THE MORPHOLOGICAL COMPOSITION OF PLANTS.
§ 180. Evolution implies insensible modifications and gradual
transitions, which render definition difficult--which make it
impossible to separate absolutely the phases of organization from
one another. And this indefiniteness of distinction, to be expected
_à priori_, we are compelled to recognize _à posteriori_, the moment
we begin to group morphological phenomena into general propositions.
Thus, on inquiring what is the morphological unit, whether of plants
or of animals, we find that the facts refuse to be included in any
rigid formula. The doctrine that all organisms are built up of cells,
or that cells are the elements out of which every tissue is developed,
is but approximately true. There are living forms of which cellular
structure cannot be asserted; and in living forms that are for the
most part cellular, there are nevertheless certain portions which
are not produced by the metamorphosis of cells. Supposing that clay
were the only material available for building, the proposition that
all houses are built of bricks, would bear about the same relation to
the truth, as does the proposition that all organisms are composed
of cells. This generalization respecting houses would be open to
two criticisms:--first, that certain houses of a primitive kind are
formed, not of bricks, but out of unmoulded clay; and second, that
though other houses consist mainly of bricks, yet their chimney-pots,
drain-pipes, and ridge-tiles, do not result from combination or
metamorphosis of bricks, but are made directly out of the original
clay. And of like natures are the criticisms which must be passed
on the generalization, that cells are the morphological units of
organisms. To continue the simile, the truth turns out to be, that the
primitive clay or protoplasm out of which organisms are built, may be
moulded either directly, or with various degrees of indirectness, into
organic structures. The physiological units which we are obliged to
assume as the components of this protoplasm, must, as we have seen, be
the possessors of those proclivities which result in the structural
arrangements of the organism. The assumption of such structural
arrangements may go on, and in many cases does go on, by the shortest
route; without the passage through what we call metamorphoses. But
where such structural arrangements are reached by a circuitous route,
the first stage is the formation of these small aggregates which, under
the name of cells, are currently regarded as morphological units.
The rationale of these truths appears to be furnished by the hypothesis
of evolution. We set out with molecules some degrees higher in
complexity than those molecules of nitrogenous colloidal substance
into which organic matter is resolvable; and we regard these very much
more complex molecules as having the implied greater instability,
greater sensitiveness to surrounding influences, and consequent
greater mobility of form. Such being the primitive physiological
units, organic evolution must begin with the formation of a minute
aggregate of them--an aggregate showing vitality by a higher degree
of that readiness to change its form of aggregation which colloidal
matter in general displays; and by its ability to unite the nitrogenous
molecules it meets with, into complex molecules like those of which
it is composed. Obviously, the earliest forms must have been minute;
since, in the absence of any but diffused organic matter, no form
but a minute one could find nutriment. Obviously, too, it must have
been structureless; since, as differentiations are producible only
by the unlike actions of incident forces, there could have been no
differentiations before such forces had had time to work. Hence,
distinctions of parts like those required to constitute a cell were
necessarily absent at first. And we need not therefore be surprised
to find, as we do find, specks of protoplasm manifesting life, and
yet showing no signs of organization. A further stage of evolution
is reached when the imperfectly integrated molecules forming one of
these minute aggregates, become more coherent; at the same time as
they pass into a state of heterogeneity, gradually increasing in its
definiteness. That is to say, we may look for the assumption by them,
of some distinctions of parts, such as we find in cells and in what
are called unicellular organisms. They cannot retain their primordial
uniformity; and while in a few cases they may depart from it but
slightly, they will, in the great majority of cases, acquire a decided
multiformity: there will result the comparatively integrated and
comparatively differentiated _Protophyta_ and _Protozoa_.
