SUMMARY OF MORPHOLOGICAL DEVELOPMENT.
§ 262. That any formula should be capable of expressing a common
character in the shapes of things so unlike as a tree and a cow, a
flower and a centipede, is a remarkable fact; and is a fact which
affords strong _primâ facie_ evidence of truth. For in proportion to
the diversity and multiplicity of the cases to which any statement
applies, is the probability that it sets forth the essential relations.
Those connexions which remain constant under all varieties of
manifestation, are most likely to be the causal connexions.
Still higher will appear the likelihood of an alleged law of organic
form possessing so great a comprehensiveness, when we remember that on
the hypothesis of Evolution, there must exist between all organisms
and their environments, certain congruities expressible in terms of
their actions and reactions. The forces being, on this hypothesis, the
causes of the forms, it is inferable, _à priori_, that the forms
must admit of generalization in terms of the forces; and hence, such
a generalization arrived at _à posteriori_, gains the further
probability due to fulfilment of anticipation.
Nearer yet to certainty seems the conclusion thus reached, on finding
that it does but assert in their special manifestations, the laws of
Evolution in general--the laws of that universal re-distribution of
matter and motion which hold throughout the totality of things, as
well as in each of its parts.
It will be useful to glance back over the various minor inferences
arrived at, and contemplate them in their _ensemble_ from these
higher points of view.
§ 263. That process of integration which every plant displays during
its life, we found reason to think has gone on during the life of the
vegetal kingdom as a whole. Protoplasm into cells, cells into folia,
folia into axes, axes into branched combinations--such, in brief, are
the stages passed through by every shrub; and such appear to have been
the stages through which plants of successively-higher kinds have been
evolved from lower kinds. Even among certain groups of plants now
existing, we find aggregates of the first order passing through various
gradations into aggregates of the second order--here forming small,
incoherent, indefinite assemblages, and there forming large, definite,
coherent fronds. Similar transitions are traceable through which these
integrated aggregates of the second order pass into aggregates of
the third order: in one species the unions of parent-fronds with the
fronds that bud out from them, being temporary, and in another species
such unions being longer continued; until, in species still higher,
by a gemmation which is habitual and regular, there is produced a
definitely-integrated aggregate of the third order--an axis bearing
fronds or leaves. And even between this type and a type further
compounded, a link occurs in the plants which cast off, in the shape of
bulbils, some of the young axes they produce. As among plants, so among
animals. A like spontaneous fission of cells ends here in separation,
there in partial aggregation, while elsewhere, by closer combination
of the multiplying units, there arises a coherent and tolerably
definite individual of the second order. By the budding of individuals
of the second order, there are in some cases produced other separate
individuals like them; in some cases temporary aggregates of such like
individuals; and in other cases permanent aggregates of them: certain
of which become so definitely integrated that the individualities of
their component members are almost lost in a tertiary individuality.
Along with this progressive integration there has gone on a progressive
differentiation. Vegetal units of whatever order, originally
homogeneous, have become heterogeneous while they have become united.
Spherical cells aggregating into threads, into laminæ, into masses, and
into special tissues, lose their sphericity; and instead of remaining
all alike assume innumerable unlikenesses--from uniformity pass
into multiformity. Fronds combining to form axes, severally acquire
definite differences between their attached ends and their free ends;
while they also diverge from one another in their shapes at different
parts of the axes they compose. And axes, uniting into aggregates of
a still higher order, become contrasted in their sizes, curvatures,
and the arrangements of their appendages. Similarly among animals.
Those components of them which, with a certain license, we class as
morphological units, while losing their minor individualities in the
major individualities formed of them, grow definitely unlike as they
grow definitely combined. And where the aggregates so produced become,
by coalescence, segments of aggregates of a still higher order, they,
too, diverge from one another in their shapes.
The morphological differentiation which thus goes hand in hand with
morphological integration, is clearly what the perpetually-complicating
conditions would lead us to anticipate. Every addition of a new unit
to an aggregate of such units, must affect the circumstances of the
other units in all varieties of ways and degrees, according to their
relative positions--must alter the distribution of mechanical strains
throughout the mass, must modify the process of nutrition, must affect
the relations of neighbouring parts to surrounding diffused actions;
that is, must initiate a changed incidence of forces tending ever to
produce changed structural arrangements.
§ 264. This broad statement of the correspondence between the general
facts of Morphological Development and the principles of Evolution
at large, may be reduced to statements of a much more specific kind.
The phenomena of symmetry and unsymmetry and asymmetry, which we have
traced out among organic forms, are demonstrably in harmony with those
laws of the re-distribution of matter and motion to which Evolution
conforms. Besides the myriad-fold illustrations of the instability of
the homogeneous, afforded by these aggregates of units of each order,
which, at first alike, lapse gradually into unlikeness; and besides
the myriad-fold illustrations of the multiplication of effects, which
these ever-complicating differentiations exhibit to us; we have also
myriad-fold illustrations of the definite equalities and inequalities
of structures, produced by definite equalities and inequalities of
forces.
The proposition arrived at when dealing with the causes of Evolution,
“that in the actions and reactions of force and matter, an unlikeness
in either of the factors necessitates an unlikeness in the effects; and
that in the absence of unlikeness in either of the factors the effects
must be alike” (_First Principles_, § 169), is a proposition which
implies all these particular likenesses and unlikenesses of parts
which we have been tracing. For have we not everywhere seen that the
strongest contrasts are between the parts that are most contrasted in
their conditions; while the most similar parts are those most similarly
conditioned? In every plant the leading difference is between the
attached end and the free end; in every branch it is the same; in every
leaf it is the same. And in every plant the leading likenesses are
those between the two sides of the branch, the two sides of the leaf,
and the two sides of the flower, where these parts are two-sided in
their conditions; or between all sides of the branch, all sides of
the leaf, and all sides of the flower, where these parts are similarly
conditioned on all sides. So, too, is it with animals which move about.
The most marked contrasts they present are those between the part in
advance and the part behind, and between the upper part and the under
part; while there is complete correspondence between the two sides.
Externally the likenesses and differences among limbs, and internally
the likenesses and differences among vertebræ, are expressible in terms
of this same law.
And here, indeed, we may see clearly that these truths are corollaries
from that ultimate truth to which all phenomena of Evolution are
referable. It is an inevitable deduction from the persistence of force,
that organic forms which have been progressively evolved, must present
just those fundamental traits of form which we find them present. It
cannot but be that during the intercourse between an organism and its
environment, equal forces acting under equal conditions must produce
equal effects; for to say otherwise is, by implication, to say that
some force can produce more or less than its equivalent effect, which
is to deny the persistence of force. Hence those parts of an organism
which are, by its habits of life, exposed to like amounts and like
combinations of actions and reactions, must develop alike; while
unlikenesses of development must as unavoidably follow unlikenesses
among these agencies. And this being so, all the specialities of
symmetry and unsymmetry and asymmetry which we have traced, are
necessary consequences.
PART V.
PHYSIOLOGICAL DEVELOPMENT.
CHAPTER I.
THE PROBLEMS OF PHYSIOLOGY.
§ 265. The questions to be treated under the above title are widely
different from those which it ordinarily expresses. We have no
alternative, however, but to use Physiology in a sense co-extensive
with that in which we have used Morphology. We must here consider
the facts of function in a manner parallel to that in which we
have, in the foregoing Part, considered the facts of structure.
As, hitherto, we have concerned ourselves with those most general
phenomena of organic form which, holding irrespective of class and
order and sub-kingdom, illustrate the processes of integration and
differentiation characterizing Evolution at large; so, now, we have
to concern ourselves with the evidences of those differentiations and
integrations of organic functions which have simultaneously arisen,
and which similarly transcend the limits of zoological and botanical
divisions. How heterogeneities of action have progressed along with
heterogeneities of structure--that is the inquiry before us; and
obviously, in pursuing it, all the specialities with which Physiology
usually deals can serve us only as materials.
Before entering on the study of Morphological Development, it
was pointed out that while facts of structure may be empirically
generalized apart from facts of function, they cannot be rationally
interpreted apart; and throughout the foregoing pages this truth has
been made abundantly manifest. Here we are obliged to recognize the
interdependence still more distinctly; for the phenomena of function
cannot even be conceived without direct and perpetual consciousness of
the phenomena of structure. Though the subject-matter of Physiology
is as broadly distinguished from the subject-matter of Morphology as
motion is from matter; yet, just as the laws of motion cannot be known
apart from some matter moved, so there can be no knowledge of function
without a knowledge of some structure as performing function.
Much more than this is obvious. The study of functions, considered from
our present point of view as arising by Evolution, must be carried on
_mainly_ by the study of the correlative structures. Doubtless, by
experimenting on the organisms which are growing and moving around
us, we may ascertain the connexions existing among certain of their
actions, while we have little or no knowledge of the special parts
concerned in those actions. In a living animal that can be conveniently
kept under observation, we may learn the way in which conspicuous
functions vary together--how the rate of a man’s pulse increases with
the amount of muscular exertion he is undergoing; or how a horse’s
rapidity of breathing is in part dependent on his speed. But though
observations of this order are indispensable--though by accumulation
and comparison of such observations we learn which parts perform which
functions--though such observations, prosecuted so as to disclose
the actions of all parts under all circumstances, constitute, when
properly generalized and co-ordinated, what is commonly understood as
Physiology; yet such observations help us but a little way towards
learning how functions came to be established and specialized.
We have next to no power of tracing up the genesis of a function
considered purely as a function--no opportunity of observing the
progressively-increasing quantities of a given action that have arisen
in any order of organisms. In nearly all cases we are able only to
show the greater growth of the part which we have found performs the
action, and to infer that greater action of the part has accompanied
greater growth of it. The tracing out of Physiological Development,
then, becomes substantially a tracing out of the development of
the organs by which the functions are known to be discharged--the
differentiation and integration of the functions being presumed to have
progressed hand in hand with the differentiation and integration of the
organs. Between the inquiry pursued in Part IV, and the inquiry to be
pursued in this Part, the contrast is that, in the first place, facts
of structure are now to be used to interpret facts of function, instead
of conversely; and, in the second place, the facts of structure to be
so used are not those of conspicuous shape so much as those of minute
texture and chemical composition.
§ 266. The problems of Physiology, in the wide sense above
described, are, like the problems of Morphology, to be considered
as problems to which answers must be given in terms of incident
forces. On the hypothesis of Evolution these specializations of
tissues and accompanying concentrations of functions, must, like the
specializations of shape in an organism and its component divisions,
be due to the actions and reactions which its intercourse with the
environment involves; and the task before us is to explain how they are
wrought--how they are to be comprehended as results of such actions and
reactions.
Or, to define these problems still more specifically:--Those extremely
unstable substances composing the protoplasm of which organisms are
mainly built, have to be traced through the various modifications in
their properties and powers, that are entailed on them by changes of
relation to agencies of all kinds. Those organic colloids which pass
from liquid to solid and from soluble to insoluble on the slightest
molecular disturbance--those albuminoid matters which, as we see in
clotted blood or the coagulable lymph poured out on abraded surfaces
and causing adhesion between inflamed membranes, assume new forms
with the greatest readiness--are to have their metamorphoses studied
in connexion with the influences at work. Those compounds which, as
we see in the quickly-acquired brownness of a bitten apple or in the
dark stains produced by the milky juice of a Dandelion, immediately
begin to alter when the surrounding actions alter, are to be everywhere
considered as undergoing modifications by modified conditions. Organic
bodies, consisting of substances that, as I here purposely remind the
reader, are prone beyond all others to change when the incident forces
are changed, we must contemplate as in all their parts differently
changed in response to the different changes of the incident forces.
And then we have to regard the concomitant differentiations of their
reactions as being concomitant differentiations of their functions.
Here, as before, we must take into account two classes of factors.
