sexual reproduction= here, probably in all cases, consists in the
coalescence of two masses of protoplasm, that is, in the fusion of
their nuclei.
[Illustration: FIG. 5.--_Cladophora glomerata. A_ The lower
cells are full of swarmspores, whilst from the upper one the
greater part have escaped through the aperture _m_. _B_ Free and
germinating swarmspores.]
[Illustration: FIG. 6.--_Œdogonium_: _a_ (free), _b_ germinating
swarmspores.]
[Illustration: FIG. 7.--_Zanardinia collaris. A_ Male
gametangia (the small-celled) and female gametangia
(large-celled). _C_ Female gamete. _D_ Male gamete. _B E_
Fertilisation. _F_ Zygote. _G_ Germinating zygote.]
The simplest and lowest form is termed =conjugation=, or =isogamous=
fertilisation, and is characterized by the fact that the two coalescing
cells (termed gametes) are equal, or almost equal, in shape and size
(the female gamete in the _Cutleriaceæ_, _e.g. Zanardinia collaris_,
Fig. 7, is considerably larger than the male gamete). The cell in
which the _gametes_ are developed is called a _gametaugium_, and the
reproductive cell formed by their union--which generally has a thick
wall and only germinates after a short period of rest--is termed a
_zygote_ or _zygospore_. The conjugation takes place in two ways:--
(_a_) In the one way the gametes are motile cells (_planogametes_,
_zoogametes_, Fig. 8), which unite in pairs during their swarming
hither and thither in the water; during this process they lie side by
side (Fig. 8 _d_), generally at first touching at the clear anterior
end, and after a time they coalesce and become a motionless _zygote_,
which surrounds itself with a cell-wall (Fig. 8 _e_). This form of
conjugation is found in _Ulothrix_ (Fig. 8 _d_), _Acetabularia_, and
other Algæ (Figs. 45, 56, 66).
[Illustration: FIG. 8.--_Ulothrix zonata_: a portion of
a thread with zoospores, of which two are formed in each
cell (zoosporangium), the dark spots upon them are the “red
eye-spots”; 1, 2, 3, 4 depict successive stages in the
development of the zoospores; _b_ a single zoospore, at _v_ the
pulsating vacuole; _c_ portion of a thread with gametes, of which
sixteen are formed in each gametangium; _d_ gametes free and in
conjugation; _e_ conjugation has been effected, and the formed
zygotes are in the resting condition.]
(_b_) Among other Algæ (_e.g. Diatomaceæ_ and _Conjugatæ_), the
conjugating cells continue to be surrounded by the cell-wall of
the mother-cell (_aplanogametes_ in an _aplanogametangium_); the
aplanogametangia generally grow out into short branches, which lie
close together and touch one another, the wall at the point of contact
is then dissolved (Fig. 39). Through the aperture thus formed, the
aplanogametes unite, as in the first instance, and form a rounded
zygote, which immediately surrounds itself with a cell-wall. Various
modifications occur; compare Figs. 37, 39, 41, 43.
[Illustration: FIG. 9.--Fertilisation in the Bladder-wrack
(_Fucus vesiculosus_).]
[Illustration: FIG. 10.--_Sphæroplea annulina._]
The highest form of the sexual reproduction is the =Egg- or Oogamous=
fertilisation. The two coalescing cells are in the main unlike each
other in form as well as size. The one which is considered as the male,
and is known as the _spermatozoid_ (_antherozoid_), developes as a rule
in large numbers in each mother-cell (_antheridium_); they are often
self-motile (except in the Florideæ, where they are named _spermatia_),
and are many times smaller than the other kind, the female, which
is known as the _egg-cell_, (_oosphere_). The egg-cell is always a
motionless, spherical, primordial cell which can either float about
freely in the water, as in the Fucaceæ (Fig. 9), or is surrounded by
a cell-wall (_oogonium_); generally only one oosphere is to be found
in each oogonium, but several occur in _Sphæroplea_ (Fig. 10). The
result of the spermatozoid coalescing with the egg-cell is, as in this
case, the formation of a oospore, which generally undergoes a period of
rest before germination (the Florideæ are an exception, a fruit-body,
_cystocarp_, being produced as the result of coalescence).
An example of fertilisation is afforded by the Alga, _Sphæroplea
annulina_ (Fig. 10). The filamentous thallus is formed of
cylindrical cells with many vacuoles (_r_ in _A_); some cells
develope egg-cells (_B_), others spermatozoids (_C_), the latter
in a particularly large number. The egg-cells are spherical,
the spermatozoids of a club- or elongated pear-shape with two
cilia at the front end (_G_; _E_ is however a swarmspore).
The spermatozoids escape from their cells through apertures
in the wall (_o_ in _C_) and enter through similar apertures
(_o_ in _B_) to the egg-cells. The colourless front end of the
spermatozoid is united at first with the “receptive spot” of the
egg-cell (see _F_), and afterwards completely coalesces with
it. The result is the formation of a oospore with wart-like
excrescences (_D_).
The female (_parthenogenesis_) or male (_androgenesis_) sexual cell
may, sometimes without any preceding fertilisation, form a new
individual (_e.g. Ulothrix zonata_, _Cylindrocapsa_, etc.).
=Systematic division of the Algæ.= The Algæ are divided into the
following ten classes:
1. SYNGENETICÆ; 2. DINOFLAGELLATA, or PERIDINEA; 3. DIATOMACEÆ;
4. SCHIZOPHYTA, FISSION-ALGÆ; 5. CONJUGATÆ; 6. CHLOROPHYCEÆ,
GREEN-ALGÆ; 7. CHARACEÆ, STONE-WORTS; 8. PHÆOPHYCEÆ; 9.
DICTYOTALES; 10. RHODOPHYCEÆ.
Among the lowest forms of the Algæ, the Syngeneticæ, the
Dinoflagellata, and the unicellular Volvocaceæ (Chlamydomoneæ),
distinct transitional forms are found approaching the animal kingdom,
which can be grouped as animals or plants according to their method of
taking food or other characteristics. Only an artificial boundary can
therefore be drawn between the animal and vegetable kingdoms. In the
following pages only those forms which possess _chromatophores_,
and have _no mouth_, will be considered as Algæ.