The production of minute aggregates of physiological units being
the first step, and the passage of such minute aggregates into more
consolidated and more complex forms being the second step, it must
naturally happen that all higher organic types, subsequently arising
by further integrations and differentiations, will everywhere bear the
impress of this earliest phase of evolution. From the law of heredity,
considered as extending to the entire succession of living things
during the Earth’s past history, it follows that since the formation
of these small, simple organisms must have preceded the formation
of larger and more complex organisms, the larger and more complex
organisms must inherit their essential characters. We may anticipate
that the multiplication and combination of these minute aggregates or
cells, will be conspicuous in the early developmental stages of plants
and animals; and that throughout all subsequent stages, cell-production
and cell-differentiation will be dominant characteristics. The
physiological units peculiar to each higher species will, speaking
generally, pass through this form of aggregation on their way towards
the final arrangement they are to assume; because those primordial
physiological units from which they are remotely descended, aggregated
into this form. And yet, just as in other cases we found reasons for
inferring (§ 131) that the traits of ancestral organization may,
under certain conditions, be partially or wholly obliterated, and the
ultimate structure assumed without passing through them; so, here,
it is to be inferred that the process of cell-formation may, in some
cases, be passed over. Thus the hypothesis of evolution prepares us for
those two radical modifications of the cell-doctrine which the facts
oblige us to make. It leads us to expect that as structureless portions
of protoplasm must have preceded cells in the process of general
evolution; so, in the special evolution of each higher organism, there
will be an habitual production of cells out of structureless blastema.
And it leads us to expect that though, generally, the physiological
units composing a structureless blastema, will display their inherited
proclivities by cell-development and metamorphosis; there will
nevertheless occur cases in which the tissue to be formed, is formed by
direct transformation of the blastema.[2]
Interpreting the facts in this manner, we may recognize that large
amount of truth which the cell-doctrine contains, without committing
ourselves to the errors involved by a sweeping assertion of it. We
are enabled to understand how it happens that organic structures are
usually cellular in their composition, at the same time that they are
not universally so. We are shown that while we may properly continue to
regard the cell as the morphological unit, we must constantly bear in
mind that it is such only in a qualified sense.
§ 181. These aggregates of the lowest order, each formed of
physiological units united into a group that is structurally single and
cannot be divided without destruction of its individuality, may, as
above implied, exist as independent organisms. The assumption to which
we are committed by the hypothesis of evolution, that such so-called
unicellular plants were at first the only kinds of plants, is in
harmony with the fact that habitats not occupied by plants of higher
orders, commonly contain these protophytes in great abundance and great
variety. The various species of _Pleurococcaceæ_, of _Desmidiaceæ_,
and _Diatomaceæ_, supply examples of morphological units living and
propagating separately, under numerous modifications of form and
structure. Figures 1, 2, and 3, represent a few of the commonest types.
[Illustration: Figs. 1, 2, 3.]
[Illustration: Figs. 4, 5, 6.]
Mostly, simple plants are too small to be individually visible
without the microscope. But, in some cases, these vegetal aggregates
of the first order grow to appreciable sizes. In the mycelium of some
fungi, we have single cells developed into long branched filaments,
or ramified tubules, that are of considerable lengths. An analogous
structure characterizes certain tribes of _Algæ_, of which _Codium
adhærens_, Fig. 4, may serve as an example. In _Botrydium_, another
alga, Fig. 5, we have a structure which is described as simulating a
higher plant, with root, stem, bud, and fruit, all produced by the
branching of a single cell. And among fungi the genus _Mucor_, Fig. 6,
furnishes an example of allied kind.[3] Here, though the size attained
is much greater than that of many organisms which are morphologically
compound, we are compelled to consider the morphological composition
as simple; since the whole can no more be separated into minor wholes,
than can the branched vascular system of an animal. In these cases we
have considerable bulk attained, not by a number of aggregates of the
first order being united into an aggregate of the second order, but by
the continuous growth of an aggregate of the first order.
§ 182. The transition to higher forms begins in a very unobtrusive
manner. Among these aggregates of the first order, an approach towards
that union by which aggregates of the second order are produced, is
indicated by mere juxtaposition. Protophytes multiply rapidly; and
their rapid multiplication sometimes causes crowding. When, instead
of floating free in the water, they form a thin film on a moist
surface, or are imbedded in a common matrix of mucilage; the mechanical
obstacles to dispersion result in a kind of feeble integration, vaguely
shadowing forth a combined group. Somewhat more definite combination is
shown us by such plants as _Palmella botryoides_. Here the members of a
family of cells, arising by the spontaneous fission of a parent-cell,
remain united by slender threads of that jelly-like substance which
envelops their surfaces. In some _Diatomaceæ_ several individuals,
instead of completely separating, hold together by their angles; and
in other _Diatomaceæ_, as the _Bacillaria_, a variable number of units
cohere so slightly, that they are continually moving in relation to one
another.