We have to bear in mind the inherited results of actions to which
antecedent organisms were exposed, and to join with these the results
of present actions. Each organism is to be considered as presenting
a moving equilibrium of functions, and a correlative arrangement
of structures, produced by the aggregate of actions and reactions
that have taken place between all ancestral organisms and their
environments. The tendency in each organism to repeat this adjusted
arrangement of functions and structures, must be regarded as from time
to time interfered with by actions to which its inherited equilibrium
is not adjusted--actions to which, therefore, its equilibrium has to be
re-adjusted. And in studying physiological development we have in all
cases to contemplate the progressing compromise between the old and the
new, ending in a restored balance or adaptation.
Manifestly our data are so scanty that nothing more than very general
and approximate interpretations of this kind are possible. If the
hypothesis of Evolution furnishes us with a rude conception of the
way in which the more conspicuous and important differentiations of
functions have arisen, it is as much as can be expected.
§ 267. It will be best, for brevity and clearness, to deal with these
physiological problems as we dealt with the morphological ones--to
carry on the inductive statement and the deductive interpretation
hand in hand: so disposing of each general truth before passing
to the next. Treating separately vegetal organisms and animal
organisms, we will in each kingdom consider:--first, the physiological
differentiations and accompanying changes of structure which arise
between outer tissues and inner tissues; next, those which arise
between different parts of the outer tissues; and, finally, those which
arise between different parts of the inner tissues. What little has
to be said concerning physiological integration must come last. For
though, in tracing up Morphological Evolution, we have to study those
processes of integration by which organic aggregates are formed, before
studying the differentiations that arise among their parts; we must,
contrariwise, in tracing up Physiological Evolution, study the genesis
of the different functions before we study the interdependence that
eventually arises among them and constitutes physiological unity.
CHAPTER II.
DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF PLANTS.
§ 268. The simplest plant presents a contrast between its peripheral
substance and its central substance. In each protophyte, be it a
spherical cell or a branched tube, or such a more-specialized form as a
Desmid, a marked unlikeness exists between the limiting layer and that
which it limits. These vegetal aggregates of the first order may differ
widely from one another in the natures of their outer coats and in the
natures of their contents. As in the Palmella-form of one of the lower
_Algæ_, there may exist a clothing of jelly; or, as in Diatom, the
walls may take the form of silicious valves variously sculptured. The
contained matter may be partly or wholly here green, there red, and in
other cases brown. But amid all these diversities there is this one
uniformity--a strong distinction between the parts in contact with the
environment and the parts not in contact with the environment.
When we remember that this trait is one which these simple living
bodies have in common with bodies that are not living--when we
remember that each inorganic mass eventually has its outer part more
or less differentiated from its inner part, here by oxidation, there
by drying, and elsewhere by the actions of light, of moisture, of
frost; we can scarcely resist the conclusion that, in the one case
as in the other, the contrast is due to the unlike actions to which
the parts are subject. Given an originally-homogeneous portion of
protoplasm, and it follows from the general laws of Evolution (First
Principles, §§ 149–155), first, that it must lose its homogeneity, and,
second, that the leading dissimilarities must arise between the parts
most-dissimilarly conditioned--that is, between the outside and the
inside. The exterior must bear amounts and kinds of force unlike the
amounts and kinds which the interior bears; and from the persistence
of force it follows inevitably that unlike effects must be wrought on
them--they must be differentiated.
What is the limit towards which the differentiation tends? We
have seen that the re-distribution of matter and motion whence,
under certain conditions, evolution results, can never cease until
equilibrium is reached--proximately a moving equilibrium, and finally
a complete equilibrium (_First Principles_, §§ 170–175). Hence, the
differentiation must go on until it establishes such differences in the
parts as shall balance the differences in the forces acting on them.
When dealing with equilibration in general, we saw that this process
is what is called adaptation (_First Principles_, § 173); and, in this
work, we saw that by it the totality of functions of an organism is
brought into correspondence with the totality of actions affecting it
(§§ 159–163). Manifestly in this case, as in all others, either death
or adjustment must eventually result. A force falling on one of these
minute aggregates of protoplasm, must expend itself in working its
equivalent of change. If this force is such that in expending itself
it disturbs beyond rectification the balance of the organic processes,
then the aggregate is disintegrated or decomposed. But if it does
not overthrow that moving equilibrium constituting the life of the
aggregate, then the aggregate continues in that modified form produced
by the expenditure of the force. Thus, by direct equilibration,
continually furthered by indirect equilibration, there must arise this
distinction between the outer part adapted to meet outer forces, and
the inner part adapted to meet inner forces. And their respective
actions, as thus meeting outer and inner forces, must be what we call
their respective functions.
§ 269. Aggregates of the second order exhibit parallel traits,
admitting of parallel interpretations. Integrated masses of cells or
units homologous with protophytes, habitually show us contrasts between
the characters of the superficial tissues and the central tissues. Such
among these aggregates of the second order as have their component
units arranged into threads or laminæ, single or double, cannot, of
course, furnish contrasts of this kind; for all their units are as much
external as internal. We must turn to the more or less massive forms.
Of these, among _Fungi_, the common Puff-ball is a good example--good
because it presents this fundamental differentiation but little
complicated by others. In it we have a cortical layer of interwoven
hyphæ obviously unlike the mass of spores which it incloses. So far as
the unlikeness between external and internal parts is concerned, we see
here a relation analogous to that existing in the simple cell; and we
see in it a similar meaning: there is a physiological differentiation
corresponding to the difference in the incidence of forces.
Under various forms the _Algæ_ show just the same relation. Where,
as in _Codium Bursa_, we have the ramified tubular branches of the
thallus aggregated into a hollow globular mass, the outer and inner
surfaces are contrasted both in colour and structure, though the
tubules composing the two surfaces are continuous with one another. In
_Rivularia_, again, we see the like, both in the radial arrangement
of the imbedded threads and in the difference of colour between the
exterior of the imbedding jelly and its interior. The more-developed
_Algæ_ of all kinds repeat the antithesis. In branched stems, when
they consist of more than single rows of cells, the outer cells become
unlike the inner, as shown in Fig. 35. Such types as _Chrysymenia
rosea_ show us this unlikeness very conspicuously. And it holds even
with ribbon-shaped fronds. Wherever one of these is composed of three,
four, or more layers, as in _Laminaria_ and _Punctaria_, the cells
of the external layers are strongly distinguished from those of the
internal layers, both by their comparative smallness and by their deep
colour.
§ 270. The higher plants variously display the like fundamental
distinction between outer and inner tissues. Each leaf, thin as it is,
exemplifies this differentiation of the parts immediately in contact
with the environment from the parts not in immediate contact with
the environment. Its epidermal cells, forming a protecting envelope,
diverge physically and chemically from the mesophyll cells, which carry
on the more active functions. And the contrast may be observed to
establish itself in the course of development. At first the component
cells of the leaf are all alike; and this unlikeness between the cells
of the outer and inner layers, arises simultaneously with the rise of
differences in their conditions--differences that have acted on all
ancestral leaves as they act on the individual leaf.
An unlikeness more marked in kind but similar in meaning, exists
between the bark of every branch and the tissues it clothes. The
phænogamic axis, especially when it undergoes what is known as
secondary thickening, is commonly characterized by an outer zone of
cells (the cork layer) differing from the inner layers in character and
function, as it differs from them in position. Subject as this outer
layer is to the unmitigated actions of forces around--to abrasions, to
extremes of heat and cold, to evaporation and soaking with water--its
units have to be brought into equilibrium with these more violent
actions, and have acquired molecular constitutions more stable
than those of the interior cells. That is to say, the forces which
differentiate the cortical part from the rest are the forces which it
has to resist, and from which it passively protects the parts within.
How clearly this heterogeneity of structure and function is consequent
upon intercourse with the environment, every tree and shrub shows. The
young shoots, alike of annuals and perennials, are quite green and soft
at their extremities. Among plants of short lives, there is usually
but a slight development of bark or none at all: such traces of it as
the surface of the axis acquires being seen only at its lowermost or
oldest portion. In long-lived plants, however, this formation of a
tough opaque coating takes place more rapidly; and shows us distinctly
the connexion between the degree of differentiation and the length of
exposure. For, in a growing twig, we see that the bark, invisible at
the bud, thickens by insensible gradations as we go downwards to the
junction of the twig with the branch; and we come to still thicker
parts of it as we descend along the branch towards the main stem.
Moreover, on examining main stems we find that while in some trees the
bark, cracked by expansion of the wood, drops off in flakes, leaving
exposed patches of the inner tissue which presently become green and
finally develop new bark; in other trees the exfoliated flakes continue
adherent, and in the course of years form a rugged fissured coat: so
producing a still more marked contrast between outside and inside. Of
course the establishment of this heterogeneity is furthered by natural
selection, which, where a protective covering is needed, gives an
advantage to those individuals in which it has become strongest. But
that this divergence of structure commences as a direct adaptation, is
clearly shown by other facts than the foregoing. There is the fact that
many of the plants which in our gardens develop bark with considerable
rapidity, do not develop it with the same rapidity in a greenhouse. And
there is the fact that plants which, in some climates, have their stems
covered only by thin semi-transparent layers, acquire thick opaque
layers when taken to other climates.
Just noting, for the sake of completeness, that in the roots of the
higher plants there arises a contrast between outer and inner parts,
parallel to the one we have traced in their branches, let me draw
attention to another differentiation of the same ultimate nature, which
the higher plants exhibit to us--a differentiation which, familiar
though it is, gains a new meaning by association with those named
above, and makes their meaning still more manifest. Each great plant
shows it. When, by the budding of axes out of axes, there is produced
one of those highly-compounded Phænogams which we call a tree, the
central part of the aggregate becomes functionally and structurally
unlike the peripheral part. On looking into a large tree, or even
a small one which has thick foliage, like the Laurel, we see that
the internal branches are almost or quite bare of leaves, while the
leaf-clad branches form an external stratum; and all our experience
unites in proving that this contrast arises by degrees, as fast as
the growth of the tree entails a contrast between the conditions
to which inner and outer branches are exposed. Now when, in these
most-composite aggregates, we see a differentiation between peripheral
and central parts demonstrably caused by a difference in the relations
of these parts to environing forces, we get support for the conclusion
otherwise reached, that there is a parallel cause for the parallel
differentiations exhibited by all aggregates of lower orders--branches,
leaves, cells.
§ 271. Before leaving this most general physiological differentiation,
it may be well to say something respecting certain secondary
unlikenesses which usually arise between interior and exterior. For the
contrast is not, as might be supposed from the foregoing descriptions,
a simple contrast: it is a compound contrast. The outer structure
itself is usually divisible into concentric structures. This is equally
true of a protophyte and of a phænogamic axis. Between the centre of
an independent vegetal cell and its surface, there are at least two
layers; and the bark coating the substance of a shoot, besides being
itself compound, includes another tissue lying between it and the
wood. What is the physical interpretation of these facts?
When a mass of something we distinguish as inert matter is exposed
to external agencies capable of working changes in it--when it is
chemically acted upon, or when, being dry, it is allowed to soak, or
when, being wet, it is allowed to dry--the changes set up progress in
an equable way from the surface towards the centre. At any time during
the process (supposing no other action supervenes) the modification
wrought, first completed at the outside, either gradually diminishes
as we approach the centre, or ceases suddenly at a certain distance
from the centre. But now suppose that the mass, instead of being
inert, is the seat of active changes--suppose that it is a portion of
complex colloidal substance, permeable by light and by fluids capable
of affecting its unstable molecules--suppose that its interior is
a source of forces continually liberated and diffusing themselves
outwards. Is it not likely that while at the centre the action of the
internally-liberated forces will dominate, and while at the surface
the action of the environing forces will dominate, there will be
between the two a certain place at which their actions balance? May
we not expect that this will be the place where the most unstable
matter exists--the place outside of which the matter becomes relatively
stable in the face of external forces, and inside of which the matter
becomes relatively stable in the face of internal forces? And must we
not conclude that though part of the adjustment is due to indirect
equilibration, the initiation of it is due to direct equilibration?
But we are here chiefly concerned with the more general interpretation,
which is independent of any such speculation as the foregoing. These
contrasted tissues and the contrasted functions they severally perform
are, beyond question, subordinated to the relations of outside and
inside. And the evidence makes it tolerably clear that the unlike
actions or forces involved by the relations of outside and inside,
determine these contrasts--partly directly and partly indirectly.
