CHAPTER III.

THE MORPHOLOGICAL COMPOSITION OF PLANTS,

_CONTINUED_.

§ 187. That advanced composition arrived at in the _Archegoniatæ_, is

carried still further in the Flowering Plants. In these most-elevated

vegetal forms, aggregation of the third order is always distinctly

displayed; and aggregates of the fourth, fifth, sixth, &c., orders are

very common.

Our inquiry into the morphology of these flowering plants, may be

advantageously commenced by studying the development of simple leaves

into compound leaves. It is easy to trace the transition, as well as

the conditions under which it occurs; and tracing it will prepare us

for understanding how, and when, metamorphoses still greater in degree

take place.

§ 188. If we examine a branch of the common bramble, when in flower

or afterwards, we shall not unfrequently find a simple or undivided

leaf, at the insertion of one of the lateral flower-bearing axes,

composing the terminal cluster of flowers. Sometimes this leaf is

partially lobed; sometimes cleft into three small leaflets. Lower down

on the shoot, if it be a lateral one, occur larger leaves, composed

of three leaflets; and in some of these, two of the leaflets may be

lobed more or less deeply. On the main stem the leaves, usually still

larger, will be found to have five leaflets. Supposing the plant to be

a well-grown one, it will furnish all gradations between the simple,

very small leaf, and the large composite leaf, containing sometimes

even seven leaflets. Figs. 50 to 64, represent leading stages of the

transition. What determines this transition? Observation shows that

the quintuple leaves occur where the materials for growth are supplied

in greatest abundance; that the leaves become less and less compound,

in proportion to their remoteness from the main currents of sap; and

that where an entire absence of divisions or lobes is observed, it is

on leaves within the flower-bunch: at the place, that is, where the

forces which cause growth are nearly equilibrated by the forces which

oppose growth; and where, as a consequence, gamogenesis is about to

be set in (§ 78). Additional evidence that the degree of nutrition

determines the degree of composition of the leaf, is furnished by

the relative sizes of the leaves. Not only, on the average, is the

quintuple leaf much larger in its total area than the triple leaf;

but the component leaflets of the one, are usually much larger than

those of the other. The like contrasts are still more marked between

triple leaves and simple leaves. This connection of decreasing size

with decreasing composition, is conspicuous in the series of figures:

the differences shown being not nearly so great as may be frequently

observed. Confirmation may be drawn from the fact that when the leading

shoot is broken or arrested in its growth, the shoots it gives off

(provided they are given off after the injury), and into which its

checked currents of sap are thrown, produce leaves of five leaflets

where ordinarily leaves of three leaflets occur. Of course incidental

circumstances, as variations in the amounts of sunshine, or of rain,

or of matter supplied to the roots, are ever producing changes in the

state of the plant as a whole; and by thus affecting the nutrition of

its leaf-buds at the times of their formation, cause irregularities

in the relations of size and composition above described. But taking

these causes into account, it is abundantly manifest that a leaf-bud of

the bramble will develop into a simple leaf or into a leaf compounded

in different degrees, according to the quantity of assimilable matter

brought to it at the time when the rudiments of its structure are

being fixed. And on studying the habits of other plants--on observing

how annuals that have compound leaves usually bear simple leaves at

the outset, when the assimilating surface is but small; and how, when

compound-leaved plants in full growth bear simple leaves in the midst

of compound ones, the relative smallness of such simple leaves shows

that the buds from which they arose were ill-supplied with sap; it

will cease to be doubted that a foliar organ may be metamorphosed into

a group of foliar organs, if furnished, at the right time, with a

quantity of matter greater than can be readily organized round a single

centre of growth. An examination of the transitions through which

a compound leaf passes into a doubly-compound leaf, as seen in the

various intermediate forms of leaflets in Fig. 65, will further enforce

this conclusion.

[Illustration: Figs. 50–64.]

[Illustration: Fig. 65.]

Here we may advantageously note, too, how in such cases the leaf-stalk

undergoes concomitant changes of structure. In the bramble-leaves above

described, it becomes compound simultaneously with the leaf--the veins

become mid-ribs while the mid-ribs become petioles. Moreover, the

secondary stalks, and still more the main stalks, bear thorns similar

in their shapes, and approaching in their sizes, to those on the stem;

besides simulating the stem in colour and texture. In the petioles of

large compound leaves, like those of the common _Heracleum_, we see

still more distinctly both internal and external approximations in

character to axes. Nor are there wanting plants whose large, though

simple, leaves, are held out far from the stems by foot-stalks that

are, near the ends, sometimes so like axes that the transverse sections

of the two are indistinguishable; as instance the _Calla palustris_.

One other fact respecting the modifications which leaves undergo,

should be set down. Not only may leaf-stalks assume to a great degree

the characters of stems, when they have to discharge the functions

of stems, by supporting many leaves or very large leaves; but they

may assume the characters of leaves, when they have to undertake the

functions of leaves. The Australian Acacias furnish a remarkable

illustration of this. Acacias elsewhere found bear pinnate leaves;

but the majority of those found in Australia bear what appear to be

simple leaves. It turns out, however, that these are merely leaf-stalks

flattened out into foliar shapes: the laminæ of the leaves being

undeveloped. And the proof is that in young plants, showing their

kinships by their embryonic characters, these leaf-like petioles bear

true leaflets at their ends. A metamorphosis of like kind occurs in

_Oxalis bupleurifolia_, Fig. 66. The fact most deserving of notice,

however, is that these leaf-stalks, in usurping the general aspects and

functions of leaf-blades, have, to some also usurped their structures:

though their venation is not like that of the leaf-blades they replace,

yet they have veins, and in some cases mid-ribs.

[Illustration: Fig. 66.]

Reduced to their most general expression, the truths above shadowed

forth are these:--That group of morphological units, or cells, which

we see integrated into the compound unit called a leaf, has, in

each higher plant, a typical form, due to the special arrangement

of these cells around a mid-rib and veins. If the multiplication of

morphological units, at the time when the leaf-bud is taking on its

main outlines, exceeds a certain limit, these units begin to arrange

themselves round secondary centres, or lines of growth, in such ways as

to repeat, in part or wholly, the typical form: the larger veins become

transformed into imperfect mid-ribs of partially independent leaves;

or into complete mid-ribs of quite separate leaves. And as there goes

on this transition from a single aggregate of cells to a group of

such aggregates, there simultaneously arises, by similarly insensible

steps, a distinct structure which supports the several aggregates thus

produced, and unites them into a compound aggregate. These phenomena

should be carefully studied; since they give us a key to more involved

phenomena.[6]

§ 189. Thus far we have dealt with leaves ordinarily so-called: briefly

indicating the homologies between the parts of the simple and the

compound. Let us now turn to the homologies among foliar organs in

general. These have been made familiar to readers of natural history

by popularized outlines of _The Metamorphosis of Plants_--a title, by

the way, which is far too extensive; since the phenomena treated of

under it, form but a small portion of those it properly includes.

Passing over certain vague anticipations which have been quoted from

ancient writers, and noting only that some clearer recognitions

were reached by Joachim Jung, a Hamburg professor, in the middle

of the 17th century; we come to the _Theoria Generationis_, which

Wolff published in 1759, and in which he gives definite forms to the

conceptions that have since become current. Specifying the views of

Wolff, Dr. Masters writes:--“After speaking of the homologous nature

of the leaves, the sepals and petals, an homology consequent on their

similarity of structure and identity of origin, he goes on to state

that the ‘pericarp is manifestly composed of several leaves, as in the

calyx, with this difference only, that the leaves which are merely

placed in close contact in the calyx, are here united together’; a

view which he corroborates by referring to the manner in which many

capsules open and separate ‘into their leaves.’ The seeds, too, he

looks upon as consisting of leaves in close combination. His reasons

for considering the petals and stamens as homologous with leaves, are

based upon the same facts as those which led Linnæus, and, many years

afterwards, Goethe, to the same conclusion. ‘In a word,’ says Wolff,

‘we see nothing in the whole plant, whose parts at first sight differ

so remarkably from each other, but leaves and stem, to which latter

the root is referrible.’” It appears that Wolff, too, enunciated the

now-accepted interpretation of compound fruits: basing it on the same

evidence as that since assigned. In the essay of Goethe, published

thirty years after, these relations among the parts of flowering plants

were traced out in greater detail, but not in so radical a way; for

Goethe did not, as did Wolff, verify his hypothesis by dissecting buds

in their early stages of development. Goethe appears to have arrived

at his conclusions independently. But that they were original with

him, and that he gave a more variously-illustrated exposition of them

than had been given by Wolff, does not entitle him to anything beyond

a secondary place, among those who have established this important

generalization.