This formation of aggregates of the second order, faintly indicated
in feeble and variable unions like the above, may be traced through
phases of increasing permanence and definiteness, as well as increasing
extent. In the yeast-plant, Fig. 7, we have cells which may exist
singly, or joined into groups of several; and which have their shapes
scarcely at all modified by their connexion. Among the _Desmidiaceæ_,
it happens in many cases that the two individuals produced by division
of a parent-individual, part as soon as they are fully formed; but in
other cases, instead of parting they compose a group of two. Allied
kinds show us how, by subsequent fissions of the adherent individuals
and their progeny, there result longer groups; and in some species,
a continuous thread of them is thus produced. Figs. 8, 9, 11, exhibit
these several stages. Fig. 10 represents a _Scenedesmus_ in which the
individuation of the group is manifest. Instead of linear aggregation,
many protophytes illustrate central aggregation; as shown in Figs. 12,
13, 14, 15. Other instances are furnished by such forms as the _Gonium
pectorale_, Fig. 16 (_a_ being the front view, and _b_ the edge view),
and the _Sarcina ventriculi_, Fig. 17. Further, we have that spherical
mode of aggregation of which the _Volvox globator_ furnishes a familiar
instance.
[Illustration: Figs. 7–17.]
[Illustration: Figs. 18–23.]
Thus far, however, the individuality of the secondary aggregate is
feebly pronounced: not simply in the sense that it is small; but also
in the sense that the individualities of the primary aggregates are
very little subordinated. But on seeking further, we find transitions
towards forms in which the compound individuality is more dominant,
while the simple individualities are more obscured. Obscuration of one
kind accompanies mere increase of size in the secondary aggregate.
In proportion to the greater number of the morphological units held
together in one mass, becomes their relative insignificance as
individuals. We see this in the irregularly-spreading lichens that
form patches on rocks; and in such creeping fungi as grow in films or
laminæ on decaying wood and the bark of trees. In these cases, however,
the integration of the component cells is of an almost mechanical
kind. The aggregate of them is scarcely more individuated than a lump
of inorganic matter: as witness the way in which the lichen extends
its curved edges in this or that direction, as the surface favours;
or the way in which the fungus grows round and imbeds the shoots and
leaves that lie in its way, just as so much plastic clay might do.
Though here, in the augmentation of mass, we see a progress towards the
evolution of a higher type, we have as yet none of that definiteness
required to constitute a compound unit, or true aggregate of the
second order. Another kind of obscuration of the morphological units,
is brought about by their more complete coalescence into the form of
some structure made by their union. This is well exemplified among
the _Confervoideæ_ and _Conjugatæ_. In Fig. 18, there are represented
the stages of a growing _Mougeotia genuflexa_, in which this merging
of the simple individualities into the compound individuality, is
shown in the history of a single plant; and in Figs. 19, 20, 21, 22,
23, are represented a series of species from this group, and that of
_Cladophora_,[4] in which we see a progressing integration. While,
in the lower types, the primitive spheroidal forms of the cells
are scarcely altered, in the higher types the cells are so fused
together as to constitute cylinders divided by septa. Here, however,
the indefiniteness is still great. There are no specific limits to
the length of any thread thus produced, and there is none of that
differentiation of parts required to give a decided individuality to
the whole.
To constitute something like a true aggregate of the second order,
capable of serving as a compound unit that may be combined with others
like itself into still higher aggregates, there must exist both mass
and definiteness.
§ 183. An approach towards plants which unite these characters, may be
traced in such forms as _Bangia ciliaris_, Fig. 24. The multiplication
of cells here takes place, not in a longitudinal direction only, but
also in a transverse direction; and the transverse multiplication being
greater towards the middle of the frond, there results a difference
between the middle and the two extremities--a character which, in
a feeble way, unites all the parts into a whole. Even this slight
individuation is, however, very indefinitely marked; since, as shown by
the figures, the lateral multiplication of cells does not go on in a
precise manner.
[Illustration: Fig. 24.]