CHAPTER III.
DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS.
§ 272. The motionless protococcoid forms of lower _Algæ_, which do
not permanently expose any parts of their surfaces to actions unlike
those which other parts are exposed to, have no parts of their
surfaces unlike the rest in function and composition. This is what
the hypothesis prepares us for. If physiological differentiations
are determined by differences in the incidence of forces, then there
will be no such differentiations where there are no such differences.
Contrariwise, it is to be expected that the most conspicuous
unlikeness of function and minute structure will arise between the
most-dissimilarly circumstanced parts of the surface. We find that
they do. The upper end and the lower end, or, more strictly speaking,
the free end and the attached end, habitually present the strongest
physiological contrasts.
Even aggregates of the first order illustrate this truth. Such
so-called unicellular plants as those delineated in Figs. 4, 5, and 6,
show us, on comparing the contents of their fixed ends and their loose
ends, that different processes are going on in them, and that different
functions are being performed by their limiting membranes. _Caulerpa
prolifera_, which “consists of a little creeping stem with roots below
and leaves above,” originating “in the growth of a body which may be
regarded as an individual cell,” supplies a still-better example. Among
aggregates of the second order a like connexion is displayed in more
various modes but with equal consistency. As before, the Puff-ball
served to exemplify the primary physiological differentiation of outer
parts from inner parts; so, here, it supplies a simple illustration of
the way in which the differentiated outer part is re-differentiated,
in correspondence with the chief contrast in its relations to the
environment. The only marked unlikeness which the cortical layer of
the Puff-ball presents, is that between the portion next the ground
and the opposite portion. The better-developed _Fungi_ exhibit a
more decided heterogeneity of parallel kind. Such incrusting _Algæ_
as _Ralfsia verrucosa_ furnish a kindred contrast; and in the higher
_Algæ_ it is uniformly repeated. Phænogams display this physiological
differentiation very conspicuously. That earth and air are unlike
portions of the environment, subjecting roots and leaves to unlike
physical forces, which entail on them unlike reactions, and that the
unlike functions and structures of their respective surfaces are fitted
to these unlike physical forces, are familiar facts which it would be
needless here to name, were it not that they must be counted as coming
within a wider group of facts.
Is this unlikeness between the outer tissues of the attached ends
and those of the free ends in plants, determined by their converse
with the unlike parts of the environment? That they result from an
equilibration partly arising in the individual and partly arising by
the survival of individuals in which it has been carried furthest, is
inferable _à priori_; and this _à priori_ argument may be adequately
enforced by arguments of the inductive order. A few typical ones must
here suffice. The gemmules of the _Marchantia_ are little disc-shaped
masses of cells composed of two or more layers. Their sides being
alike, there is nothing to determine which side falls lowermost when
one of them is detached. Whichever side falls lowermost, however,
presently begins to send out rootlets, while the uppermost side
begins to assume those characters which distinguish the face of the
frond. When this differentiation has commenced, the tendency to its
complete establishment becomes more and more decided; as is proved by
the fact that if the positions of the surfaces be altered, the gemmule
bends itself so as to re-adjust them: the change towards equilibrium
with environing forces having been once set up, there is acquired,
as it were, an increasing momentum which resists any counter-change.
But the evidence shows that at the outset, the relations to earth
and air alone determine the differentiation of the under surface
from the upper. The experiences of the gardener, multiplying his
plants by cuttings and layers, constitute another class of evidences
not to be omitted: they are commonplace but instructive examples of
physiological differentiation. While circumstanced as it usually is,
the meristematic tissue of each branch in a Phænogam continues to
perform its ordinary function--regularly producing on its outer side
the cortical substances, and on its inner side the vascular and woody
tissues. But change the conditions to those which the underground part
of the plant is exposed to, and there begins another differentiation
resulting in underground structures. Contact with water often suffices
alone to produce this result, as in the branches of some trees when
they droop into a pool, or as occasionally with a cutting placed in a
bottle of water; and when the light is excluded by imbedding the end
of the cutting, or the middle of the still-attached branch, in the
earth, this production of tissues adapted to the function of absorbing
moisture and mineral constituents proceeds still more readily.
With such cases may be grouped those in which this development of
underground organs by an above-ground tissue, is not exceptional but
habitual. Creeping plants furnish good illustrations. From the shoots
of the Ground-Ivy, rootlets are put out into the soil in a manner
differing but little from that in which they are put out by an imbedded
layer; save that the process follows naturally-induced conditions
instead of following artificially-induced conditions. But in the common
Ivy which, instead of running along the surface of the earth, runs
up inclined or vertical surfaces, we see the process interestingly
modified without being essentially changed. The rootlets, here
differentiated by their conditions into organs of attachment much more
than organs of absorption, still develop on that side of the shoot next
the supporting surface, and do not develop where the shoot, growing
away from the tree or wall, is surrounded equally on all sides by light
and air: thus showing, undeniably, that the production of the rootlets
is determined by the differential incidence of forces. Though survival
of the fittest doubtless furthered this transition yet it clearly
did not initiate it. That greenness which may be observed in these
Ivy-branch rootlets while they are quite young, soft, and unshaded,
introduces us to facts which are the converse of the foregoing facts;
and proves that the parts ordinarily imbedded in the soil and adapted
to its actions, acquire, often in very marked degrees, the superficial
structures of the aërial parts, when they are exposed to light and air.
This may be witnessed in Maize, which, when luxuriant, sends out from
its nodes near the ground, clusters of roots that are thick, succulent,
and of the same colour as the leaves. Examples more familiar to us
in England occur in every field of turnips. On noting how green is
the uncovered part of a turnip-root, and how manifestly the area over
which the greenness extends varies with the area exposed to light, as
well as with the degree of the exposure, it will be seen that beyond
question, root-tissue assumes to a considerable extent the appearances
and function of leaf tissue, when subject to the same agencies. Let
us not forget, too, that where exposed roots do not approach in
superficial character towards leaves, they approach in superficial
character towards stems: becoming clothed with a thick, fissured bark,
like that of the trunk and branches. But the most conclusive evidence
is furnished by the actual substitutions of surface-structures and
functions, that occur in aërial organs which have taken to growing
permanently under ground, and in underground organs which have taken to
growing permanently in the air. On the one hand, there is the rhizome
exemplified by Ginger--a stem which, instead of shooting up vertically,
runs horizontally below the surface of the soil, and assumes the
character of a root, alike in colour, texture, and production of
rootlets; and there is that kind of swollen underground axis, bearing
axillary buds, which the Potato exemplifies--a structure which, though
homologically an axis, simulates a tuberous root in surface-character,
and when exposed to the air, manifests no greater readiness to develop
chlorophyll than a tuberous root does. On the other hand, there are the
aërial roots of certain Orchids which, habitually green at their tips,
continue green throughout their whole lengths when kept moist; which
have become leaf-like not only by this development of chlorophyll, but
also by the acquirement of stomata; and which do not bury themselves in
the soil when they have the opportunity.[46] Thus we have aërial organs
so completely changed to fit underground actions, that they will not
resume aërial functions; and underground organs so completely changed
to fit aërial actions, that they will not resume underground functions.
That the physiological differentiation between the part of a plant’s
surface which is exposed to light and air and the part which is exposed
to darkness and moisture and solid matter, is primarily due to the
unlike actions of these unlike parts of the environment, is, then,
clearly implied by observed facts--more clearly, indeed, than was to
be expected. Considering how strong must be the inherited tendency
of a plant to assume those special characters, physiological as well
as morphological, which have resulted from an enormous accumulation
of antecedent actions, it may be even thought surprising that this
tendency can be counteracted to so great an extent by changed
conditions. Such a degree of modifiability becomes comprehensible only
when we remember how little a plant’s functions are integrated, and how
much, therefore, the functions going on in each part may be altered
without having to overcome the momentum of the functions throughout
the whole plant. But this modifiability being as great as it is, we
can have no difficulty in understanding how, by the cumulative aid
of natural selection, this primary differentiation of the surface in
plants has become what we see it.
§ 273. We will leave now these contrasts between the free surfaces of
plants and their attached or imbedded surfaces, and turn our attention
to the secondary contrasts existing between different parts of their
free surfaces. Were a full statement of the evidence practicable,
it would be proper here to dwell on that which is furnished by the
inferior classes. It might be pointed out in detail that where, as
among the _Algæ_, the free surfaces are not dissimilarly conditioned,
there is no systematic differentiation of them--that the frond of
an _Ulva_, the ribbon-shaped divisions of a _Laminaria_, and the
dichotomous expansions of the _Fuci_ which clothe the rocks between
tide-marks, are alike on both sides; because, swayed about in all
directions as they are by the waves and tides, their sides are equally
affected. Conversely, from the _Fungi_ might be drawn abundant proof
that even among Thallophytes, unlikenesses arise between different
parts of the free surfaces when their circumstances are unlike.
In such laterally-growing kinds as are shown in Fig. 196_b_, the
honey-combed under surface and the smooth leathery upper surface,
have their contrasts related to contrasted conditions; and in the
adjacently-figured Agarics, and other stalked genera, the pileus
exhibits a parallel difference, explicable in a parallel way. But
passing over Cryptogams it must suffice if we examine more at length
these traits as they are displayed by Phænogams. Let us first note the
dissimilarities between the outer tissues of stems and leaves.
That these dissimilarities arose by degrees, as fast as the units
of which the phænogamic axis is composed became integrated, is a
conclusion in harmony with the truth that in every shoot of every
plant, they are at first slight and become gradually marked. Already,
in briefly tracing the contrasts between the outer and inner tissues
of plants, some facts have been named showing, by implication, how
the cessation of the leaf-function in axes is due to that change of
conditions entailed by the discharge of other functions. Here we have
to consider more closely facts of this class, together with others
immediately to the point. On pulling off from a stem of grass the
successive sheaths of its leaves, the more-inclosed parts of which are
of a fainter green than the outer parts, it will be found that the
tubular axis eventually reached is of a still fainter green; but when
the axis eventually shoots up into a flowering stem, its exposed part
acquires as bright a green as the leaves. In other Monocotyledons, the
leaf-sheaths of which are successively burst and exfoliated by the
swelling axis, it may be observed that where the dead sheaths do not
much obstruct the light and air, the surface of the axis underneath
is full of chlorophyll. _Dendrobium_ is an example. But when the dead
sheaths accumulate into an opaque envelope, the chlorophyll is absent,
and also, we may infer, the function which its presence habitually
implies. Carrying with us this evidence, we shall recognize a like
relation in Dicotyledons. While its outer layer remains tolerably
transparent, an exogenous stem or branch continues to show, by the
formation of chlorophyll, that it shares in the duties of the leaves;
but in proportion as a bark which the light cannot penetrate is
produced by the adherent flakes of dead skin, or by the actual deposit
of a protective substance, the differentiation of duties becomes
more decided. Cactuses and Euphorbias supply us with converse facts
having the same implication. The succulent axes so strangely combined
in these plants, maintain for a long time the translucency of their
outer layers and their greenness; and they so efficiently perform the
offices of leaves that leaves are not produced. In some cases, axes
that are not succulent participate largely in the leaf-function, or
entirely usurp it--still, however, by fulfilling the same essential
conditions. Occasionally, as in _Statice brassicæfolia_, stems become
fringed; and the fringes they bear assume, along with the thinness of
leaves, their darker green and general aspect. In the genus _Ruscus_,
the flattened axis simulates so closely the leaf-structure, that were
it not for the flower borne on its mid-rib, or edge, its axial nature
would hardly be suspected. And let us not omit to note that where axes
usurp the characters of leaves, in their attitudes as well as in their
shapes and thicknesses, there are contrasts between their under and
upper surfaces, answering to the contrasts between the relations of
these surfaces to the light. Of this _Ruscus androgynus_ furnishes a
striking example. In it the difference which the unaided eye perceives
is much less conspicuous than that disclosed by the microscope; for
I find that while the face of the pseudo-leaf has no stomata, the
back is abundantly supplied with them. One more illustration must
be added. Equally for the morphological and physiological truths
which it enforces, the _Mühlenbeckia platyclada_ is one of the most
instructive of plants. In it the simulation of forms and usurpation
of functions, are carried out in a much more marvellous way than
among the _Cactaceæ_. Imagine a growth resembling in outline a very
long willow-leaf, but without a mid-rib, and having its two surfaces
alike. Imagine that across this thin, green, semi-transparent
structure, there are from ten to thirty divisions, which prove to be
the successive nodes of an axis. Imagine that along the edges of this
leaf-shaped aggregate of internodes, there arise axillary buds, some
of which unfold into flowers, and others of which shoot up vertically
into growths like the one which bears them. Imagine a whole plant thus
seemingly composed of jointed willow-leaves growing from one another’s
edges, and some conception will be formed of the _Mühlenbeckia_.