Were it not that these pages may be read by some to whom Biology, in

all its divisions, is a new subject of study, it would be needless to

name the evidence on which this now-familiar generalization rests. For

the information of such it will suffice to say, that the fundamental

kinship existing among all the foliar organs of a flowering plant, is

shown by the transitional forms which may be traced between them, and

by the occasional assumption of one another’s forms. “Floral leaves, or

bracts, are frequently only to be distinguished from ordinary leaves

by their position at the base of the flower; at other times the bracts

gradually assume more and more of the appearance of the sepals.” The

sepals, or divisions of the calyx, are not unlike undeveloped leaves:

sometimes assuming quite the structure of leaves. In other cases, they

acquire partially or wholly the colours of the petals--as, indeed, the

bracts and uppermost stem-leaves occasionally do. Similarly, the petals

show their alliances to the foliar organs lower down on the axis, and

to those higher up on the axis. On the one hand, they may develop into

ordinary leaves that are green and veined; and, on the other hand, as

so commonly seen in double flowers, they may bear anthers on their

edges. All varieties of gradation into neighbouring foliar organs may

be witnessed in stamens. Flattened and tinted in various degrees, they

pass insensibly into petals, and through them prove their homology with

leaves; into which, indeed, they are transformed in flowers that become

wholly foliaceous. The style, too, is occasionally changed into petals

or into green leaflets; and even the ovules are now and then seen to

take on leaf-like forms. Thus we have clear evidence that in Phænogams,

all the appendages of the axis are homologues: they are all modified

leaves.

Wolff established, and Goethe further illustrated, another general law

of structure in flowering plants. Each leaf commonly contains in its

axil a bud, similar in structure to the terminal bud. This axillary bud

may remain undeveloped; or it may develop into a lateral shoot like

the main shoot; or it may develop into a flower. If a shoot bearing

lateral flowers be examined, it will be found that the internode, or

space which separates each leaf with its axillary flower from the leaf

and axillary flower above it, becomes gradually less towards the upper

end of the shoot. In some plants, as in the fox-glove, the internodes

constitute a regularly-diminishing series. In other plants, the series

they form suddenly begins to diminish so rapidly, as to bring the

flowers into a short spike: instance the common orchis. And again, by

still more sudden dwarfing of the internodes, the flowers are brought

into a cluster; as they are in the cowslip. On contemplating a clover

flower, in which this clustering has been carried so far as to produce

a compact head; and on considering what must happen if, by a further

arrest of axial development, the foot-stalks of the florets disappear;

it will be seen that there must result a crowd of flowers, seated

close together on the end of the axis. And if, at the same time, the

internodes of the upper stem-leaves also remain undeveloped, these

stem-leaves will be grouped into a common involucre: we shall have

a composite flower, such as the thistle. Hence, to modifications in

the developments of foliar organs, have to be added modifications in

the developments of axial organs. Comparisons disclose the gradations

through which axes, like their appendages, pass into all varieties

of size, proportion, and structure. And we learn that the occurrence

of these two kinds of metamorphosis, in all conceivable degrees

and combinations, furnishes us with a proximate interpretation of

morphological composition in Phænogams.

I say a proximate interpretation, because there remain to be solved

certain deeper problems; one of which at once presents itself to be

dealt with under the present head. Leaves, petals, stamens, &c., being

shown to be homologous foliar organs; and the part to which they are

attached, proving to be an indefinitely-extended axis of growth, or

axial organ; we are met by the questions,--What is a foliar organ? and

What is an axial organ? The morphological composition of a Phænogam is

undetermined, so long as we cannot say to what lower structures leaves

and shoots are homologous; and how this integration of them originates.

To these questions let us now address ourselves.

§ 190–1. Already, in § 78, reference has been made to the occasional

development of foliar organs into axial organs: the special case there

described being that of a fox-glove, in which some of the sepals were

replaced by flower-buds. The observation of these and some analogous

monstrosities, raising the suspicion that the distinction between

foliar organs and axial organs is not absolute, led me to examine into

the matter; and the result has been the deepening of this suspicion

into a conviction. Part of the evidence is given in Appendix A.

Some time after having reached this conviction, I found on looking

into the literature of the subject, that analogous irregularities had

suggested to other observers, beliefs similarly at variance with the

current morphological creed. Difficulties in satisfactorily defining

these two elements, have served to shake this creed in some minds.

To others, the strange leaf-like developments which axes undergo in

certain plants, have afforded reasons for doubting the constancy of

this distinction which vegetal morphologists usually draw. And those

not otherwise rendered sceptical, have been made to hesitate by such

cases as that of the Nepaul-barley, in which the glume, a foliar

organ, becomes developed into an axis and bears flowers. In his

essay--“Vegetable Morphology: its History and Present Condition,”[7]

whence I have already quoted, Dr. Masters indicates sundry of the

grounds for thinking that there is no impassable demarcation between

leaf and stem. Among other difficulties which meet us if we assume that

the distinction is absolute, one is implied by this question:--“What

shall we say to cases such as those afforded by the leaves of _Guarea_

and _Trichilia_, where the leaves after a time assume the condition

of branches and develop young leaflets from their free extremities,

a process less perfectly seen in some of the pinnate-leaved kinds of

_Berberis_ or _Mahonia_, to be found in almost every shrubbery?”

A class of facts on which it will be desirable for us here to dwell

a moment, before proceeding to deal with the matter deductively, is

presented by the _Cactaceæ_. In this remarkable group of plants,

deviating in such varied ways from the ordinary phænogamic type, we

find many highly instructive modifications of form and structure. By

contemplating the changes here displayed within the limits of a single

order, we shall greatly widen our conception of the possibilities of

metamorphosis in the vegetal kingdom, taken as a whole. Two different,

but similarly-significant, truths are illustrated. First, we are shown

how, of these two components of a flowering plant, commonly regarded

as primordially distinguished, one may assume, throughout numerous

species, the functions, and to a great degree the appearance, of the

other. Second, we are shown how, in the same individual, there may

occur a re-metamorphosis: the usurped function and appearance being

maintained in one part of the plant, while in another part there is a

return to the ordinary appearance and function. We will consider these

two truths separately. Some of the _Euphorbiaceæ_, which simulate

Cactuses, show us the stages through which such abnormal structures are

arrived at. In _Euphorbia splendens_, the lateral axes are considerably

swollen at their distal ends, so as often to be club-shaped: still,

however, being covered with bark of the ordinary colour, and still

bearing leaves. But in kindred plants, as _Euphorbia neriifolia_,

this swelling of the lateral axes is carried to a far greater extent;

and, at the same time, a green colour and a fleshy consistence have

been acquired: the typical relations nevertheless being still shown

by the few leaves that grow out of these soft and swollen axes. In

the _Cactaceæ_, which are thus resembled by plants not otherwise

allied to them, we have indications of a parallel transformation.

Some kinds, not commonly brought to England, bear leaves; but in the

species most familiar to us, the leaves are undeveloped and the axes

assume their functions. Passing over the many varieties of form and

combination which these green succulent growths display, we have to

note that in some genera, as in _Phyllocactus_, they become flattened

out into foliaceous shapes, having mid-ribs and something approaching

to veins. So that here, and in the genus _Epiphyllum_, which has this

character still more marked, the plant appears to be composed of fleshy

leaves growing one upon another. And then, in _Rhipsalis_, the same

parts are so leaf-like, that an uncritical observer would regard them

as leaves. These which are axial organs in their homologies, have

become foliar organs in their analogies. When, instead of comparing

these strangely-modified axes in different genera of Cactuses, we

compare them in the same individual, we meet with transformations no

less striking. Where a tree-like form is produced by the growth of

these foliaceous shoots, one on another; and where, as a consequence,

the first-formed of them become the main stem that acts as support

to secondary and tertiary stems; they lose their green, succulent

character, acquire bark, and become woody. In resuming the functions

of axes they resume the structures of axes, from which they had

deviated. In Fig. 71 are shown some of the leaf-like axes of _Rhipsalis

rhombea_ in their young state; while Fig. 72 represents the oldest

portion of the same plant, in which the foliaceous characters are

quite obliterated, and there has resulted an ordinary stem-structure.

One further fact is to be noted. At the same time that their

leaf-like appearances are lost, the axes also lose their separate

individualities. As they become stem-like, they also become integrated;

and they do this so effectually that their original points of junction,

at first so strongly marked, are effaced, and a consolidated trunk is

produced.

[Illustration: Figs. 71–72.]