From some such type as this there appear to arise, through slight
differences in the modes of growth, two closely-allied groups of
plants, having individualities somewhat more pronounced. If, while
the cells multiply longitudinally, their lateral multiplication goes
on in one direction only, there results a flat surface, as in the
genus _Ulva_ (Sea-lettuce) or in the upper part of the thallus of
_Enteromorpha Linza_, Fig. 25; or where the lateral multiplication is
less uniform in its rate, in types like Fig. 26. But where the lateral
multiplication occurs in two directions transverse to one another, a
hollow frond may be produced--sometimes irregularly spheroidal, and
sometimes irregularly tubular; as in _Enteromorpha intestinalis_,
Fig. 27. And often, as in _Enteromorpha compressa_, Fig. 28, and
other species, this tubular frond becomes branched. Figs. 29 and 30
are magnified portions of such fronds, showing the simple cellular
aggregation which allies them with the preceding forms.
[Illustration: Figs. 25–30.]
In the common _Fuci_ of our coasts, other and somewhat higher stages
of this integration are displayed. We have fronds preserving something
like constant breadths and dividing dichotomously with approximate
regularity. Though the subdivisions so produced are not to be regarded
as separate fronds, but only as extensions of one frond, they
foreshadow a higher degree of composition; and by the comparatively
methodic way in which they are united, give to the aggregate a more
definite, as well as a more complex, individuality. Many of the higher
lichens exhibit an analogous advance. While in the lowest lichens, the
different parts of the thallus are held together only by being all
attached to the supporting surface, in the higher lichens the thallus
is so far integrated that it can support itself by attachment to such
surface at one point only. And then, in still more developed kinds,
we find the thallus assuming a dichotomously-branched form, and so
gaining a more specific character as well as greater size.
Where, as in types like these, the morphological units show an inherent
tendency to arrange themselves in a manner which is so far constant
as to give characteristic proportions, we may say that there is a
recognizable compound individuality. Considering the Thallophytes which
grow in this way apart from their kinships, and wholly with reference
to their morphological composition, we might not inaptly describe them
as pseudo-foliar.
§ 184. Another mode in which aggregation is so carried on as to produce
a compound individuality of considerable definiteness, is variously
displayed among other families of _Algæ_. When the cells, instead
of multiplying longitudinally alone, and instead of all multiplying
laterally as well as longitudinally, multiply laterally only at
particular places, they produce branched structures.
Indications of this mode of aggregation occur among the _Confervoideæ_,
as shown in Figs. 22, 23. Though, in some of the more-developed
_Algæ_ which exhibit the ramified arrangement in a higher degree, the
component cells are, like those of the lower _Algæ_, united together
end to end, in such way as but little to obscure their separate
forms, as in _Cladophora Hutchinsiæ_, Fig. 31; they nevertheless
evince greater subordination to the whole of which they are parts,
by arranging themselves more methodically. Still further pronounced
becomes the compound individuality when, while the component cells of
the branches unite completely into jointed cylinders, the component
cells of the stem form an axis distinguished by its relative thickness
and complexity. Such types of structures are indicated by Figs. 32,
33--figures representing small portions of plants which are quite
tree-like in their entire outlines. On examining Figs. 34, 35, 36,
which show the structures of the stems in these types, it will be
seen, too, that the component cells in becoming more coherent,
have undergone changes of form which obscure their individualities
more than before. Not only are they much elongated, but they are so
compressed as to be prismatic rather than cylindrical. This structure,
besides displaying integration of the morphological units carried on
in two directions instead of one; and besides displaying this higher
integration in the greater merging of the individualities of the
morphological units in the general individuality; also displays it in
the more pronounced subordination of the branches and branchlets to the
main stem. This differentiation and consolidation of the stem, brings
all the secondary growths into more marked dependence; and so renders
the individuality of the aggregate more decided.
[Illustration: Figs. 31–36.]
We might not inappropriately call this type of structure pseud-axial.
It simulates that of the higher plants in certain superficial
characters. We see in it a primary axis along which development may
continue indefinitely, and from which there bud out, laterally,
secondary axes of like nature, bearing like tertiary axes; and this is
a mode of growth with which Phænogams make us familiar.