The two facts which have meaning for us here are--first, that the
performance of leaf-functions by these axes goes along with the
assumption of a leaf-like translucency; and, second, that these
flattened axes, retaining their upright attitudes, and therefore
keeping their two faces similarly conditioned, have these two faces
alike in colour and texture.
That physiological differentiation of the surface which arises in
Phænogams between axial organs and foliar organs, is thus traceable
with tolerable clearness to those differences between their conditions
which integration has entailed--partly in the way above described and
partly in other ways still to be named. By its relative position, as
being shaded by the leaves, the axis is less-favourably circumstanced
for performing those assimilative actions effected by the aid of light.
Further, that relatively-small ratio of surface to mass in the axis,
which is necessitated by its functions as a support and a channel
for circulation, prevents it from taking in, with the same facility
as the leaves, those surrounding gases from which matter is to be
assimilated. Both these special causes, however, in common with that
previously assigned, fall within the general cause. And in the fact
that where the differential conditions do not exist, the physiological
differentiation does not arise, or is obliterated, we have clear proof
that it is determined by unlikenesses in the relations of the parts to
the environment.
§ 274. From this most general contrast between aërial
surface-tissues--those of axes and those of folia--we pass now to the
more special contrasts of like kind existing in folia themselves.
Leaves present us with superficial differentiations of structure and
function; and we have to consider the relations between these and the
environing forces.
Over the whole surface of every phænogamic leaf, as over the fronds
of the _Pteridophyta_, there extends a simple or compound epidermal
layer, formed of cells that are closely united at their edges and
devoid (in the Flowering Plants) of that granular colouring matter
(chlorophyll) contained in the layers of cells they inclose: the result
being that the membrane formed of them is comparatively transparent. On
the submerged leaves of aquatic Phænogams, this outer layer is thin,
delicate, and permeable by water; but on leaves exposed to the air, and
especially on their upper surfaces, is comparatively strong, dense,
often smooth and impermeable by water: being thus fitted to prevent the
rapid escape of the contained juices by evaporation. Similarly, while
the leaves of terrestrial plants which live in temperate climates,
usually have comparatively thin coats thus composed, in climates that
are both hot and dry, leaves are commonly clothed with a very thick
cuticle. Nor is this all. The outside of an aërial leaf differs from
that of a submerged leaf by containing a deposit of waxy substance.
Whether this be exuded by the exposed surfaces of the cells, as some
contend, or whether it is deposited within the cells, as thought by
others, matters not in so far as the general result is concerned. In
either case a waterproof coating is formed at the outermost sides of
these outermost cells; and in many cases produces that polish by which
the upper surface of the leaf is more or less distinguished from the
under surface. This external pellicle presents us with another contrast
of allied meaning. On the upper surfaces of leaves subject to the
direct action of the sun’s rays, there are either few or none of those
minute openings, or stomata, through which gases can enter or escape;
but on the under surfaces these stomata are abundant: a distribution
which, while permitting free absorption of the needful carbonic acid,
puts a check on the exit of watery vapour. Two general exceptions to
this arrangement may be noted. Leaves that float on the water have all
their stomata on their upper sides, and leaves that are submerged have
no stomata--modifications obviously appropriate to the conditions.
What is to be said respecting the genesis of these differentiations?
For the last there seems no direct cause: its cause must be indirect.
The unlike actions to which the upper and under surfaces of leaves are
subject, have no apparent tendency to produce unlikeness in the number
of their breathing holes. Here the natural selection of spontaneous
variations furnishes the only feasible explanation. For the first,
however, there is a possible cause in the immediate actions of incident
forces, which survival of the fittest continually furthers.
The fluid exhaling through the walls of the cells next the air, will be
likely to leave behind suspended substances on their outer surfaces.
On remembering the pellicle which is apt to form on thick solutions or
emulsions as they dry, and how this pellicle as it grows retards the
further drying, it will be perceived that the deposit of waxy matter
next to the outer surfaces of the cuticular cells in leaves, is not
improbably initiated by the evaporation which it eventually checks.
Should it be so, there results a very simple case of equilibration.
Where the loss of water is too great, this waxy pellicle left behind by
the escaping water will protect most those individuals of the species
in which it is thickest or densest; and by inheritance and continual
survival of the fittest, there will be established in the species that
thickness of the layer which brings the evaporation to a balance with
the supply of water.
Another superficial differentiation, still more familiar, has to be
noted. Every child soon learns to distinguish by its colour the upper
side of a leaf from its under side, if the leaf is one that has grown
in such way as to establish the relations of upper and under. The upper
surfaces of leaves are habitually of a deeper green than the under.
Microscopic examination shows that this deeper green results from the
closer clustering of those parenchyma-cells full of chlorophyll that
are in some way concerned with the assimilative actions; while beneath
them are more numerous intercellular passages communicating with those
openings or stomata through which is absorbed the needful air. Now when
it is remembered that the formation of chlorophyll is clearly traceable
to the action of light--when it is remembered that leaves are pale
where they are much shaded and colourless when developed in the dark,
as in the heart of a Cabbage--when it is remembered that succulent axes
and petioles, like those of Sea-kale and Celery, remain white while
the light is kept from them and become green when exposed; it cannot
be questioned that this greater production of chlorophyll next to the
upper surface of a leaf, is directly consequent on the greater amount
of light received. Here, as in so many other cases, we must regard
the differentiation as in part due to direct equilibration and in
part to indirect equilibration. Familiar facts compel us to conclude
that from the beginning, each individual foliar organ has undergone
a certain immediate adaptation of its surfaces to the incidence of
light; that when there arose a mode of growth which exposed the leaves
of successive generations in similar ways, this immediately-produced
adaptation, ever tending to be transmitted, was furthered by the
survival of individuals inheriting it in the greatest degree; and that
so there was gradually established that difference between the two
surfaces which each leaf displays before it unfolds to the light, but
which becomes more marked when it has unfolded.[47]
From the ordinary cases let us now pass to the exceptional cases. We
will look first at those in which the two faces of the leaves differ
but little, or not at all--their circumstances being similar or equal.
Leaves that grow in approximately-upright attitudes, and attitudes
which do not maintain the relative positions of the two surfaces with
constancy, may be expected to display an unusual likeness between the
two surfaces; and among them we see it. The Grasses may be named as a
group exemplifying this relation; and if, instead of comparing them as
a group with other groups, we compare those dwarf kinds of them which
spread out their leaves horizontally, with the large aspiring kinds,
such as _Arundo_, we trace a like antithesis: in the one the contrast
of upper and under is very obvious, while in the other it is scarcely
to be detected. Leaves of various other Monocotyledons that grow in a
similar way, similarly show us a near approach to uniformity of the
two surfaces; as instance the genus _Clivia_ and the thinner-leaved
kinds of _Yucca_. Where the contrast of upper and under is greatly
diminished by the assumption of a rounded or cylindrical form, instead
of a flattened form, the same thing happens. The genus _Kleinia_
furnishes illustrations. It may be remarked, too, that even within the
limits of this genus there are instructive variations; for while in
_Kleinia ficoides_ the leaves, shaped like pea-pods, are broadest in a
vertical direction, and have their lateral surfaces alike in conditions
and structure, in other species the leaves, broader horizontally than
vertically, exhibit unlikeness between the upper and under sides.
Equally to the point is the evidence furnished by vertically-growing
leaves that are cylindrical, as those of _Sanseviera cylindrica_, or as
those of the Rush-tribe: the similarly-placed surface has all around
a similar character. Of kindred meaning, and still more conclusive,
are the cases in which the under side of the leaf, being more exposed
to light than the upper side, usurps the character and function of
the upper side. If a common Flag be pulled to pieces, it will be seen
that what answers to the face in other leaves, forms merely the inside
of the sheath including the younger leaves, and is obliterated higher
up. The two surfaces of the blade answer to the two under halves of a
leaf that has been, as it were, folded together lengthways, with the
two halves of its upper surface in contact. And here, in default of an
upper surface, the under surface acquires its character and discharges
its function. A like substitution occurs in _Aristea corymbosa_; and
there are some of the Orchids, as _Lockhartia_, which display it in a
very obvious way.
When joined with the foregoing evidence, the evidence which another
kind of substitution supplies is of great weight. I refer to that which
occurs in the Australian Acacias, already instanced as throwing light
on morphological changes. In these plants the leaves properly so-called
are undeveloped, and the foot-stalks, flattened out into foliaceous
shapes, acquire veins and mid-ribs, and so far simulate leaves as
ordinarily to be taken for them: a fact in itself of much physiological
significance. But that which it concerns us especially to note, is
the absence of distinction between the two faces of these phyllodes,
as they are named, and the cause of its absence. These transformed
petioles do not flatten themselves out horizontally, so as to acquire
under and upper sides, as most true leaves do; but they flatten
themselves out vertically: the result being that their two sides are
similarly circumstanced with respect to light and other agencies; and
there is consequently nothing to cause their differentiation. And then
we find an analogous case where differential conditions arise, and
where some differentiation results. In _Oxalis bupleurifolia_, Fig. 66,
there is a similar flattening out of the petiole into a pseudo-leaf;
but in it the flattening takes place in the same plane as the leaf, so
as to produce an under and an upper surface; and here the two surfaces
of the pseudo-leaf are slightly unlike--in contour if in nothing else.
§ 275. We now come to such physiological differentiations among the
outer tissues of plants, as are displayed in the contrasts between
foliar organs on the same axis, or on different axes--contrasts between
the seed-leaves and the leaves subsequently formed, between submerged
and aërial leaves in certain aquatic plants, between leaves and bracts,
and between bracts and sepals. To deal even briefly with these implies
information which even a professed botanist would have to increase by
special inquiries, before attempting interpretations. Here it must
suffice to say something respecting those marked unlikenesses existing
between the tissues of the more characteristic parts of flowers, and
the tissues of the homologous foliar organs.
It was pointed out in § 196, that the terminal folia of a phænogamic
axis have sundry characters in common with such fronds as those
out of which we concluded that the phænogamic axis has arisen by
integration--common characters of a kind to be expected. In their
simple cellular composition, comparative want of chlorophyll, and
deficiency of vascular structures, the undeveloped ends of leaf-shoots
and the developed ends of flower-shoots, approach to the fronds of the
simpler Archegoniates. We also noted between them another resemblance.
It is said of the _Jungermanniaceæ_, that “though under certain
circumstances of a pure green, they are inclined to be shaded with
red, purple, chocolate, or other tints;” and answering to this we have
the facts that such colours commonly occur in the terminal folia of a
phænogamic axis, when arrest of its development leads to the formation
of a flower, and that very frequently they are visible at the ends
of leaf-axes. In the unfolding parts of shoots, more or less of red,
or copper-colour, or chocolate-colour, may generally be seen: often,
indeed, it characterizes the leaves for some time after they are
unfolded. Occasionally the traces of it are permanent; and, as in the
scarlet terminal leaves of _Poinsettia pulcherrima_, we see that it may
become, and continue, extremely conspicuous. The question, then, now
to be asked is--has this colouring by which the immature part of the
phænogamic axis is characterized, anything to do with the colouring of
flowers? Has this difference between undeveloped folia and folia that
are further developed, been increased by natural selection where an
advantage accrued from it, until it has ended in the strong contrast we
now see? I think we may not irrationally infer that this has happened.