Joined with the facts previously specified, these facts help us to

conceive how, in the evolution of flowering plants in general, the

morphological components that were once distinct, may become extremely

disguised. We may rationally expect that during so long a course of

modification, much greater changes of form, and much more decided

fusions of parts, have taken place. Seeing how, in an individual

plant, the single leaves pass into compound leaves, by the development

of their veins into mid-ribs while their petioles begin to simulate

axes; and seeing that leaves ordinarily exhibiting definitely-limited

developments, occasionally produce other leaves from their edges; we

are led to suspect the possibility of still greater changes in foliar

organs. When, further, we find that within the limits of one natural

order, petioles usurp the functions and appearances of leaves, at

the same time that in other orders, as in _Ruscus_, lateral axes so

simulate leaves that their axial nature would by most not be suspected,

did they not bear flowers on their mid-ribs or edges; and when, among

Cactuses, we perceive that such metamorphoses and re-metamorphoses

take place with great facility; our suspicion that the morphological

elements of Phænogams admit of profound transformations, is deepened.

And then, on discovering how frequent are the monstrosities which do

not seem satisfactorily explicable without admitting the development

of foliar organs into axial organs; we become ready to entertain

the hypothesis that during the evolution of the phænogamic type, the

distinction between leaves and axes has arisen by degrees.

With our preconceptions loosened by such facts, and carrying with us

the general idea which such facts suggest, let us now consider in what

way the typical structure of a flowering plant may be interpreted.

§ 192. To proceed methodically, we must seek a clue to the structures

of Phanerogams, in the structures of those inferior plants that

approach to them--_Archegoniatæ_. The various divisions of this

class present, along with sundry characters which ally them with

Thallophytes, other characters by which the phænogamic structure is

shadowed forth. While some of the inferior _Hepaticæ_ or Liverworts,

severally consist of little more than a thallus-like frond, among

the higher members of this group, and still more among the Mosses

and Ferns, we find a distinctly marked stem.[8] Some Archegoniates

(or rather Rhizoids) have foliar expansions that are indefinite in

their forms; and some have quite definitely-shaped leaves. Roots are

possessed by all the more-developed genera of the class; but there are

other genera, as _Sphagnum_, which have no roots. Here the fronds are

formed of only a single layer of cells; and there a double layer gives

them a higher character--a difference exhibited between closely-allied

genera of one group, the Mosses. Equally varied are the developments

of the foliar organs in their detailed structures: now being without

mid-ribs or veins; now having mid-ribs but no veins; now having

both mid-ribs and veins. Nor must we omit the similarly-significant

circumstance, that whereas in the lower Archegoniates the reproductive

elements are immersed here and there in the thallus-like frond, they

are, in the higher orders, seated in well-specialized and quite

distinct fructifying organs, having analogies with the flowers of

Phænogams. Thus, many facts imply that if the Phænogamic type is to

be analyzed at all, we must look among the Archegoniates for its

morphological components, and the manner of their integration.

Already we have seen among the lower _Cryptogamia_, how, as they became

integrated and definitely limited, aggregates acquire the habit of

budding out other aggregates, on reaching certain stages of growth.

Cells produce other cells endogenously or exogenously; and fronds

give origin to other fronds from their edges or surfaces. We have

seen, too, that the new aggregates so produced, whether of the first

order or the second order, may either separate or remain connected.

Fissiparously-multiplying cells in some cases part company, while in

other cases they unite into threads or laminæ or masses; and fronds

originating proliferously from other fronds, sometimes when mature

disconnect themselves from their parents, and sometimes continue

attached to them. Whether they do or do not part, is clearly determined

by their nutrition. If the conditions are such that they can severally

thrive better by separating after a certain development is reached,

it will become their habit then to separate; since natural selection

will favour the propagation of those which separate most nearly at

that time. If, conversely, it profits the species for the cells or

fronds to continue longer attached, which it can only do if their

growths and subsequent powers of multiplication are thereby increased,

it must happen, through the continual survival of the fittest, that

longer attachment will become an established characteristic; and,

by persistence in this process, permanent attachment will result

when permanent attachment is advantageous. That disunion is really

a consequence of relative innutrition, and union a consequence of

relative nutrition, is clear _à posteriori_. On the one hand, the

separation of the new individuals, whether in germs or as developed

aggregates, is a dissolving away of the connecting substance; and

this implies that the connecting substance has ceased to perform its

function as a channel of nutriment. On the other hand, where, as

we see among Phænogams, there is about to take place a separation

of new individuals in the shape of germs, at the point where the

nutrition is the lowest, a sudden increase of nutrition will cause the

impending separation to be arrested; and the fructifying elements,

reverting towards the ordinary form, thereupon develop in connexion

with the parent. Turning to the Archegoniates, we find among them

many indications of this transition from discontinuous development

to continuous development. Thus the Liverworts give origin to new

plants by cells which they throw off from their surfaces; as, indeed,

we have seen that much higher plants do. “According to Bischoff,”

says Schleiden, “both the cells of the stem (_Jungermannia_ [now

_Lophocolea_] _bidentata_) and those of the leaves (_J. exsecta_)

separate themselves as propagative cells from the plant, and isolated

cells shoot out and develop while still connected with the parent

plant into small cellular bodies (_Metzgeria furcata_), which separate

from the plant, and grow into new plants, as in _Mnium androgynum_

among the Mosses.” Now in the way above explained, these propagative

cells and proliferous buds, may continue developing in connexion with

the parent to various degrees before separating; or the buds which

are about to become fructifying organs may similarly, under increased

nutrition, develop into young fronds. As Sir W. Hooker says of the male

fructification in _Metzgeria furcata_,--“It has the appearance of being

a young shoot or innovation (for in colour and texture I can perceive

no difference) rolled up into a spherical figure.” On finding in this

same plant, that sometimes the proliferously-produced frond buds out

from itself another frond before separating from the parent, as shown

in Fig. 46, it becomes clear that this long-continued connexion may

readily pass into permanent connexion. And when we see how, even among

Phænogams, buds may either detach themselves as bulbils, or remain

attached and become shoots; we can scarcely doubt that among inferior

plants, less definite in their modes of organization, such transitions

must continually occur.

[Illustration: Figs. 73–76.]

Let us suppose, then, that Fig. 73 is the frond of some primitive

Archegoniate, similar in general characters to _Pellia epiphylla_,

Fig. 43; bearing, like it, the fructifying buds on its upper surface,

and having a slightly-marked mid-rib and rootlets. And suppose

that, as shown, a secondary frond is proliferously produced from

the mid-rib, and continues attached to it. Evidently the ordinary

discontinuous development, can thus become a continuous development,

only on condition that there is an adequate supply, to the secondary

frond, of such materials as are furnished by the rootlets: the

remaining materials being obtainable by itself from the air. Hence,

that portion of the mid-rib lying between the secondary frond and

the chief rootlets, having its function increased, will increase in

bulk. An additional consequence will be a greater concentration of

the rootlets--there will be extra growth of those which are most

serviceably placed. Observe, next, that the structure so arising

is likely to be maintained. Such a variation implying, as it does,

circumstances especially favourable to the growth of the plant, will

give to the plant extra chances of leaving descendants; since the area

of frond supported by a given area of the soil, being greater than

in other individuals, there may be a greater production of spores.

And then, among the more numerous descendants thus secured by it, the

variation will give advantages to those in which it recurs. Such a

mode of growth having, in this manner, become established, let us ask

what is next likely to result. If it becomes the habit of the primary

frond to bear a secondary frond from its mid-rib, this secondary frond,

composed of physiological units of the same kind, will inherit the

habit; and supposing that the supply of mineral matters obtained by

the rootlets suffices for the full development of the secondary frond,

there is a likelihood that the growth from it of a tertiary frond,

will become an habitual characteristic of the variety. Along with the

establishment of such a tertiary frond, as shown in Fig. 74, there

must arise a further development of mid-rib in the primary frond, as

well as in the secondary frond--a development which must bring with it

a greater integration of the two; while, simultaneously, extra growth

will take place in such of the rootlets as are most directly connected

with this main channel of circulation. Without further explanation it

will be seen, on inspecting Figs. 75 and 76, that there may in this

manner result an integrated series of fronds, placed alternately on

opposite sides of a connecting vascular structure. That this connecting

vascular structure will, as shown in the figures, become more distinct

from the foliar surfaces as these multiply, is no unwarranted

assumption; for we have seen in compound-leaved plants, how, under

analogous conditions, mid-ribs become developed into separate

supporting parts, which acquire some of the characters of axes while

assuming their functions. And now mark how clearly the structure thus

built up by integration of proliferously-growing fronds, corresponds

with the structure of the more-developed _Jungermanniaceæ_. Each of

the fronds successively produced, repeating the characters of its

parent, will bear roots; and will bear them in homologous places, as

shown. Further, the united mid-ribs having but very little rigidity,

will be unable to maintain an erect position. Hence there will result

the recumbent, continuously-rooted stem, which these types exhibit: an

embryo phænogam having the weakness of an embryo.[9]

