§ 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.

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.

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.

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.

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.

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.

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.