Facts, very numerous and varied, united to warrant us in concluding
that gamogenesis commences where the forces which conduce to growth are
nearly equilibrated by the forces which resist growth (§ 78); and the
induction that in plants, fertilized germs are produced at places where
there is an approach towards this balance, we found to be in harmony
with the deduction that an advantage to the species must be gained by
sending off migrating progeny from points where nutrition is failing.
Other things equal, failure of nutrition may be expected in parts which
have the most remote or most indirect access to the materials furnished
by the roots--materials which have to be carried great distances by
a very imperfect apparatus. The ends of lateral axes are therefore
the probable points of fructification, in aggregates of the third
order that have taken to growing vertically. But if these points at
which nutrition is failing, are also the points at which the colours
inherited from lower types are likely to recur in more marked degrees
than elsewhere; then we may infer that the organs of fructification
will not unfrequently co-exist with such colours at the ends of such
axes. How may the resulting contrast between the older fronds and the
fronds next the germ-producing organs be increased? If uninterfered
with it would be likely to diminish. These traits inherited from remote
ancestry might be expected slowly to fade away. How, then, is the
intensification of them to be explained?
If a contrast of the kind described favours the propagation of a race
in which it exists, it will be maintained and increased; and if we
take into account an agency of which Mr. Darwin has shown the great
importance--the agency of insects--we shall have little difficulty
in understanding how such a contrast may facilitate propagation. We
cannot, of course, here assume the agency of insects so specialized
in their habits as Bees and Butterflies; for their specialized habits
imply the pre-existence of the contrast to be explained. But there is
an insect-agency of a more general kind which may be fairly counted
upon as coming into action. Various small Flies and Beetles wander
over the surfaces of plants in search of food. It is a legitimate
assumption that they will frequent most those parts in which they find
most food, or food most to their liking--especially if at the same time
they gain the advantage of concealment. Now the ends of axes, formed
of young, soft, and closely-packed folia, are the parts which more
than any others offer these several advantages. They afford shelter
from enemies; they frequently contain exuded juices; and when they do
not, their tissues are so tender as to be easily pierced in search
of the sap. If, then, from the first, as at present, these ends of
axes have been favourite haunts of small insects; and if, where the
closely-clustered folia contained the generative organs, the insects
frequenting them occasionally carried adherent fructifying cells from
one plant to another, and so aided fertilization; it would follow
that anything which made such terminal clusters more attractive to
such insects, or more conspicuous to them, or both, would further the
multiplication of the race, and would so be continually increased by
the extra multiplication of individuals in which it was greatest.
Here we find the clue. This contrast of colour between the folia next
to the fructifying parts and all other folia, must constantly have
facilitated insect-agency; supposing the insects to have had the power
of distinguishing between colours. That Bees and Butterflies have this
power is manifest. They may be watched flying from flower to flower,
disregarding all other parts of the plants. And if the less-specialized
insects possessed some degree of such discrimination, then the initial
contrasts of colour above described would be maintained and increased.
Let such a connexion be once established, and it must tend to become
more decided. Insects most able to discern the parts of plants which
afford what they seek, will be those most likely to survive and leave
offspring. Plants presenting most of the desired food, and showing most
clearly where it lies, will have their fertilization and multiplication
furthered in the greatest degree. And so the mutual adaptation will
become ever closer; while it is rendered at the same time more varied
by the special requirements of the insects and of the plants in each
locality, under each change of conditions. Of course, the genesis
of the sweet secretions and the odours of flowers, has a parallel
interpretation. The simultaneous production of honey, or some kindred
substance, is implied above; since, unless a bait co-existed with
the colour, the colour would not attract insects, and would not be
maintained and intensified by natural selection. Gums, and resins, and
balsams, are familiar products of plants; apparently, in many cases,
excreted as useless matters from various parts of their surfaces. These
substances, admitting of wide variations in quality, as they do, afford
opportunities for the action of natural selection wherever any of
them, attractive to insects, happen to be produced near the organs of
fructification. And this action of natural selection once set up, may
lead to the establishment of a local excretion, to the production of an
excretion more and more attractive, and to the disposal of the organ
containing it in such a way as most to facilitate the carrying away of
pollen. Similarly and simultaneously with odours. Odours, like colours,
draw insects to flowers. After observing how Bees come swarming into a
house where honey is largely exposed, or how Wasps find their way into
a shop containing much ripe fruit, it cannot be questioned that insects
are to a considerable extent guided by scent. Being thus sensitive
to the aromatic substances which flowers exhale, they may, when the
flowers are in large masses, be attracted by them from distances
at which the flowers themselves are invisible. And manifestly, the
flowers which so attract them from the greatest distances, increasing
thereby their chances of efficient fertilization, will be most likely
to perpetuate themselves. That is to say, survival of the fittest
must tend to produce perfumes that are both more powerful and more
attractive.
These physiological differentiations, then, which mark off the
foliar organs constituting flowers from other foliar organs, are
the consequences of indirect equilibration. They are not due to the
immediate actions of unlike incident forces on the parts of the
individual plant; but they are due to the actions of such unlike
incident forces on the aggregate of individuals, generation after
generation.[48]
§ 276. The unity of interpretation which we here find for phenomena
of such various orders, could hardly be found were the phenomena
otherwise caused. That the stronger and the feebler contrasts among the
different parts of the outer tissues in plants, should so constantly
occur along with stronger and feebler contrasts among the incident
forces, is in itself weighty evidence that unlike outer actions have
caused unlike inner actions, and correspondingly-unlike structures;
either by changing the functional equilibrium in the individual, or by
changing it in the race, or by both.
Even in the absence of more direct proof, there would be great
significance in the marked differences that habitually exist between
the exposed and imbedded parts of plants, between the stems and the
leaves, and between the upper and under surfaces of the leaves. The
significance of these differences is increased when we discover that
they vary in degree as the differences in the conditions vary in
degree. Still greater becomes the force of the evidence on finding
that these strongly-contrasted parts may, when placed in one another’s
conditions, and kept in them from generation to generation, permanently
assume one another’s functions, and, in a great degree, one another’s
structures. Even more conclusive yet is the argument rendered, by the
discovery that, where these substitutions of function and structure
take place, the superinduced modifications differ in different
circumstances; just as the original modifications do. The fact that
a flattened stem simulating a vertically-growing leaf has its two
surfaces alike, while when it simulates a horizontally-growing leaf its
upper and under surfaces differ, is a fact which, standing alone, might
prove little, but proves much when joined with all the other evidence.
And its profound meaning becomes the more obvious on discovering that
the same thing happens with petioles when they usurp leaf-functions.
Finally, when we remember how rapidly analogous modifications of
function and structure arise in the superficial tissues of individual
plants, the general inference can scarcely be resisted. When we meet
with so striking a case as that of the _Begonia_-leaf, a fragment
of which stuck in the ground produces roots from its under surface
and leaves from its upper surface--when we see that though, in this
case, the typical structure of the plant presently begins to control
the organizing process, yet the initial differentiations are set
up by the differential actions of the environment; the presumption
becomes extremely strong that the heterogeneities of surface which
we have considered, result, as alleged, directly or indirectly from
heterogeneities in the incident forces.
CHAPTER IV.
DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS.[49]
§ 277. In passing from plants formed of threads or thin laminæ, to
plants having some massiveness, we find that after the external and
internal parts have become distinguished from one another, there arise
distinctions among the internal parts themselves, as well as among the
external parts themselves: the primarily-differentiated parts are both
re-differentiated.
From types of very low organisation illustrations of this may be
drawn. In the thinner kinds of _Laminaria_ there exists but the
single contrast between the outer layer of cells and an inner layer;
but in larger species of the same genus, as _L. digitata_, there
are three unlike layers on each side of a central layer differing
from them--augmentation of bulk is accompanied by multiplication of
concentric internal structures, having their unlikenesses obviously
related to unlikenesses in their conditions. In _Furcellaria_ and
various _Algæ_ of similarly swollen forms, the like relation may be
traced.
Just indicating the generality of this contrast, but not attempting to
seek in these lower types for any more specific interpretation of it,
let us pass to the higher types. The argument will be amply enforced by
the evidence obtained from them. We will look first at the conditions
which they have to fulfil; and then at the ways in which the functions
and structures adapting them to these conditions arise.
§ 278. A terrestrial plant that grows vertically needs no marked
modification of its internal tissues, so long as the height it reaches
is very small. As we before saw, the spiral or cylindrical rolling
up of a simple cellular frond, or the more bulky growth of a simple
cellular axis, may give the requisite strength; and the requisite
circulation may be carried on through the unchanged cellular tissue.
But in proportion as the height to be attained and the mass to be
supported increase, the supporting part must acquire greater bulk or
greater density, or both; and some modification that shall facilitate
the transfer of nutritive liquids must take place. Hence, in the
inner tissues of plants we may expect to find that structural changes
answering to these requirements become marked, as the growth of the
aërial part becomes great. Facts correspond with these expectations.
Among the humbler Cormophytes, which creep over or raise themselves
but little above, the surfaces they flourish upon, there is scarcely
any internal differentiation: the vascular and woody structures, if
not in all cases absolutely unrepresented, are rarely and very feebly
indicated. But among the higher types--the Ferns and Lycopodiums--which
raise their fronds to considerable heights, there are vascular bundles
and hard tissues like wood; and by the Tree-Ferns massive axes are
developed. That the relation which thus shows itself among Cryptogams
is habitual among Phænogams, scarcely needs saying.
Phænogams, however, are not universally thus characterized in a decided
way. Besides the comparative want of woody tissue in flowering plants
of humble growth, and besides the paucity of vessels in ordinary
water-plants, there are cases of much more marked divergence from
this typical internal structure. These exceptional cases occur under
exceptional conditions, and are highly instructive. They are of two
kinds. One group of them is furnished by certain plants which are
parasitic on the exposed roots of trees--parasitic not partially,
as the Mistletoe, but to the extent of subsisting wholly on the sap
they absorb. Fungus-like in colour and texture, and having scales for
leaves, these _Balanophoræ_ and _Rafflesiaceæ_ are recognizable as
Phænogams by scarcely any other traits than their fructifications.
Along with their aborted leaves and absence of chlorophyll, there is
a great degradation of those internal tissues by which Phænogams are
commonly distinguished. Though Dr. [now Sir J.] Hooker has shown that
they are not, as some botanists thought, devoid of spiral vessels; yet,
as shown by the mistake previously made in classifying them, their
appliances for circulation are rudimentary. And this trait goes along
with a greatly-simplified distribution of nutriment. In the absence
of leaves there can be but little down-current of sap, such as leaves
usually supply to roots: there cannot be much beyond an upward current
of the absorbed juices. The other cases occur where circulation is
arrested or checked in a different way; namely, in plants that are
wholly submerged. These are the _Podostemaceæ_. Clothing as they do
the submerged rocks, their roots play the part of rhizomes, being
attached to the substratum by hairs and other processes, and having
the leaf-bearing and flower-bearing shoots on their surfaces. The
latter spread out more or less horizontally and are also fixed to the
substratum in the same manner as the roots. Observe then the connexion
of facts. One of these _Podostemaceæ_ needs no internal stiffening
substance, for it exists in a medium of its own specific gravity; and
being in a position to absorb water over its entire surface, it has no
need for a circulation of crude sap--nor, indeed, in the absence of
evaporation from any part of its surface, could any active circulation
take place. Here, accordingly, the tracheal and mechanical elements are
undeveloped. Though spiral vessels are not entirely absent, yet they
are so rare as to do no more than verify the inference of phænogamic
relationship drawn from the flowers.
The method of agreement, the method of difference, and the method of
concomitant variations, thus unite in proving a direct relation between
the demand for support and circulation, and the existence of these
vascular woody bundles which the higher plants habitually possess.