A natural concomitant of the mode of growth here described, is that

the stem, while it increases longitudinally, increases scarcely at

all transversely: hence the old name Acrogens. Clearly the transverse

development of a stem is the correlative, partly of its function as

a channel of circulation, and partly of its function as a mechanical

support. That an axis may lift its attached leaves into the air,

implies thickness and solidity proportionate to the mass of such

leaves; and an increase of its sap-vessels, also proportionate to the

mass of such leaves, is necessitated when the roots are all at one end

and the leaves at the other. But in the generality of Acrogens, these

conditions, under which arises the necessity for transverse growth

of the axis, are absent wholly or in great part. The stem habitually

creeps below the surface, or lies prone upon the surface; and where

it grows in a vertical or inclined direction, does this by attaching

itself to a vertical or inclined object. Moreover, throwing out

rootlets, as it mostly does, at intervals throughout its length, it

is not called upon in any considerable degree, to transfer nutritive

materials from one of its ends to the other. Hence this peculiarity

which gives their name to the Acrogens, now called Archegoniates, is a

natural accompaniment of the low degree of specialization reached in

them. And that it is an incidental and not a necessary peculiarity, is

demonstrated by two converse facts. On the one hand, in those higher

Acrogens which, like the tree-ferns, lift large masses of foliage into

the air, there is just as decided a transverse expansion of the axis

as in dicotyledonous trees. On the other hand, in those Dicotyledons

which, like the common Dodder, gain support and nutriment from the

surfaces over which they creep, there is no more lateral expansion of

the axis than is habitual among Acrogens or Archegoniates. Concluding,

as we are thus fully justified in doing, that the lateral expansion

accompanying longitudinal extension, which is a general characteristic

of Phanerogams as distinguished from Archegoniates, is nothing more

than a concomitant of their usually-vertical growth;[10] let us now

go on to consider how vertical growth originates, and what are the

structural changes it involves.

§ 193. Plants depend for their prosperity mainly on air and light: they

dwindle where they are smothered, and thrive where they can expand

their leaves into free space and sunshine. Those kinds which assume

prone positions, consequently labour under disadvantages in being

habitually interfered with by one another--they are mutually shaded

and mutually injured. Such of them, however, as happen, by variations

in mode of growth, to rise higher than others, are more likely to

flourish and leave offspring than others. That is to say, natural

selection will favour the more upright-growing forms. Individuals

with structures which lift them above the rest, are the fittest for

the conditions; and by the continual survival of the fittest, such

structures must become established. There are two essentially-different

ways in which the integrated series of fronds above described, may

be modified so as to acquire the stiffness needful for maintaining

perpendicularity. We will consider them separately.

[Illustration: Figs. 77, 78.]

A thin layer of substance gains greatly in power of resisting a

transverse strain, if it is bent round so as to form a tube: witness

the difference between the pliability of a sheet of paper when

outspread, and the rigidity of the same sheet of paper when rolled up.

Engineers constantly recognize this truth, in devising appliances by

which the greatest strength shall be obtained at the smallest cost of

material; and among organisms, we see that natural selection habitually

establishes structures conforming to the same principle, wherever

lightness and stiffness are to be combined. The cylindrical bones of

mammals and birds, and the hollow shafts of feathers, are examples.

The lower plants, too, furnish cases where the strength needful for

maintaining an upright position, is acquired by this rolling up of a

flat thallus or frond. In Fig. 77 we have an _Alga_ which approaches

towards a tubular distribution of substance; and which has a consequent

rigidity. Sundry common forms of lichen, having the thallus folded

into a branched tube, still more decidedly display the connexion

between this structural arrangement and this mechanical advantage.

And from the particular class of plants we are here dealing with--the

Archegoniates--a type is shown in Fig. 78, _Riella helicophylla_,

similarly characterized by a thin frond that is made stiff enough to

stand, by an incurving which, though it does not produce a hollow

cylinder, produces a kindred form. If, then, as we have seen, natural

selection or survival of the fittest will favour such among these

recumbent Archegoniates as are enabled, by variations in their

structures, to maintain raised postures; it will favour the formation

of fronds that curve round upon themselves, and curve round upon the

fronds growing out of them. What, now, will be the result should such a

modification take place in the group of proliferous fronds represented

in Fig. 76? Clearly, the result will be a structure like that shown

in Fig. 79. And if this inrolling becomes more complete, a form like

_Jungermannia cordifolia_, represented in Fig. 80, will be produced.

[Illustration: Figs. 79, 80.]

[Illustration: Figs. 81–89.]

[Illustration: Figs. 90, 91.]

When the successive fronds are thus folded round so completely that

their opposite edges meet, these opposite edges will be apt to unite:

not that they will grow together after being formed, but that they will

develop in connexion; or, in botanical language, will become “adnate.”

That foliar surfaces which, in their embryonic state, are in close

contact, often join into one, is a familiar fact. It is habitually

so with sepals or divisions of the calyx. In all campanulate flowers

it is so with petals. And in some tribes of plants it is so with

stamens. We are therefore well warranted in inferring that, under the

conditions above described, the successive fronds or leaflets will,

by union of their remote edges, first at their points of origin and

afterwards higher up, form sheaths inserted one within another, and

including the axis. This incurving of the successive fronds, ending

in the formation of sheaths, may be accompanied by different sets of

modifications. Supposing Fig. 81 to be a transverse section of such

type (_a_ being the mid-rib, and _b_ the expansion of an older frond;

while _c_ is a younger frond proliferously developed within it), there

may begin two divergent kinds of changes, leading to two contrasted

structures. If, while frond continues to grow out of frond, the

series of united mid-ribs continues to be the channel of circulation

between the uppermost fronds and the roots--if, as a consequence,

the compound mid-rib, or rudimentary axis, continues to increase in

size laterally; there will arise the series of transitional forms

represented by the transverse sections 82, 83, 84, 85; ending in the

production of a solid axis, everywhere wrapped round by the foliar

surface of the frond, as an outer layer or sheath. But if, on the

other hand, circumstances favour a form of plant which maintains its

uprightness at the smallest cost of substance--if the vascular bundles

of each succeeding mid-rib, instead of remaining concentrated, become

distributed all round the tube formed by the infolded frond; then the

structure eventually reached, through the transitional forms 86, 87,

88, 89, will be a hollow cylinder.[11] And now observe how the two

structures thus produced, correspond with two kinds of Monocotyledons.

Fig. 90 represents a species of _Dendrobium_, in which we see clearly

how each leaf is but a continuation of the external layer of a solid

axis--a sheath such as would result from the infolded edges of a frond

becoming adnate; and on examining how the sheath of each leaf includes

the one above it, and how the successive sheaths include the axis, it

will be manifest that the relations of parts are just such as exist

in the united series of fronds shown in Fig. 79--the successive nodes

answering to the successive points of origin of the fronds. Conversely,

the stem of a grass, Fig. 91, displays just such relations of parts,

as would result from the development of the type shown in Fig. 79,

if instead of the mid-ribs thickening into a solid axis, the matter

composing them became evenly distributed round the foliar surfaces,

at the same time that the incurved edges of the foliar surfaces

united. The arrangements of the tubular axis and its appendages, thus

resulting, are still more instructive than those of the solid axis.