§ 185. Some of the larger _Algæ_ supply examples of an integration
still more advanced; not simply inasmuch as they unite much greater
numbers of morphological units into continuous masses, but also
inasmuch as they combine the pseudo-foliar structure with the
pseud-axial structure. Our own shores furnish an instance of this in
the common _Laminaria_; and certain gigantic _Laminariaceæ_ of the
Antarctic seas, furnish yet better instances. In _Necrocystis_ the
germ develops a very long slender stem, which eventually expands into
a large bladder-like or cylindrical air-vessel; and the surface of
this bears numerous leaf-shaped expansions. Another kind, _Lessonia
fuscescens_, Fig. 37, shows us a massive stem growing up through water
many feet deep--a stem which, bifurcating as it approaches the surface,
flattens out the ends of its subdivisions into fronds like ribands.
These, however, are not true foliar appendages, since they are merely
expanded continuations of the stem. In _Egregia_ branches of the
thallus not only take the form of leaves, but these are differentiated
into several categories in accordance with a division of labour. In
any of these _Laminariaceæ_ the whole plant, great as may be its size,
and made up though it seems to be of many groups of morphological
units, united into a compound group by their marked subordination to a
connecting mass, is nevertheless a single thallus, which is added to by
intercalary growth at the “transition place,” at the junction of the
stem-like and leaf-like portions. The aggregate is still an aggregate
of the second order.
[Illustration: Fig. 37.]
[Illustration: Figs. 38–40.]
But among certain of the highest _Algæ_, we _do_ find something more
than this union of the pseud-axial with the pseudo-foliar structure.
In addition to pseud-axes of comparative complexity; and in addition
to pseudo-folia that are like leaves, not only in their general shapes
but in having mid-ribs and even veins; there are the beginnings of
a higher stage of integration. Figs. 38, 39, and 40, show some of
the steps. In _Rhodymenia palmata_, Fig. 38, the parent-frond is
comparatively irregular in form, and without a mid-rib; and along with
this very imperfect integration, we see that the secondary fronds
growing from the edges are distributed very much at random, and are by
no means specific in their shapes. A considerable advance is displayed
by _Phyllophora rubens_, Fig. 39. Here the frond, primary, secondary,
or tertiary, betrays some approach towards regularity in both form
and size; by which, as also by its partially-developed mid-rib, there
is established a more marked individuality; and at the same time,
the growth of the secondary fronds no longer occurs anywhere on the
edge, in the same plane as the parent-frond, but from the surface at
specific places. _Delesseria sanguinea_, Fig. 40, illustrates a much
more definite arrangement of the same kind. The fronds of this plant,
quite regularly shaped, have their parts decidedly subordinated to the
whole; and from their mid-ribs grow other fronds which are just like
them. Each of these fronds is an organized group of those morphological
units which we distinguish as aggregates of the first order. And in
this case, two or more such aggregates of the second order, well
individuated by their forms and structures, are united together; and
the plant composed of them is thus rendered, in so far, an aggregate of
the third order.
Just noting that in certain of the most developed _Algæ_, as the
_Sargassum_, or common gulf-weed, this tertiary degree of composition
is far more completely displayed, so as to produce among Thallophytes a
type of structure closely simulating that of the higher plants, let us
now pass to the consideration of these higher plants.
§ 186. Having the surface of the soil for a support and the air for a
medium, terrestrial plants are mechanically circumstanced in a manner
widely different from that in which aquatic plants are circumstanced.
Instead of being buoyed up by a surrounding fluid of specific gravity
equal to their own, they have to erect themselves into a rare fluid
which yields no appreciable support. Further, they are dissimilarly
conditioned in having two sources of nutriment in place of one. Unlike
the _Algæ_, which derive all the materials for their tissues from
the water bathing their entire surfaces, and use their roots only
for attachment, most of the plants which cover the Earth’s surface,
absorb part of their food through their imbedded roots and part through
their exposed leaves. These two marked unlikenesses in the relations
to surrounding conditions, profoundly affect the respective modes of
growth. We must duly bear them in mind while studying the further
advance of composition.
[Illustration: Figs. 41–44.]
[Illustration: Figs. 45–49.]