The question which we have to consider is--Under what influences are
these structures, answering to these requirements, developed? How are
these internal differentiations caused? The inquiry may be conveniently
divided. Though the supporting tissues and the tissues concerned in the
circulation of liquids are closely connected, and indeed entangled,
with one another, we may fitly deal with them apart. Let us take first
the supporting tissue.
§ 279. Many commonplace facts indicate that the mechanical strains to
which upright-growing plants are exposed, themselves cause increase
of the dense deposits by which such plants are enabled to resist
such strains. There is the fact that the massiveness of a tree-trunk
varies according to the stress habitually put upon it. If the contrast
between the slender stem of a tree growing in a wood and the bulky stem
of a kindred tree growing in the fields, be ascribed to difference
of nutrition rather than difference of exposure to winds; there is
still the fact that a tree trained against a wall has a less bulky
stem than a tree of the same kind growing unsupported; and that
between the long weak branches of the one and the stiff ones of the
other there are decided contrasts. If it be objected that a tree so
trained and branches so borne have relatively less foliage, and that
therefore these unlikenesses also are due to unlikenesses of general
nutrition, which may in part be true; there are still such cases as
those of garden plants, which when held up by tying them to sticks
have weaker stems than when they are unpropped, and sink down if their
props are taken away. Again, there is the evidence supplied by roots.
Though the contrast between the feeble roots of a sheltered tree
and the strong roots of an exposed tree, may, like the contrast of
their stems, be mainly due to difference of nutrition, and therefore
supplies but doubtful evidence, we get tolerably clear evidence where
trees growing on inclined rocky surfaces, send into crevices that
afford little moisture or nutriment, roots which nevertheless become
thick where they are so directed as to bear great strains. Suspicion
thus raised is strengthened into conviction by special evidences
occurring in the places where they are to be expected. The Cactuses,
with their succulent growths that pass into woody growths slowly
and irregularly, give us the opportunity of tracing the conditions
under which the wood is formed. Good examples occur in the genus
_Cereus_, and especially in forms like _C. crenulatus_. Here,
from a massive vertically-growing rod of fleshy tissue, two inches
or more in diameter, there grow at intervals lateral rods similarly
bulky, which, quickly curving themselves, take vertical directions.
One of these heavy branches puts great strains on its own substance
and that of the stem at their point of junction; and here both of them
become brown and hard, while they continue green and succulent all
around. Such differentiations may be traced internally before they
are visible on the surface. If a joint of an _Opuntia_ be sliced
through longitudinally, the greater resistance to the knife all around
the narrow neck, indicates there a larger deposit of lignin than
elsewhere; and a section of the tissue placed under the microscope,
exhibits at the narrowest part a concentration of the woody and
vascular bundles. Clear evidence of another kind has been noted by
Mr. Darwin, in the organs of attachment of climbing plants. Speaking
of _Solanum jasminoides_ he says:--“When the flexible petiole
of a half-or a quarter-grown leaf has clasped any object, in three
or four days it increases much in thickness, and after several weeks
becomes wonderfully hard and rigid; so that I could hardly remove one
from its support. On comparing a thin transverse slice of this petiole
with one from the next or older leaf beneath, which had not clasped
anything, its diameter was found to be fully doubled, and its structure
greatly changed.... This clasped petiole had actually become thicker
than the stem close beneath; and this was chiefly due to the greater
thickness of the ring of wood, which presented, both in transverse
and longitudinal sections, a closely similar structure in the petiole
and axis. The assumption by a petiole of this structure is a singular
morphological fact; but it is a still more singular physiological fact
that so great a change should have been induced by the mere act of
clasping a support.”
If there is a direct relation between mechanical stress and the
formation of wood, it ought to explain for us the internal distribution
of the wood. Let us see whether it does this.
When seeking in mechanical actions and reactions the cause of that
indurated structure which forms the vertebrate axis (§§ 254–7), it
was pointed out that in a transversely-strained mass, the greatest
pressures and tensions are thrown on the molecules of the concave and
convex surfaces. Hence, supposing the transversely-strained mass to
be a cylinder, bent backwards and forwards not in one plane but now
in this plane and now in that, its peripheral layers will be those on
which the greatest stress falls. An ordinary dicotyledonous axis is
such a cylinder so strained. The maintenance of its attitude either as
a lateral shoot or a vertical shoot, implies subjection to the bendings
caused by its own weight and by the ever-varying wind. These bendings
imply tensions and pressures falling most severely first on one side of
its outer layers and then on another. And if the dense substance able
to resist these tensions and pressures is deposited most where they are
greatest, we ought to find it taking the shape of a cylindrical casing.
This is just what we do find. On cutting across a shoot in course of
formation, we see its central space either unoccupied or occupied only
by soft tissue. That the layer of hard tissue surrounding this is not
the outermost layer, is true: there lies beyond it the cambium layer,
from which it is formed, the phloëm, and the cortex. But outside of the
soft phloëm there is frequently another layer of dense tissue now known
as the pericyclic fibres, having frequently a tenacity greater even
than that of the wood--a layer which, while it protects the cambium
and offers additional resistance to the transverse strain, admits of
being fissured as fast as the cylinder of wood thickens. That is to
say, the deposit of resisting substance is as completely peripheral as
the exogenous mode of growth permits. So, too, in general arrangement
is it with the ordinary monocotyledonous stem. Different as is here
the internal structure, there yet holds the same general distribution
of tissues, answering to the same mechanical conditions. The vascular
woody bundles, more abundant towards the outside of the stem than
near the centre, produce a harder casing surrounding a softer core.
In the supporting structures of leaves we find significant deviations
from this arrangement. While axes are on the average exposed to equal
strains on all sides, most leaves, spreading out their surfaces
horizontally, have their petioles subject to strains that are not alike
in all directions; and in them the hard tissue is differently arranged.
Its transverse section is not ring-shaped but crescent-shaped: the two
horns being directed towards the upper surface of the petiole. That
this arrangement is one which answers to the mechanical conditions,
is not easy to demonstrate: we must satisfy ourselves by noting that
here, where the distribution of forces is different, the distribution
of resisting tissue is different. And then, showing conclusively the
connexion between these differences, we have the fact that in petioles
growing vertically and supporting peltate leaves--petioles which are
therefore subject to equal transverse strains on all sides--the
vascular bundles are arranged cylindrically, as in axes.
Such, then, are some of the reasons for concluding that the development
of the supporting tissue in plants, is caused by the incident forces
which this tissue has to resist. The individuals in which this direct
balancing of inner and outer actions progresses most favourably, are
those which, other things equal, are most likely to prosper; and, by
habitual survival of the fittest, there is established a systematic and
constant distribution of a deposit adapted to the circumstances of each
type.
§ 280. The function of circulation may now be dealt with. We have to
consider here by what structures this is discharged; and what connexion
exists between the demand for them and the genesis of them.
The contrast between the rates at which a dye passes through simple
cellular tissue and cellular tissue of which the units have been
elongated, indicates one of the structural changes required to
facilitate circulation. If placed with its cut surface in a coloured
liquid, the parenchyma of a potato or the medullary mass of a
cabbage-stalk, will absorb the liquid with extreme slowness; but if
the stalk of a fungus be similarly placed, the liquid runs up it, and
especially up its loose central substance, very quickly. On comparing
the tissues which thus behave so differently, we find that whereas in
the one case the component cells, packed close together, have deviated
from their primitive sphericity only as much as mutual pressure
necessitates, in the other case they are drawn out into long tubules
with narrow spaces among them--the greatest dimensions of the tubules
and the spaces being in the direction which the dye takes so rapidly.
That which we should infer, then, from the laws of capillary action,
is experimentally shown: liquid moving through tissues follows the
lines in which the elements of the tissues are most elongated. It does
this for two reasons. That narrowing of the cells and intercellular
spaces which accompanies their elongation, facilitates capillarity;
and at the same time fewer of the septa formed by the joined ends of
the cells have to be passed through in a given distance. Hence the
general fact that the establishment of a rudimentary vascular system,
is the formation of bundles of cells lengthened in the direction
which the liquid is to take. This we see very obviously among the
lower Cormophytes. In one of the lichen-like Liverworts, the veins
which, branching through its frond, serve as communications with its
scattered rootlets, are formed of cells longer than those composing the
general tissue of the frond: the lengths of these cells corresponding
in their directions with the lengths of the veins. So, too, is it
with the mid-ribs of such fronds as assume more definite shapes; and
so, too, is it with the creeping stems which unite many such fronds.
That is to say, the current which sets towards the growing part from
the part which supplies certain materials for growth, sets through
a portion of the tissues composed of units that are longer in the
line of the current than at right angles to that line. The like is
true of Phænogams. Omitting all other characteristics of those parts
of them through which chiefly the currents of sap flow, we find the
uniform fact to be that they consist of cells and intercellular spaces
distinguished from others by their lengths. It is thus with veins,
and mid-ribs, and petioles; and if we wish proof that it is thus with
stems, we have but to observe the course taken by a coloured solution
into which a stem is inserted.
What is the original cause of this differentiation? Is it possible
that this modification of cell-structure which favours the transfer of
liquid towards each place of demand, is itself caused by the current
which the demand sets up? Does the stream make its own channel? There
are various reasons for thinking that it does. In the first place,
the simplest and earliest channels, such as we see in the Liverworts,
do not develop in any systematic way, but branch out irregularly,
following everywhere the irregular lobes of the fronds as these spread;
and on examining under a magnifier the places at which the veins are
lost in the cellular tissue, it will be seen that the cells are there
slightly longer than those around: suggesting that the lengthening of
them which produces an extension of the veins, takes place as fast
as the growth of the tissue beyond causes a current to pass through
them. In the second place, a disappearance of the granular contents of
these cells accompanies their union into a vein--a result which the
transmission of a current may not improbably bring about. But be the
special causes of this differentiation what they may, the evidence
favours very much the conclusion that the general cause is the setting
up of a current towards a place where the sap is being consumed. In the
histological development of the higher plants we find confirmation.
The more finished distributing canals in Phænogams are formed of cells
previously lengthened. At parts of which the typical structure is
fixed, and the development direct, this fact is not easy to trace;
the cells rapidly take their elongated structures in anticipation of
their predetermined functions. But in places where new vessels are
required in adaptation to a modifying growth, we may clearly trace
this succession. The swelling root of a turnip, continually having its
vascular system further developed, and the component vessels lengthened
as well as multiplied, gives us an opportunity of watching the process.
In it we see that the reticulated cells which unite to form ducts,
arise in the midst of bundles of cells that have previously become
elongated, and that they arise by transformation of such elongated
cells; and we also see that these bundles of elongated cells have an
arrangement suggestive of their formation by passing currents.
Are there grounds for thinking that these further transformations by
which strings of elongated cells pass into vessels lined with spiral,
annular, reticulated, or other frameworks, are also in any way
determined by the currents of sap carried? There are some such grounds.
As just indicated, the only places where we may look for evidence
with any rational hope of finding it, are places where some local
requirement for vessels has arisen in consequence of some local
development which the type does not involve. In these cases we
find such evidence. Good illustrations occur in those genera of
the _Cactaceæ_, which simulate leaves, like _Epiphyllum_ and
_Phyllocactus_. A branch of one of these is outlined in Fig. 256. As
before explained this is a flattened axis; and the notches along its
edges are the seats of the axillary buds. Most of these axillary buds
are arrested; but occasionally one of them grows. Now if, taking an
_Epiphyllum_-shoot which bears a lateral shoot, we compare the parts
of it that are near the aborted axillary buds with the part that is
near the developed axillary bud, we find a conspicuous difference. In
the neighbourhood of an aborted axillary bud there is no external sign
of any internal differentiation; and on holding up the branch against
the light, the uniform translucency shows that there is no greater
amount of dense tissue near it than in other parts of the succulent
mass. But where an axillary bud has developed, a prominent rounded
ridge joins the mid-rib of the lateral branch with the mid-rib of the
parent branch. In the midst of this rounded ridge an opaque core may be
seen. And on cutting through it, this opaque core proves to be full of
vascular bundles imbedded in woody deposits. Clearly, these clusters
of vessels imply transformations of the tissues, caused by the passage
of increased currents of sap. The vessels were not there when the
axillary bud was formed; they would not have developed had the axillary
bud proved abortive; but they arise as fast as growth of the axillary
bud draws the sap along the lines in which they lie. Verification
is obtained by examining the internal structures. If longitudinal
sections be made through a growing bud of _Opuntia_ or _Cereus_, it
will be found that the vessels in course of formation converge towards
the point of growth, as they would do if the sap-currents determined
their formation; that they are most developed near their place of
convergence, which they would be if so produced; and that their
terminations in the tissue of the parent shoot are partially-formed
lines of irregular elongated cells, like those out of which the vessels
of a leaf or bud are developed.