For while, even more clearly than in the _Dendrobium_, we see at the

point _b_, a continuity of structure between the substance of the

axis below the node, and the substance of the sheath above the node:

we see that this sheath, instead of having its edges united as in

_Dendrobium_, has them simply overlapping, so as to form an incomplete

hollow cylinder which may be taken off and unrolled; and we see that

were the overlapping edges of this sheath united all the way from the

node _a_ to the node _b_, it would constitute a tubular axis, like that

which precedes it or like that which it includes. And then, giving an

unexpected conclusiveness to the argument, it turns out that in one

family of grasses, the overlapping edges of the sheaths _do_ unite:

thus furnishing us with a demonstration that tubular structures _are_

produced by the incurving and joining of foliar surfaces; and that so,

hollow axes may be interpreted as above, without making any assumption

unwarranted by fact. One further correspondence between the type thus

ideally constructed, and the monocotyledonous type, must be noted. If,

as already pointed out, the transverse growth of an axis arises when

the axis comes to be a channel of circulation between all the roots at

one of its extremities and all the leaves at the other; and if this

lateral bulging must increase as fast as the quantity of foliage to

be brought in communication with the roots increases--especially if

such foliage has at the same time to be raised high above the earth’s

surface; what must happen to a plant constructed in the manner just

described? The elder fronds or foliar organs, ensheathing the younger

ones, as well as the incipient axis serving as a bond of union, are

at first of such circumference only as suffices to inclose these

undeveloped parts. What, then, will take place when the inclosed

parts grow--when the axis thickens while it elongates? Evidently the

earliest-formed sheaths, not being large enough for the swelling axis,

must burst; and evidently each of the later-formed sheaths must, in

its turn, do the like. There must result a gradual exfoliation of

the successive sheaths, like that indicated as beginning in the above

figure of _Dendrobium_; which, at _a_, shows the bud of the undeveloped

parts just visible above the enwrapping sheaths, while at _b_, and _c_,

it shows the older sheaths in process of being split open. That is to

say, there must result the mode of growth which helped to give the name

Endogens to this class.

[Illustration: Figs. 92–94.]

[Illustration: Figs. 95–99.]

The other way in which an integrated series of fronds may acquire

the rigidity needful for maintaining an erect position, has next to

be considered. If the successive fronds do not acquire such habit

of curling as may be taken advantage of by natural selection, so as

to produce the requisite stiffness; then, the only way in which the

requisite stiffness appears producible, is by the thickening and

hardening of the fused series of mid-ribs. The incipient axis will not,

in this case, be inclosed by the rolled-up fronds; but will continue

exposed. Survival of the fittest will favour the genesis of a type, in

which those portions of the successive mid-ribs that enter into the

continuous bond, become more bulky than the disengaged portions of the

mid-ribs: the individuals which thrive and have the best chances of

leaving offspring, being, by the hypothesis, individuals having axes

stiff enough to raise their foliage above that of their fellows. At

the same time, under the same influences, there will tend to result an

elongation of those portions of the mid-ribs, which become parts of the

incipient axis; seeing that it will profit the plant to have its leaves

so far removed from one another, as to prevent mutual interferences.

Hence, from the recumbent type there will evolve, by indirect

equilibration (§ 167), such modifications as are shown in Figs. 92,

93, 94; the first of which is a slight advance on the ideal type

represented in Fig. 76, arising in the way described; and the others

of which are actual plants--_Haplomitrium Hookeri_, and _Plagiochila

decipiens_. Thus the higher Archegoniates show us how, along with

an assumption of the upright attitude, there does go on, as we see

there must go on, a separation of the leaf-producing parts from the

root-producing parts; a greater development of that connecting portion

of the successive fronds, by which they are kept in communication with

the roots, and raised above the ground; and a consequent increased

differentiation of such connecting portion from the parts attached

to it. And this lateral bulging of the axis, directly or indirectly

consequent on its functions as a support and a channel, being here

unrestrained by the early-formed fronds folded round it, goes on

without the bursting of these. Hence arises a leading character of

what is called exogenous growth--a growth which is, however, still

habitually accompanied by exfoliation, in flasks, of the outermost

layers, continually being cracked and split by the accumulation of

layers within them. And now if we examine plants of the exogenous type,

we find among them many displaying the stages of this metamorphosis. In

Fig. 95, is shown a form in which the continuity of the axis with the

mid-rib of the leaf, is manifest--a continuity that is conspicuous in

the common thistle. Here the foliar expansion, running some distance

down the axis, makes the included portion of the axis a part of its

mid-rib; just as in the ideal types above drawn. By the greater growth

of the internodes, which are very variable, not only in different

plants but in the same plant, there results a modification like that

delineated in Fig. 96. And then, in such forms as Fig. 97, there is

shown the arrangement that arises when, by more rapid development of

the proximal end of the mid-rib, the distal part of the foliar surface

is separated from the part which embraces the axis: the wings of the

mid-rib still serving, however, to connect the two portions of the

foliar surface. Such a separation is, as pointed out in § 188, an

habitual occurrence; and in some compound leaves, an actual tearing

of the inter-venous tissue is caused by extra growth of the mid-rib.

Modifications like this, and the further one in Fig. 98, we may expect

to be established by survival of the fittest, among those plants

which produce considerable masses of leaves; since the development of

mid-ribs into foot-stalks, by throwing the leaves further away from

the axes, will diminish the shading of the leaves, one by another. And

then, among plants of bushy growth, in which the assimilating surfaces

become still more liable to intercept one another’s light, natural

selection will continue to give an advantage to those which carry their

assimilating surfaces at the ends of the petioles, and do not develop

assimilating surfaces close to the axis, where they are most shaded.

Whence will result a disappearance of the stipules and the foliar

fringes of the mid-ribs; ending in the production of the ordinary

stalked leaf, Fig. 99, which is characteristic of trees. Meanwhile, the

axis thickens in proportion to the number of leaves it has to carry,

and to put in communication with the roots; and so there comes to be a

more marked contrast between it and the petioles, severally carrying a

leaf each.[12]

§ 194. When, in the course of the process above sketched out, there has

arisen such community of nutrition among the fronds thus integrated

into a series, that the younger ones are aided by materials which the

older ones have elaborated; the younger fronds will begin to show, at

earlier and earlier periods of development, the structures about to

originate from them. Abundant nutrition will abbreviate the intervals

between the successive prolifications; so that eventually, while each

frond is yet imperfectly formed, the rudiment of the next will begin to

show itself. All embryology justifies this inference. The analogies it

furnishes lead us to expect that when this serial arrangement becomes

organic, the growing part of the series will show the general relations

of the forthcoming parts, while they are very small and unspecialized.

What will in such case be the appearances they assume? We shall have no

difficulty in perceiving what it will be, if we take a form like that

shown in Fig. 92, and dwarf its several parts at the same time that we

generalize them. Figs. 100, 101, 102, and 103, will show the result;

and in Fig. 104, which is the bud of a dicotyledon, we see how clear is

the morphological correspondence: _a_ being the rudiment of a foliar

organ beginning to take shape; _b_ being the almost formless rudiment

of the next foliar organ; and _c_ being the quite-undifferentiated

part whence the rudiments of subsequent foliar organs are to arise.

[Illustration: Figs. 100–104.]

[Illustration: Figs. 105–106.]

And now we are prepared for entering on a still-remaining question

respecting the structure of Phænogams--what is the origin of axillary

buds? As the synthesis at present stands, it does not account for

these; but on looking a little more closely into the matter, we shall

find that the axillary buds are interpretable in the same manner as the

terminal buds. So to interpret them, however, we must return to that

process of proliferous growth with which we set out, for the purpose

of observing some facts not before named. _Delesseria hypoglossum_,

Fig. 105, represents a seaweed of the same genus as one outlined in

Fig. 40; but of a species in which proliferous growth is carried much

further. Here, not only does the primary frond bud out many secondary

fronds from its mid-rib; but most of the secondary fronds similarly bud

out several tertiary fronds; and even by some of the tertiary fronds,

this prolification is repeated. Besides being shown that the budding

out of several fronds from one frond, may become habitual; we are also

shown that it may become a habit inherited by the fronds so produced,

and also by the fronds they produce: the manifestation of the tendency

being probably limited only by failure of nutrition. That under fit

conditions an analogous mode of growth will occur in fronds of the

acrogenic type, like those we set out with, is shown by the case of

_Metzgeria furcata_, Figs. 45, 46, in which such compound prolification

is partially displayed. Let us suppose, then, that the frond _a_,

Fig. 106, produces not only a single secondary frond _b_, but also

another such secondary frond _b′_. Let us suppose, further, that the

frond _b_ is in like manner doubly proliferous: producing both _c_

and _c′_. Lastly, let us suppose that in the second frond _b′_ which

_a_ produces, as well as in the second frond _c′_ which _b_ produces,

the doubly-proliferous habit is manifested. If, now, this habit grows

organic--if it becomes, as it naturally will become, the characteristic

of a plant of luxuriant growth, the unfolding parts of which can be

fed by the unfolded parts; it will happen with each lateral series,

as with the main series, that its successive components will begin to

show themselves at earlier and earlier stages of development. And in

the same way that, by dwarfing and generalizing the original series, we

arrive at a structure like that of the terminal bud; by dwarfing and

generalizing a lateral series, as shown in Figs. 107–110, we arrive at

a structure answering in nature and position to the axillary bud.

[Illustration: Figs. 107–110.]