The class of plants to which we now turn--that of the
_Archegoniatæ_--is nearly related by its lower members to the classes
above dealt with: so much so, that some of the inferior liverworts are
quite licheniform, and are often mistaken for lichens. Passing over
these, let us recommence our synthesis with such members of the class
as repeat those indications of progress towards a higher composition,
which we have just observed among the more-developed _Algæ_. The
_Jungermanniaceæ_ furnish us with a series of types, clearly indicating
the transition from an aggregate of the second order to an aggregate
of the third order. Figs. 41 and 42, indicate the structure among the
lowest of this group. Here there is but an incomplete development of
the second order of aggregate. The frond grows as irregularly as the
thallus of a lichen: it is indefinite in size and outline, spreading
hither or thither as the conditions favour. Moreover, it lacks the
differentiations required to subordinate its parts to the whole: it
is uniformly cellular, having neither mid-rib nor veins; and it puts
out rootlets indifferently from all parts of its under surface. In
Fig. 43, _Pellia epiphylla_, we have an advance on this type. There
is here, as shown in the transverse section, Fig. 44, a thickening
of the frond along its central portion, producing something like an
approach towards a mid-rib; and from this the rootlets are chiefly
given off. The outline, too, is much less irregular; whence results
greater distinctness of the individuality. A further step is displayed
in _Metzgeria furcata_, Fig. 45. The frond of this plant, comparatively
well integrated by the distribution of its substance around a decided
mid-rib, and by its comparatively-definite outlines, produces secondary
fronds. There is what is called proliferous growth; and occasionally,
as shown in Fig. 46, representing an enlarged portion, the growth is
doubly-proliferous. In these cases, however, the tertiary aggregate, so
far as it is formed, is but very feebly integrated; and its integration
is but temporary. For not only do these younger fronds that bud out
from the mid-ribs of older fronds, develop rootlets of their own; but
as soon as they are well grown and adequately rooted, they dissolve
their connexions with the parent-fronds, and become quite independent.
From these transitional forms we pass, in the higher _Jungermanniaceæ_,
to forms composed of many fronds that are permanently united by a
continuous stem. A more-developed aggregate of the third order is
thus produced. But though, along with increased definiteness in
the secondary aggregates, there is here an integration of them so
extensive and so regular, that they are visibly subordinated to the
whole they form; yet the subordination is really very incomplete.
In some instances, as in _Radula complanata_, Fig. 47, the leaflets
develop roots from their under surfaces, just as the primitive frond
does; and in the majority of the group, as in _J. capitata_, Fig. 48,
roots are given off all along the connecting stem, at the spots where
the leaflets or frondlets join it: the result being that though the
connected frondlets form a physical whole, they do not form, in any
decided manner, a physiological whole; since successive portions of
the united series, carry on their functions independently of the rest.
Finally, the most developed members of the group, whether lineally
descended from the less developed or from an early type common to the
two, present us with tertiary aggregates which are physiologically as
well as physically integrated.[5] Not lying prone like the kinds thus
far described, but growing erect, the stem and attached leaflets become
dependent upon a single root or group of roots; and being so prevented
from carrying on their functions separately, are made members of a
compound individual: there arises a definitely-established aggregate of
the third degree of composition.
The facts as arranged in the above order are suggestive. Minute
aggregates, or cells, the grouping of which we traced in § 182, showed
us analogous phases of indefinite union, which appeared to lead the
way towards definite union. We see here among compound aggregates, as
we saw there among simple aggregates, the establishment of a specific
form, and a size that falls within moderate limits of variation. This
passage from less definite extension to more definite extension,
seems in the one case, as the other, to be accompanied by the result,
that growth exceeding a certain rate, ends in the formation of a new
aggregate, rather than an enlargement of the old. And on the higher
stage, as on the lower, this process, irregularly carried out in the
simpler types, produces in them unions that are but temporary; while in
the more-developed types, it proceeds in a systematic way, and ends in
the production of a permanent aggregate that is doubly compound.
Must we then conclude that as cells, or morphological units, are
integrated into a unit of a higher order, which we call a thallus
or frond; so, by the integration of fronds, there is evolved a
structure such as the above-delineated species possess? Whether this
is the interpretation to be given of these plants, we shall best see
when considering whether it is the interpretation to be given of
plants which rank above them. Thus far we have dealt only with the
Cryptogamia. We have now to deal with the Phanerogamia or Phænogamia.