Concluding, then, that sap-vessels arise along the lines of least
resistance, through which currents are drawn or forced, the question
to be asked is--What physical process produces them? Their component
cells, united end to end more or less irregularly in ways determined
by their original positions, form a channel much more permeable, both
longitudinally and laterally, than the tissue around. How is this
greater permeability caused? The idea, first propounded I believe by
Wolff, that the adjoined ends of the cells are perforated or destroyed
by the passing current, is one for which much is to be said. Whether
these septa are dissolved by the liquids they transmit, or whether they
are burst by those sudden gushes which, as we shall hereafter see, must
frequently take place along these canals, need not be discussed: it is
sufficient for us that the septa do, in many cases, disappear, leaving
internal ridges showing their positions; and, in other cases, become
extremely porous. Though it is manifest that this is not the process
of vascular development in tissues that unfold after pr-determined
types, since, in these, the dehiscences or perforations of septa occur
before such direct actions can have come into play; yet it is still
possible that the disappearances of septa which now arise by repetition
of the type were established in the type by such direct actions. Be
this as it may, however, a simultaneous change undergone by these
longitudinally-united cells must be otherwise caused. Frame-works are
formed in them--frameworks which, closely fitting their inner surfaces,
may consist either of successive rings, or continuous spiral threads,
or networks, or structures between spirals and networks, or networks
with openings so far diminished that the cells containing them are
distinguished as fenestrated. Their differences omitted, however, these
structures have the common character that, while supporting the coats
of the vessels, they also give special facilities for the passage of
liquids, both through the sides of the transformed cells and through
their united ends, where these are not destroyed.
To attempt any physical interpretation of this change is scarcely
safe: the conditions are so complex. There are reasons for suspecting,
however, that it arises from a vacuolation of the substance deposited
on the cell-wall. If rapidly deposited, as it is likely to be along
lines where sap is freely supplied, this may, in passing from the
state of a soluble colloid to that of an insoluble colloid, so
contract as to leave uncovered spaces on the cell-membrane; and this
change, originally consequent on a physico-chemical action, may be
so maintained and utilized by natural selection, as to result in
structures of definite kinds, regularly formed in growing parts in
anticipation of functions to be afterwards discharged. But, without
alleging any special cause for this metamorphosis, we may reasonably
conclude that it is in some way consequent upon the carrying of sap.
If we examine tissues such as that in the interior of a growing
turnip that has not yet become stringy, we may, in the first place,
find bundles of elongated cells not having yet developed in them
those fenestrated or reticulated structures by which the ducts are
eventually characterized. Along the centres of adjacent bundles we may
find incomplete lines of such cells--some that are partially or wholly
transformed, with some between them that are not transformed. In other
bundles, completed chains of such transformed cells are visible. And
then, in still older bundles, there are several complete chains running
side by side. All which facts imply a metamorphosis of the elongated
cells, indirectly caused by the continued action of the currents
carried.
§ 281. Here, however, presents itself a further problem. Taking
it as manifest that there is a typical distribution of supporting
tissue adapted to meet the mechanical strains a plant is exposed to
by its typical mode of growth, and also that there goes on special
adaptation of the supporting tissue to the special strains the
individual plant has to bear; and taking it as tolerably evident that
the sap-channels are originally determined by the passage of currents
along lines of least resistance; there still remains the ultimate
question--Through what physical actions are established these general
and special adjustments of supporting tissue to the strains borne,
and these distributions of nutritive liquid required to make possible
such adjustments? Clearly, if the external actions produce internal
reactions; and if this play of actions and reactions results in a
balancing of the strains by the resistances; we may rationally suspect
that the incident forces are directly conducive to the structural
changes by which they are met. Let us consider how they must work.
When any part of a plant is bent by the wind, the tissues on its convex
surface are subject to longitudinal tension, and these extended outer
layers compress the layers beneath them. Such of the vessels or canals
in these subjacent layers as contain sap, must have some of this sap
expelled. Part of it will be squeezed through the more or less porous
walls of the canals into the surrounding tissue, thus supplying it with
assimilable materials; while part of it, and probably the larger part,
will be thrust along the canals longitudinally upwards and downwards.
When the branch or twig or leaf-stalk recoils, these vessels, relieved
from pressure, expand to their original diameters. As they expand, the
sap rushes back into them from above and below. In whichever of these
directions least has been expelled by the compression, from that
direction most must return during the dilation; seeing that the force
which more efficiently resisted the thrusting back of the sap is the
same force which urges it into the expanded vessels again, when they
are relieved from pressure. At the next bend of the part a further
portion of sap will be squeezed out, and a further portion thrust
forwards along the vessels. This rude pumping process thus serves for
propelling the sap to heights which it could not reach by capillary
action, at the same time that it incidentally serves to feed the parts
in which it takes place. It strengthens them, too, just in proportion
to the stress to be borne; since the more severe and the more repeated
the strains, the greater must be the exudation of sap from the vessels
or ducts into the surrounding tissue, and the greater the thickening
of this tissue by secondary deposits. By this same action the movement
of the sap is determined either upwards or downwards, according to the
conditions. While the leaves are active and evaporation is going on
from them, these oscillations of the branches and petioles urge forward
the sap into them; because so long as the vessels of the leaves are
being emptied, the sap in the compressed vessels of the oscillating
parts will meet with less resistance in the direction of the leaves
than in the opposite direction. But when evaporation ceases at night,
this will no longer be the case. The sap drawn to the oscillating
parts, to supply the place of the exuded sap, must come from the
directions of least resistance. A slight breeze will bring it back from
the leaves into the gently-swaying twigs, a stronger breeze into the
bending branches, a gale into the strained stem and roots--roots in
which longitudinal tension produces, in another way, the same effects
that transverse tension does in the branches.
Two possible misinterpretations must be guarded against. It is not to
be supposed that this force-pump action causes movement of the sap
towards one point rather than another: it is simply an aid to its
movement. From the stock of sap distributed through the plant, more
or less is everywhere being abstracted--here by evaporation, here by
the unfolding of the parts into their typical shapes, here by both. The
result is a tension on the contained liquid columns, which is greatest
now in this direction and now in that. This tension it is which must be
regarded as the force that determines the current upwards or downwards;
and all which the mechanical actions do is to facilitate the transfer
to the places of greatest demand. Hence it happens that in a plant
prevented from oscillating, but having a typical tendency to assume
a certain height and bulk, the demands set up by its unfolding parts
will still cause currents; and there will still be alternate ascents
and descents, according as the varying conditions change the direction
of greatest demand--the only difference being that, in the absence of
oscillations, the growth will be less vigorous. Similarly, it must not
be supposed that mechanical actions are here alleged to be the sole
causes of wood formation in the individual plant. The tendency of the
individual plant to form wood at places where wood has been habitually
formed by ancestral plants, is manifestly a cause, and, indeed, the
chief cause. In this, as in all other cases, inherited structures
repeat themselves irrespective of the circumstances of the individual:
absence of the appropriate conditions resulting simply in imperfect
repetition of the structures. Hence the fact that in trained trees and
hothouse shrubs, dense substance is still largely deposited; though
not so largely as where the normal mechanical strains have acted.
Hence, too, the fact, that in such plants as the Elephant’s-foot or the
_Welwitschia mirabilis_, which for untold generations can have
undergone no oscillations, there is an extensive formation of wood
(though not to any considerable height above the ground), in repetition
of an ancestral type: natural selection having here maintained the
habit as securing some other advantage than that of support.
Still, it must be borne in mind that though intermittent mechanical
strains cannot be assigned as the direct causes of these internal
differentiations in plants that are artificially sheltered or
supported, they are assignable as the indirect causes; since the
inherited structures, repeated apart from such strains, are themselves
interpretable as accumulated results of such strains acting on
successive generations of ancestral plants. This will become clear on
combining the several threads of the argument and bringing it to a
close, which we may now do.
§ 282. To put the co-operative actions in their actual order,
would require us to consider them as working on individuals small
modifications that become conspicuous and definite only by inheritance
and gradual increase; but it will aid our comprehension without leading
us into error, if we suppose the whole process resumed in a single
continuously-existing plant.
As the plant erects the integrated series of fronds whose united
parts form its rudimentary axis, the increasing area of frond surface
exposed to the sun’s rays entails an increasing draught upon the
liquids contained in the rudimentary axis. The currents of sap so
produced, once established along certain lines of cells that offer
least resistance, render them by their continuous passage more and
more permeable. This establishment of channels is aided by the wind.
Each bend produced by it while yet the tissue is undifferentiated,
squeezes towards the place of growth and evaporation the liquids
that are passing by osmosis from cell to cell; and when the lines of
movement become defined, each bend helps, by forcing the liquid along
these lines, to remove obstructions and make continuous canals. As fast
as this transfer of sap is facilitated, so fast is the plant enabled
further to raise itself, and add to its assimilating surfaces; and so
fast do the transverse strains, becoming greater, give more efficient
aid. The canals thus formed can be neither in the centre of the
rudimentary axis nor at its surface: for at neither of these places
can the transverse strains produce any considerable compressions.
They must arise along a tract between the outside of the axis and its
core--a tract along which there occur the severest squeezes between the
stretched outer layers and the internal mass. Just that distribution
which we find, is the distribution which these mechanical actions tend
to establish.
As the plant gains in height, and as the mass of its foliage
accumulates, the strains thrown upon its axis, and especially the
lower part of its axis, rapidly increase. Supposing the forms to
remain similar, the strains must increase in the ratio of the cubes of
the dimensions; or even in a somewhat higher ratio. One consequence
must be that the compressions to which the vessels at the lower
part of the incipient stem are subject, become greater as fast as
the height to which the sap has to be raised becomes greater; and
another consequence must be that the local exudation of sap produced
by the pressure is proportionately augmented. Hence the materials for
interstitial nutrition being there supplied more abundantly, we may
expect thickening of the surrounding tissues to show itself there
first: in other words, wood will be formed round the vessels of the
lower part of the incipient stem. The resulting greater ability of this
lower part of the stem to bear strains, renders possible an increase of
height; and while after an increase of height the lowest part becomes
still further strained, and still further thickens, the part above it,
exposed to like actions, undergoes a like thickening. This induration,
while it spreads upwards, also spreads outwards. As fast as the rude
cylinder of dense matter formed in this way, begins to inclose the
original vessels, it begins to play the part of a resistant mass, which
more and more prevents the contained vessels from being squeezed;
while between it and the outer layers the greatest compression occurs
at each bend. Thus at the same time that the original vessels become
useless, the peripheral cells of the developing wood become those which
have their liquid contents squeezed out longitudinally and laterally
with increasing force; and, consequently, amid them are formed new
sap-channels, from which there is the most active local exudation,
producing the greatest deposit of dense matter.
Thus fusing together, as it were, the individualities of successive
generations of plants, and recognizing as all-important that
facilitation of the process which natural selection has all along
given, we are enabled to interpret the chief internal differentiations
of plants as consequent on an equilibration between inner and outer
forces. Here, indeed, we see illustrated in a way more than usually
easy to follow, the eventual balancing of outer actions by inner
reactions. The relation between the demand for liquid and the formation
of channels that supply liquid, as well as that between the incidence
of strains and the deposit of substance which resists strains, are
among the clearest special examples of the general truth that the
moving equilibrium of an organism, if not overthrown by an incident
force, must eventually be adjusted to it.