Facts confirming these interpretations are afforded by the structure

and distribution of buds. The phænogamic axis in its primordial form,

being an integrated series of folia; and the development of that part

by which these folia are held together at considerable distances

from one another, taking place afterwards; it is inferable from the

general principles of embryology, that in its rudimentary stages, the

phænogamic shoot will have its foliar parts more clearly marked out

than its axial parts. This we see in every bud. Every bud consists

of the rudiments of leaves packed together without any appreciable

internodal spaces; and the internodal spaces begin to increase with

rapidity, only when the foliar organs have been considerably developed.

Moreover, where nutrition falls short, and arrest of development

takes place--that is, where a flower is formed--the internodes remain

undeveloped: the unfolding ceases before the later-acquired characters

of the phænogamic shoot are assumed. Lastly, as the hypothesis leads us

to expect, axillary buds make their appearances later than the foliar

organs which they accompany; and where, as at the ends of shoots, these

foliar organs show failure of chlorophyll, the axillary buds are not

produced at all. That these are inferable traits of structure, will be

manifest on inspecting Figs. 106–110; and on observing, first, that

the doubly-proliferous tendency of which the axillary bud is a result,

implies abundant nutrition; and on observing, next, that the original

place of secondary prolification, is such that the foliar surface on

which it occurs, must grow to some extent before the bud appears.

On thus looking at the matter--on contemplating afresh the ideal type

shown in Fig. 106, and noting how, by the conditions of the case, the

secondary prolifications must cease before that primary prolification

which produces the main axis; we are enabled to reconcile all the

phenomena of axillary gemmation. We see harmony among the several

facts--first, that the axillary bud becomes a lateral, leaf-bearing

axis if there is abundant material for growth; second, that its

development is arrested, or it becomes a flower-bearing axis, if the

supply of sap is but moderate; third, that it is absent when the

nutrition is failing. We are no longer committed to the gratuitous

assumption that, in the phænogamic type, there must exist an axillary

bud to each foliar organ; but we are led to conclude, _à priori_,

that which we find, _à posteriori_, that axillary buds are as normally

absent in flowers as they are normally present lower down the axis. And

then, to complete the argument, we are prepared for the corollary that

axillary prolification may naturally arise even at the ends of axes,

should the failing nutrition which causes the dwarfing of the foliar

organs to form a flower, be suddenly changed into such high nutrition

as to transform the components of the flower into appendages that are

green, if not otherwise leaf-like--a condition under which only, this

phenomenon is proved to occur.

§ 195. One more question presents itself, when we contrast the early

stages of development in the two classes of Phænogams; and a further

answer, supplied by the hypothesis, gives to the hypothesis a further

probability. It is characteristic of a monocotyledon, to have a single

seed-leaf or cotyledon; and it is characteristic of a dicotyledon, to

have at least two cotyledons, if not more than two. That is to say, the

monocotyledonous mode of germination everywhere co-exists with the

endogenous mode of growth; and along with the exogenous mode of growth, there always goes either a dicotyledonous or

polycotyledonous germination. Why is this? Such correlations cannot be

accidental--cannot be meaningless. A true theory of the phænogamic

types in their origin and divergence, should account for the connexion

of these traits. Let us see whether the foregoing theory does this.

The higher plants, like the higher animals, bequeath to their offspring

more or less of nutriment and structure. Superior organisms of either

kingdom do not, as do all inferior organisms, cast off their progeny

in the shape of minute portions of protoplasm, unorganized and without

stocks of material for them to organize; but they either deposit

along with the germs they cast off, certain quantities of albuminoid

substance to be appropriated by them while they develop themselves,

or else they continue to supply such substance while the germs

partially develop themselves before their detachment. Among plants

this constitutes one distinction between seeds and spores. Every seed

contains a store of food to serve the young plant during the first

stages of its independent life; and usually, too, before the seed is

detached, the young plant is so far advanced in structure, that it

bears to the attached stock of nutriment much the same relation that

the young fish bears to the appended yelk-bag at the time of leaving

the egg. Sometimes, indeed, the development of chlorophyll gives the

seed-leaves a bright green, while the seed is still contained in the

parent-pod. This early organization of the phænogam must be supposed

rudely to indicate the type out of which the phænogamic type arose.

On the foregoing hypothesis, the seed-leaves therefore represent the

primordial fronds; which, indeed, they simulate in their simple,

cellular, unveined structures. And the question here to be asked is--do

the different relations of the parts in young monocotyledons and

dicotyledons correspond with the different relations of the primordial

fronds, implied by the endogenous and the exogenous modes of growth? We

shall find that they do.

[Illustration: Figs. 111–122.]

Starting, as before, with the proliferous form shown in Fig. 111, it

is clear that if the strength required for maintaining the vertical

attitude, is obtained by the rolling up of the fronds, the primary

frond will more and more conceal the secondary frond within it. At the

same time, the secondary frond must continue to be dependent on the

first for its nutrition; and, being produced within the first, must

be prevented by defective supply of light and air, from ever becoming

synchronous in its development with the first. Hence, this infolding

which leads to the endogenous mode of growth, implies that there must

always continue such pre-eminence of the first-formed frond or its

representative, as to make the germination monocotyledonous. Figs.

111 to 115, show the transitional forms that would result from the

infolding of the fronds. In Fig. 116 (a vertical section of the form

represented in Fig. 115) are exhibited the relations of the successive

fronds to each other. The modified relations that would result, if

the nutrition of the embryo admitted of anticipatory development of

the successive fronds, are shown in Fig. 117. And how readily the

structure may pass into that of the monocotyledonous germ, will be

seen on inspecting Fig. 118; which is a vertical section of an actual

monocotyledon at an early stage--the incomplete lines at the left of

its root, indicating its connexion with the seed.[13] Contrariwise,

where the strength required for maintaining an upright attitude is not

obtained by the rolling up of the fronds, but by the strengthening

of the continuous mid-rib, the second frond, so far from being less

favourably circumstanced than the first, becomes in some respects even

more favourably circumstanced: being above the other, it gets a greater

share of light, and it is less restricted by surrounding obstacles.

There is nothing, therefore, to prevent it from rapidly gaining an

equality with the first. And if we assume, as the truths of embryology

entitle us to do, an increasing tendency towards anticipation in the

development of subsequent fronds--if we assume that here, as in other

cases, structures which were originally produced in succession will,

if the nutrition allows and no mechanical dependence hinders, come to

be produced simultaneously; there is nothing to prevent the passage of

the type represented in Fig. 111, into that represented in Fig. 122.

Or rather, there is everything to facilitate it; seeing that natural

selection will continually favour the production of a form in which the

second frond grows in such way as not to shade the first, and in such

way as allows the axis readily to assume a vertical position.

Thus, then, is interpretable the universal connexion between

monocotyledonous germination and endogenous growth; as well as

the similarly-universal connexion between exogenous growth and

the development of two or more cotyledons. That it explains these

fundamental relations, adds very greatly to the probability of the

hypothesis.

§ 196. While we are in this manner enabled to discern the kinship that

exists between the higher vegetal types themselves, as well as between

them and the lower types; we are at the same time supplied with a

rationale of those truths which vegetal morphologists have established.

Those homologies which Wolff indicated in their chief outlines and

Goethe followed out in detail, have a new meaning given to them when we

regard the phænogamic axis as having been evolved in the way described.

Forming the modified conception which we are here led to do, respecting

the units of which a flowering plant is composed, we are no longer

left without an answer to the question--What is an axis? And we are

helped to understand the naturalness of those correspondences which the

successive members of each shoot display. Let us glance at the facts

from our present standpoint.

[Illustration: Figs. 123–129.]

The unit of composition of a Phænogam, is such portion of a shoot as

answers to one of the primordial fronds. This portion is neither one

of the foliar appendages nor one of the internodes; but it consists of

a foliar appendage together with the preceding internode, including

the axillary bud where this is developed. The parts intercepted by

the dotted lines in Fig. 123, constitute such a segment; and the true

homology is between this and any other foliar organ with the portion

of the axis below it. And now observe how, when we take this for the

unit of composition, the metamorphoses which the phænogamic axis

displays, are inferable from known laws of development. Embryology

teaches us that arrest of development shows itself first in the absence

of those parts that have arisen latest in the course of evolution;

that if defect of nutrition causes an earlier arrest, parts that are

of more ancient origin abort; and that the part alone produced when

the supply of materials fails near the outset, is the primordial

part. We must infer, therefore, that in each segment of a Phænogam,

the foliar organ, which answers to the primordial frond, will be the

most constant element; and that the internode and the axillary bud,

will be successively less constant. This we find. Along with a smaller

size of foliar surface implying lower nutrition, it is usual to see a

much-diminished internode and a less-pronounced axillary bud, as in

Fig. 124. On approaching the flower, the axillary bud disappears; and

the segment is reduced to a small foliar surface, with an internode

which is in most cases very short if not absent, as in 125 and 126.

In the flower itself, axillary buds and internodes are both wanting:

there remains only a foliar surface (127), which, though often larger

than the immediately-preceding foliar surface, shows failing nutrition

by absence of chlorophyll. And then, in the quite terminal organs of

fructification (129), we have the foliar part itself reduced to a

mere rudiment. Though these progressive degenerations are by no means

regular, being in many cases varied by adaptations to particular

requirements, yet it cannot, I think, be questioned, that the general

relations are as described, and that they are such as the hypothesis

leads us to expect. Nor are we without a kindred explanation of certain

remaining traits of foliar organs in their least-developed forms.

Petals, stamens, pistils, &c., besides reminding us of the primordial

fronds by their diminished sizes, and by the want of those several

supplementary parts which the preceding segments possess, also remind

us of them by their histological characters: they consist of simple

cellular tissue, scarcely at all differentiated. The fructifying cells,

too, which here make their appearance, are borne in ways like those in

which the lower Acrogens bear them--at the edge of the frond, or at the

end of a peduncle, or immersed in the general substance; as in Figs.

128 and 129. Nay, it might even be said that the colours assumed by

these terminal folia, call to mind the plants out of which we conclude

that Phænogams have been evolved; for it is said of the fronds 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.”

As thus understood, then, the homologies among the parts of the

phænogamic axis are interpretable, not as due to a needless adhesion

to some typical form or fulfilment of a predetermined plan; but as

the inevitable consequences of the mode in which the phænogamic axis

originates.

§ 197. And now it remains only to observe, in confirmation of

the foregoing synthesis, that it at once explains for us various

irregularities. When we see leaves sometimes producing leaflets from

their edges or extremities, we recognize in the anomaly a resumption of

an original mode of growth: fronds frequently do this. When we learn

that a flowering plant, as the _Drosera intermedia_, has been known to

develop a young plant from the surface of one of its leaves, we are

at once reminded of the proliferous growths and fructifying organs in

the Liverworts. The occasional production of bulbils by Phænogams,

ceases to be so surprising when we find it to be habitual among the

inferior Acrogens, and when we see that it is but a repetition,

on a higher stage, of that self-detachment which is common among

proliferously-produced fronds. Nor are we any longer without a solution

of that transformation of foliar organs into axial organs, which not

uncommonly takes place. How this last irregularity of development is to

be accounted for, we will here pause a moment to consider. Let us first

glance at our data.

The form of every organism, we have seen, must depend on the structures

of its physiological [or constitutional] units. Any group of such

units will tend to arrange itself into the complete organism, if

uncontrolled and placed in fit conditions. Hence the development of

fertilized germs; and hence the development of those self-detached

cells which characterize some plants. Conversely, physiological units

which form a small group involved in a larger group, and are subject to

all the forces of the larger group, will become subordinate in their

structural arrangements to the larger group--will be co-ordinated into

a part of the major whole, instead of co-ordinating themselves into a

minor whole. This antithesis will be clearly understood on remembering

how, on the one hand, a small detached part of a hydra soon moulds

itself into the shape of an entire hydra; and how, on the other hand,

the cellular mass that buds out in place of a lobster’s lost claw,

gradually assumes the form of a claw--has its parts so moulded as to

complete the structure of the organism: a result which we cannot but

ascribe to the forces which the rest of the organism exerts upon it.

Consequently, among plants, we may expect that whether any portion of

protoplasm moulds itself into the typical form around an axis of its

own, or is moulded into a part subordinate to another axis, will depend

on the relative mass of its physiological units--the accumulation of

them that has taken place before the assumption of any structural

arrangement. A few illustrations will make clear the validity of this

inference. In the compound leaf, Fig. 65, the several lateral growths

_a_, _b_, _c_, _d_, are manifestly homologous; and on comparing a

number of such leaves together, it will be seen that one of these

lateral growths may assume any degree of complexity, according to the

degree of its nutrition. Every fern-leaf exemplifies the same general

truth still better. Whether each sub-frond remains an undeveloped

wing of the main frond, or whether it organizes itself into a group

of frondlets borne by a secondary rib, or whether, going further, as

it often does, it gives rise to tertiary ribs bearing frondlets, is

determined by the supply of materials for growth; since such higher

developments are most marked at points where the nutrition is greatest;

namely, next the stem. But the clearest evidence is afforded among the

_Algæ_, which, not drawing nutriment from roots, have their parts much

less mutually dependent; and are therefore capable of showing more

clearly, how any part may remain an appendage or may become the parent

of appendages, according to circumstances. In the annexed Fig. 130,

representing a branch of _Ptilota plumosa_, we see how a wing grows

into a wing-bearing branch if its nutrition passes a certain point.

This form, so strikingly like that of the feathery crystallizations of

many inorganic substances, implies that, as in such crystallizations,

the simplicity or complexity of structure at any place depends on the

quantity of matter that has to be arranged at that place in a given

time.[14]

[Illustration: Fig. 130.]

Hence, then, we are not without an interpretation of those

over-developments which the phænogamic axis occasionally undergoes.

Fig. 104, represents the phænogamic bud in its rudimentary state. The

lateral process _b_, which ordinarily becomes a foliar appendage,

differs very little from the terminal process _c_, which is to become

an axis--differs mainly in having, at this period when its form is

being determined, a smaller bulk. If while thus undifferentiated, its

nutrition remains inferior to that of the terminal process, it becomes

moulded into a part that is subordinate to the general axis. But if,

as sometimes happens, there is supplied to it such an abundance of

the materials needful for growth, that it becomes as large as the

terminal process; then we may naturally expect it to begin moulding

itself round an axis of its own: a foliar organ will be replaced by

an axial organ. And this result will be especially liable to occur,

when the growth of the axis has been previously undergoing that arrest

which leads to the formation of a flower; that is when, from defect

of materials, the terminal process has almost ceased to increase, and

when some concurrence of favourable causes brings a sudden access of

sap which reaches the lateral processes before it reaches the terminal

process.[15]

§198. The general conclusion to which these various lines of evidence

converge, is, then, that the shoot of a flowering plant is an aggregate

of the third degree of composition. Taking as aggregates of the first

order, those small portions of protoplasm which ordinarily assume

the forms under which they are known as cells; and considering as

aggregates of the second order, those assemblages of such cells which,

in the lower cryptogams, compose the various kinds of thallus; then

that structure, common to the higher cryptogams and to phænogams,

in which we find a series of such groups of cells bound up into a

continuous whole, must be regarded as an aggregate of the third

order. The inference drawn from analysis, and verified by a synthesis

which corresponds in a remarkable manner with the facts, is that

those compound parts which, in Monocotyledons and Dicotyledons are

called axes, have really arisen by integration of such simple parts

as in lower plants are called fronds. Here, on a higher level,

appears to have taken place a repetition of the process already

observed on lower levels. The formation of those small groups of

physiological units which compose the lowest protophytes, is itself

a process of integration; and the consolidation of such groups into

definitely-circumscribed and coherent cells or morphological units, is

a completing of the process. In those coalescences by which many such

cells are joined into threads, and discs, and solid or flattened-out

masses, we see these morphological units aggregating into units of a

compound kind: the different phases of the transition being exemplified

by groups of various sizes, various degrees of cohesion, and various

degrees of definiteness. And now we find evidences of a like process

on a larger scale: the compound groups are again compounded. Moreover,

as before, there are not wanting types of organization by which the

stages of this higher integration are shadowed forth. From fronds that

occasionally produce other fronds from their surfaces, we pass to those

that habitually produce them; from those that do so in an indefinite

manner, to those that do so in a definite manner; and from those that

do so singly, to those that do so doubly and triply through successive

generations of fronds. Even within the limits of a sub-class, we find

gradations between fronds irregularly proliferous, and groups of such

fronds united into a regular series.

Nor does the process end here. The flowering plant is rarely

uniaxial--it is nearly always multiaxial. From its primary shoot there

grow out secondary shoots of like kind. Though occasionally among

Phænogams, and frequently among the higher Cryptogams, the germs of

new axes detach themselves under the form of bulbils, and develop

separately instead of in connexion with the parent axis; yet in most

Phænogams the germ of each new axis maintains its connexion with the

parent axis: whence results a group of axes--an aggregate of the fourth

order. Every tree, by the production of branch out of branch, shows

us this integration repeated over and over again; forming an aggregate

having a degree of composition too complex to be any longer defined.