The processes here traced out are, of course, not to be taken as the
only differentiating processes to which the inner tissues of plants
have been subject. Besides the chief changes we have considered,
various less conspicuous changes have taken place. These must be
passed over as arising in ways too involved to admit of specific
interpretations; even supposing them to have been produced by causes of
the kind assigned. But the probability, or rather indeed the certainty,
is that some of them have not been so produced. Here, as in nearly
all other cases, indirect equilibration has worked in aid of direct
equilibration; and in many cases indirect equilibration has been the
sole agency. Besides ascribing to natural selection the rise of various
internal modifications of other classes than those above treated, we
must ascribe some even of these to natural selection. It is so with the
dense deposits which form thorns and the shells of nuts: these cannot
have resulted from any inner reactions immediately called forth by
outer actions; but must have resulted immediately through the effects
of such outer actions on the species. Let it be understood, therefore,
that the differentiations to which the foregoing interpretation
applies, are only those most conspicuous ones which are directly
related to the most conspicuous incident forces. They must be taken as
instances on the strength of which we may conclude that other internal
differentiations have had a natural genesis, though in ways that we
cannot trace.
CHAPTER V.
PHYSIOLOGICAL INTEGRATION IN PLANTS.
§ 283. A good deal has been implied on this topic in the preceding
chapters. Here, however, we must for a brief space turn our attention
immediately to it.
Plants do not display integration in such distinct and multiplied
ways as do animals. But its advance may be traced both directly and
indirectly--directly in the increasing co-ordination of actions, and
indirectly in the effect of this upon the powers and habits.
Let us group the facts under these heads: ascending in both cases from
the lower to the higher types.
§ 284. The inferior _Algæ_, along with little unlikeness of parts,
show us little mutual dependence of parts. Having surfaces similarly
circumstanced everywhere, much physiological division of labour
cannot arise; and therefore there cannot be much physiological unity.
Among the superior _Algæ_, however, the differentiation between the
attached part and the free part is accompanied by some integration.
There is evidently a certain transfer of materials, which is doubtless
facilitated by the elongated forms of the cells in the stem, and
probably leads to the formation of dense tissue at the places of
greatest strain, in a way akin to that recently explained in other
cases. And where there is this co-ordination of actions, the parts are
so far mutually dependent that each dies if detached from the other.
That though the organization is so low neither part can reproduce the
other and survive by so doing, is probably due to the circumstance
that neither part contains any considerable stock of untransformed
protoplasm, out of which new tissues may be produced.
Fungi and Lichens present no very significant advances of integration.
We will therefore pass at once to the Archegoniates. In those of them
which, either as single fronds or strings of fronds, spread over
surfaces, and which, rooting themselves as they spread, do not need
that each part should receive aid from remote parts, there is no
developed vascular system serving to facilitate transfer of nutriment:
the parts being little differentiated there is but little integration.
But along with assumption of the upright attitude and the accompanying
specializations, producing vessels for distributing sap and hard tissue
for giving mechanical support, there arises a decided physiological
division of labour; rendering the aërial part dependent on the embedded
part and the embedded part dependent on the aërial part. Here, indeed,
as elsewhere, these concomitant changes are but two aspects of the
same change. Always the gain of power to discharge a special function
involves a loss of power to perform other functions; and always,
therefore, increased mutual dependence constituting physiological
integration, must keep pace with that increased fitting of particular
parts to particular duties which constitutes physiological
differentiation.
Making a great advance among the Archegoniates, this physiological
integration reaches its climax among Phænogams. In them we see
interdependence throughout masses that are immense. Along with
specialized appliances for support and transfer, we find an exchange of
aid at great distances. We see roots giving the vast aërial growth a
hold tenacious enough to withstand violent winds, and supplying water
enough even during periods of drought; we see a stem and branches of
corresponding strength for upholding the assimilating organs under
ordinary and extraordinary strains; and in these assimilating organs
we see elaborate appliances for yielding to the stem and roots the
materials enabling them to fulfil their offices. As a consequence of
which greater integration accompanying the greater differentiation,
there is ability to maintain life over an immense period under marked
vicissitudes.
Even more conspicuously exemplified in Phænogams, is that physiological
integration which holds together the functions not of the individual
only but of the species as a whole. The organs of reproduction, both
in their relations to other parts of the individual bearing them and
in their relations to corresponding parts of other individuals, show
us a kind of integration conducing to the better preservation of the
race; as those already specified conduce to the better preservation
of the individual. In the first place, this greater co-ordination of
functions just described, itself enables Phænogams to bequeath to
the germs they cast off, stores of nutriment, protective envelopes,
and more or less of organization: so giving them greater chances of
rooting themselves. In the second place, certain differentiations
among the parts of fructification, the meaning of which Mr. Darwin
has so admirably explained, give to the individuals of the species a
kind of integration that makes possible a mutual aid in the production
of vigorous offspring. And it is interesting to observe how, in
that dimorphism by which in some cases this mutual aid is made more
efficient, the greater degree of integration is dependent on the
greater degree of differentiation--not simply differentiation of
the fructifying organs from other parts of the plant bearing them,
but differentiation of these fructifying organs from the homologous
organs of neighbouring individuals of the same race. Another form of
this co-ordination of functions which conduces to the maintenance of
the species, may be here named--partly for its intrinsic interest.
I refer to the strange processes of multiplication occurring in the
genus _Bryophyllum_. It is well known that the succulent leaves of
_B. calycinum_, borne on foot-stalks so brittle that they are easily
snapped by the wind, send forth from their edges when they fall to the
ground, buds which root themselves and grow into independent plants.
The correlation here obviously furthering the preservation of the race,
is more definitely established in another species of the genus--_B.
proliferum_. This plant, shooting up to a considerable height, and
having a stem containing but little woody fibre, habitually breaks
near the bottom while still in flower; and is thus generally prevented
from ripening its seeds. The multiplication is, however, secured in
another way. Before the stem is broken young plants have budded out
from the pedicels of the flowers, and have grown to considerable
lengths; and on the fall of the parent they forthwith commence their
separate lives. Here natural selection has established a remarkable
kind of co-ordination between a special habit of growth and decay, and
a special habit of proliferation.
§ 285. The advance of physiological integration among plants as we
ascend to the higher types, is implied by their greater constancy of
structure, as well as by the stricter limitations of their habitats and
modes of life. “Complexity of structure is generally accompanied with a
greater tendency to permanence in form,” says Dr. [now Sir J.] Hooker;
or, conversely, “the least complex are also the most variable.” This is
the second aspect under which we have to contemplate the facts.
The differences between the simpler _Algæ_ and _Fungi_ are so feebly
marked that botanists have had great difficulty in framing definitions
of these classes. This structural indefiniteness is accompanied by
functional indefiniteness. _Algæ_, which are mostly aquatic, include
many small forms that frequent the damp places preferred by _Fungi_.
Among _Fungi_, there are kinds which lead submerged lives like the
_Algæ_. Besides this indistinctness of the classes, there is great
variability in the shapes and modes of life of their species--a
variability so great that what were at first taken to be different
species, or different genera, or even different orders, have proved
to be merely varieties of one species. So inconstant in structure are
the _Algæ_ that Schleiden quotes with approval the opinion of Kutzing,
that “there are no species but merely forms of _Algæ_:” an opinion
which though now rejected sufficiently implies extreme indefiniteness.
In all which facts we see that these lowest types of plants, little
differentiated, are also but little integrated.
Archegoniates present a like relation between the small specialization
of functions which constitutes physiological differentiation, and
the small combination of functions which constitutes physiological
integration. “Mosses,” says Mr. Berkeley, “are no less variable than
other cryptogams, and are therefore frequently very difficult to
distinguish. Not only will the same species exhibit great diversity
in the size, mode of branching, form and nervation of the leaves, but
the characters of even the peristome itself are not constant.” And
concerning the classification of the remaining group, _Filicales_,
he says:--“Not only is there great difficulty in arranging ferns
satisfactorily, but it is even more difficult to determine the limits
of species.”
After this vagueness of separation as well as inconstancy of structure
and habit among the lower plants, the stability of structure and
habit and divisibility of groups among the higher plants, appear
relatively marked. Though Phænogams are much more variable than most
botanists have until lately allowed, yet the definitions of species and
genera may be made with far greater precision, and the forms are far
less capable of change, than among Cryptogams. And this comparative
fixity of type, implying, as it does, a closer combination of the
component functions, we see to be the accompaniment of the greater
differentiation of those functions and of the structures performing
them. That these characters are correlatives is further shown by the
fact that the higher plants are more restricted in their habitats
than the lower plants, both in space and time. “The much narrower
delimitation in area of animals than plants,” says Sir J. Hooker, “and
greater restriction of Faunas than Floras, should lead us to anticipate
that plant-types are, geologically speaking, more ancient and permanent
than the higher animal types are, and so I believe them to be, and I
would extend the doctrine even to plants of highly complex structure.”
“Those classes and orders which are the least complex in organization
are the most widely distributed.”
§ 286. Thus that which the general doctrine of evolution leads us
to anticipate, we find implied by the facts. The physiological
division of labour among parts, can go on only in proportion to the
mutual dependence of parts; and the mutual dependence of parts can
progress only as fast as there arise structures by which the parts are
efficiently combined, and the mutual utilization of their actions made
easy.
To say definitely by what process is brought about this co-ordination
of functions which accompanies their specialization, is hardly
practicable. Direct and indirect equilibration doubtless co-operate
in establishing it. We may see, for example, that every increase of
fitness for function produced in the aërial part of a plant by light,
as well as every increase of fitness for function produced in its
imbedded part by the direct action of the moist earth, must conduce
to an increased current of the liquid evaporated from the one and
supplied by the other--must serve, therefore, to aid the formation
of sap-channels in the ways already described; that is--must serve
to develop the structures through which mutual aid of the parts is
given: the additional differentiation tends immediately to bring
about the additional integration. Contrariwise, it is obvious that
an interdependence such as we see between the secretion of honey and
the fertilization of germs, or between the deposit of albumen in the
cotyledons of an embryo-plant and its subsequent striking root, is
a kind of integration in the actions of the individual or of the
species, which no differentiation has a direct tendency to initiate.
Hence we must regard the total results as due to a plexus of influences
acting simultaneously on the individual and on the species: some
chiefly affecting the one and some chiefly affecting the other.
* * * * *
[NOTE.--In _Nature_ for June 11, 1896, Dr. Maxwell Masters, in an
essay on “Plant Breeding,” names an instructive fact concerning the
production of varieties by selection of slightly divergent forms. He
says:--
“To the untrained eye, the primordial differences noted are often very
slight; even the botanist, unless his attention be specially directed
to the matter, fails to see minute differences which are perceptible
enough to the raiser or his workmen. Nor must it be thought that these
variations, difficult as they are to recognise in the beginning, are
unimportant. On the contrary, they are interesting, physiologically,
as the potential origin of new species, and very often they are
commercially valuable also. These apparently trifling morphological
differences are often associated with physiological variations which
render some varieties, say of wheat, much better enabled to resist
mildew and disease generally than others. Some, again, prove to be
better adapted for certain soils or for some climates than others; some
are less liable to injury from predatory birds than others, and so on.”
Thus we are shown that, to a much greater degree than might be
supposed, minute changes of forms and functions in one part of a plant
are correlated with changes of forms and functions throughout it. The
interdependence--that is to say, the physiological integration--is very
close at the same time that it is very complex.
Here while naming these facts in illustration of physiological
integration in plants I name them because they illustrate an important
truth bearing upon the general question of heredity which I have dealt
with in Appendix G, and to which I now especially draw attention.]
CHAPTER VI.
DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF ANIMALS.
§ 287. What was said respecting the primary physiological
differentiation in plants, applies with little beyond change of terms
to animals. Among _Protozoa_, as among _Protophyta_, the first definite
contrast of parts is that between outside and inside. The speck of
jelly or sarcode which appears to constitute the simplest animal,
proves, on closer examination, to be a mass of substance containing a
nucleus--a periplast in the midst of which there is a minute endoplast,
consisting of a spherical membrane and its contents.