* * * * *

[NOTE.--A criticism passed on the general argument set forth in the

foregoing sections, runs as follows:--“I have already pointed out that

the process of evolution by which you believe the Liverworts with a

distinct axis and appendages to have been produced from the thalloid

forms is not founded on sound evidence either in comparative morphology

or development. But even if we admit that such an integration of a

proliferously-produced colony might have given rise to the leafy

_Jungermanniaceæ_, there are even more weighty objections to the

supposition that the same process produced the shoot structures of

the flowering plants. In the first place the flowering plant-body is

_not homologous with the liverwort plant-body_, since they represent

different generations. The liverwort plant-body or _gametophyte_,

_i.e._, the generation bearing sexual organs, is homologous with the

prothallus of ferns and other Pteridophytes, and in the Flowering

Plants with reduced structures contained within the spores (embryo-sac

and pollen-grain) but still giving rise to sexual cells. The liverwort

spore-capsule and its accessory parts (in fact everything produced

from the fertilized egg) is homologous with the sporogonium of the

mosses, and, as most botanists think, with the leafy plant-body

of Pteridophytes and Phanerogams. This generation is called the

_sporophyte_ and from the spores which it produces are developed

the gametophytes of the next generation. These generalizations were

first established by Hofmeister, and all subsequent work has tended

to establish them more firmly. The only doubtful question is (and the

doubt is mainly, I think, peculiar to myself, certainly not being

shared by the majority of botanists) whether the sporophyte of Mosses

and Liverworts is really homologous with that of Pteridophytes and

Phanerogams, whether it may not rather be regarded as a parallel

development along another line of descent from the Green Algæ.

“Hence we must look for the origin of the shoot-structure of flowering

plants in the sporophytes of the Pteridophytes, from which group there

is no reason to doubt that the phanerogams have arisen in descent.

The various groups of Pteridophytes vary much in the organization of

these shoot-systems, as a mental glance at the types exhibited by the

Ferns, Horse-tails, Club-mosses, _Ophioglossaceæ_, and the isolated

Isoetes will convince you at once. It may be that some of these groups

are independent in descent, _i.e._, that the _Pteridophyta_ are

polyphyletic, and the current hypothesis with regard to the phanerogams

is that they have arisen by two, if not three, separate lines of

descent from different groups of Pteridophytes (this is indicated in

the classificatory diagram on p. 377 of vol. I). I should not, however,

care to pin my faith to these or to any such lines of ancestry. Still

I think we must look for the ancestors of the Flowering Plants among

the Pteridophytes, and the latter always have a good distinction

between axis and appendages. The problem of the evolution of these

differentiated sporophytic shoots is undoubtedly the great outstanding

problem of morphology. Various attempts have been made to solve it,

of which probably the most important is the theory of Profs. Bown

and Campbell, who derive the Pteridophytes from some Liverwort like

_Anthoceros_, but the sporophyte of course from the sporophytic portion

of the plant (not much more than a spore-capsule), the prothallus

of the Fern representing the vegetative thallus of Anthoceros. I am

not wholly convinced by these undoubtedly ingenious hypotheses, in

support of which an immense amount of facts have been collected; but

my position would, I know, simply ‘put us to ignorance again’ on this

question.

“I have discussed this at some length in order to bring out clearly

the immense difficulty of constructing a wellgrounded theory of

the origin of the differentiated shoot-system of the higher plant.

I confess I don’t think it can be done at all with the materials at

present at our disposal. Of course it is just possible to suppose that

some ancestral sporophyte had the structure of a proliferous thalloid

liverwort gametophyte, and that from it was evolved the phanerogamic

shoot in the ways you suggest. This gives us absolutely no clue,

however, to any Pteridophytic shoot, which ought to be intermediate

(more or less) between the hypothetical ancestor and the Phanerogam,

and is furthermore, as far as I can see, not supported by an atom of

evidence of any kind. It is true that your theory fits in well with the

phenomena exhibited by phanerogamic shoots themselves, but this fact

you will see must lose much of its significance if the hypothesis lacks

foundation.

“With regard to your method of explaining the fundamental characters

of ‘Exogens’ and ‘Endogens,’ this of course is part of the same

hypothesis; but I may point out that since Von Mohl and Sanio, between

1855 and 1865, showed (1) that the growth at the stem apex of a

monocotyledon was _not_ endogenous, and (2) that the ‘thickening ring’

near the apex of a dicotyledon was not to be confused, as had been

done up till then, with the ring of _secondary meristem_ or _true

cambium_, which arose lower down, and only in woody or practically

woody stem, the terms ‘Exogen’ and ‘Endogen’ have necessarily fallen

into disuse, since they imply a false conception of what happens. Both

monocotyledons and dicotyledons have a ‘thickening ring,’ which gives

rise to the primary vascular cylinder of the stem. When the stem is

of considerable thickness, as in Palms, &c., it grows by the active

cell-division of its outer layers, so that both classes are ‘exogenous’

in this sense; while the addition of a centrifugal zone of secondary

wood is confined to certain Dicotyledons (Trees, shrubs, &c.).

“The distinction between the embryos, moreover, is not absolute. The

single cotyledon is usually terminal in monocotyledons, but not always

(_Dioscoraceæ_ have lateral cotyledons), but the plumule may push

through it (Grasses) or make its exit sideways (Palms), or be formed at

the side (_Alisma_); and Dicotyledons very similarly.

“The occurrence of completely sheathing leaves in grasses is perhaps

correlated with the absence of cambium, but grasses are an aberrant

type among monocotyledons, and secondary thickening is only found in

very few genera of this class, so that the correlation is, so to speak,

negative and indirect.... It is clear that the greater part of the

discussion will have to be re-written.”

For the reasons assigned in the preface I cannot undertake to re-write

the discussion, as suggested. It must stand for what it is worth. All I

can do is here to include along with it the foregoing criticisms.

I may, however, indicate the line of defence I should take were I to

go again into the matter. The objections are based on the structure of

existing Liverworts and Phænogams. But I have already referred to the

probability--or, indeed, the certainty--that in conformity with the

general principle set forth in the note to Chapter I, we must conclude

that the early types of Liverworts out of which the Phænogams are

supposed to have evolved, as well as the early types of Phænogams in

which the stages of evolution were presented, no longer exist. We must

infer that forms simpler than any now known, and more intermediate in

their traits, were the forms concerned; and if so, it may be held that

the incongruities with the hypothesis which are presented by existing

forms, do not negative it. The scepticism my critic himself expresses

respecting the current interpretation is a partial justification of

this view. Moreover, his admission that the theory set forth “fits in

well with the phenomena exhibited by phanerogamic shoots,” must, I

think, be regarded as weighty evidence. On the Evolution hypothesis

we are obliged to suppose that the Monocotyledons and Dicotyledons

respectively arose by integration of fronds; and if to the question

after what manner the integration took place, there is an hypothesis

which renders it comprehensible, and agrees both with the structures of

the two kinds of shoots and the structures of the two kinds of seeds,

as well as with various of the other phenomena the two types present,

it has strong claims for acceptance.

Reconsideration suggests the following remarks.

1. Alternation of generations is a means of furthering multiplication.

To be effective each member of either generation must be a

self-supporting centre of growth or diffusion or both. Hence if, as in

the Liverworts, one of the so-called alternating generations is not

independent, but a permanent growth on the other--a parasite--it is a

misuse of words to call the arrangement Alternation of generations.

(Since this was written I have found that Sir Edward Fry takes the

same view. He approvingly quotes Professor Bower, who says that “the

alternation of generations is not an accurate statement of facts or a

useful analogy.”)

2. The alternating of sexual and non-sexual processes is not

fundamentally distinctive; for, as shown by sundry Archegoniates,

it is an inconstant trait, and as shown by Klebs’ experiments on

_Vaucheria_, the conditions may be varied so as to determine its

occurrence or non-occurrence. Nay, the same individual may reproduce in

either way.

3. Still more significant is the fact that in some of the marine

Thallophytes, there is a process like that which in a moss or a fern

is considered an alternation of generations, whereas in others, as

the Brown Wrack (_Fucus_), each generation is sexual. Thus the

presence or absence of this mode of genesis cannot be a cardinal

distinction.

4. With these facts before us, it is not only a reasonable supposition

but a highly probable supposition, that there have existed plants of

the Liverwort type in which the so-called alternation of generations

did not take place. If so, nearly all the foregoing objections to my

hypothesis fall to the ground.]