§ 180. Evolution implies insensible modifications and gradual transitions, which render definition difficult—which make it impossible to separate absolutely the phases of organization from one another. And this indefiniteness of distinction, to be expected à priori, we are compelled to recognize à posteriori, the moment we begin to group morphological phenomena into general propositions. Thus, on inquiring what is the morphological unit, whether of plants or of animals, we find that the facts refuse to be included in any rigid formula. The doctrine that all organisms are built up of cells, or that cells are the elements out of which every tissue is developed, is but approximately true. There are living forms of which cellular structure cannot be asserted; and in living forms that are for the most part cellular, there are nevertheless certain portions which are not produced by the metamorphosis of cells. Supposing that clay were the only material available for building, the proposition that all houses are built of bricks, would bear about the same relation to the truth, as does the proposition that all organisms are composed of cells. This generalization respecting houses would be open to two criticisms:—first, that certain houses of a primitive kind are formed, not of bricks, but out of unmoulded clay; and second, that though other houses consist mainly of bricks, yet their chimney-pots, drain-pipes, and ridge-tiles, do not result from combination or metamorphosis of bricks, but are made directly out of the original clay. And of like natures are the criticisms which must be passed on the generalization, that cells are the morphological units of organisms. To continue the simile, the truth turns out to be, that the primitive clay or protoplasm out of which organisms are built, may be moulded either directly, or with various degrees of indirectness, into organic structures. The physiological units which we are obliged to assume as the components of this protoplasm, must, as we have seen, be the possessors of those proclivities which result in the structural arrangements of the organism. The assumption of such structural arrangements may go on, and in many cases does go on, by the shortest route; without the passage through what we call metamorphoses. But where such structural arrangements are reached by a circuitous route, the first stage is the formation of these small aggregates which, under the name of cells, are currently regarded as morphological units.

The rationale of these truths appears to be furnished by the hypothesis of evolution. We set out with molecules some degrees higher in complexity than those molecules of nitrogenous colloidal substance into which organic matter is resolvable; and we regard these very much more complex molecules as having the implied greater instability, greater sensitiveness to surrounding influences, and consequent greater mobility of form. Such being the primitive physiological units, organic evolution must begin with the formation of a minute aggregate of them—an aggregate showing vitality by a higher degree of that readiness to change its form of aggregation which colloidal matter in general displays; and by its ability to unite the nitrogenous molecules it meets with, into complex molecules like those of which it is composed. Obviously, the earliest forms must have been minute; since, in the absence of any but diffused organic matter, no form but a minute one could find nutriment. Obviously, too, it must have been structureless; since, as differentiations are producible only by the unlike actions of incident forces, there could have been no differentiations before such forces had had time to work. Hence, distinctions of parts like those required to constitute a cell were necessarily absent at first. And we need not therefore be surprised to find, as we do find, specks of protoplasm manifesting life, and yet showing no signs of organization. A further stage of evolution is reached when the imperfectly integrated molecules forming one of these minute aggregates, become more coherent; at the same time as they pass into a state of heterogeneity, gradually increasing in its definiteness. That is to say, we may look for the assumption by them, of some distinctions of parts, such as we find in cells and in what are called unicellular organisms. They cannot retain their primordial uniformity; and while in a few cases they may depart from it but slightly, they will, in the great majority of cases, acquire a decided multiformity: there will result the comparatively integrated and comparatively differentiated Protophyta and Protozoa. The production of minute aggregates of physiological units being the first step, and the passage of such minute aggregates into more consolidated and more complex forms being the second step, it must naturally happen that all higher organic types, subsequently arising by further integrations and differentiations, will everywhere bear the impress of this earliest phase of evolution. From the law of heredity, considered as extending to the entire succession of living things during the Earth’s past history, it follows that since the formation of these small, simple organisms must have preceded the formation of larger and more complex organisms, the larger and more complex organisms must inherit their essential characters. We may anticipate that the multiplication and combination of these minute aggregates or cells, will be conspicuous in the early developmental stages of plants and animals; and that throughout all subsequent stages, cell-production and cell-differentiation will be dominant characteristics. The physiological units peculiar to each higher species will, speaking generally, pass through this form of aggregation on their way towards the final arrangement they are to assume; because those primordial physiological units from which they are remotely descended, aggregated into this form. And yet, just as in other cases we found reasons for inferring (§ 131) that the traits of ancestral organization may, under certain conditions, be partially or wholly obliterated, and the ultimate structure assumed without passing through them; so, here, it is to be inferred that the process of cell-formation may, in some cases, be passed over. Thus the hypothesis of evolution prepares us for those two radical modifications of the cell-doctrine which the facts oblige us to make. It leads us to expect that as structureless portions of protoplasm must have preceded cells in the process of general evolution; so, in the special evolution of each higher organism, there will be an habitual production of cells out of structureless blastema. And it leads us to expect that though, generally, the physiological units composing a structureless blastema, will display their inherited proclivities by cell-development and metamorphosis; there will nevertheless occur cases in which the tissue to be formed, is formed by direct transformation of the blastema.[2]

Interpreting the facts in this manner, we may recognize that large amount of truth which the cell-doctrine contains, without committing ourselves to the errors involved by a sweeping assertion of it. We are enabled to understand how it happens that organic structures are usually cellular in their composition, at the same time that they are not universally so. We are shown that while we may properly continue to regard the cell as the morphological unit, we must constantly bear in mind that it is such only in a qualified sense.

§ 181. These aggregates of the lowest order, each formed of physiological units united into a group that is structurally single and cannot be divided without destruction of its individuality, may, as above implied, exist as independent organisms. The assumption to which we are committed by the hypothesis of evolution, that such so called unicellular plants were at first the only kinds of plants, is in harmony with the fact that habitats not occupied by plants of higher orders, commonly contain these protophytes in great abundance and great variety. The various species of Pleurococcaceæ, of Desmidiaceæ, and Diatomaceæ, supply examples of morphological units living and propagating separately, under numerous modifications of form and structure. Figures 1, 2, and 3, represent a few of the commonest types.

Mostly, simple plants are too small to be individually visible without the microscope. But, in some cases, these vegetal aggregates of the first order grow to appreciable sizes. In the mycelium of some fungi, we have single cells developed into long branched filaments, or ramified tubules, that are of considerable lengths. An analogous structure characterizes certain tribes of Algæ, of which Codium adhærens, Fig. 4, may serve as an example. In Botrydium, another alga, Fig. 5, we have a structure which is described as simulating a higher plant, with root, stem, bud, and fruit, all produced by the branching of a single cell. And among fungi the genus Mucor, Fig. 6, furnishes an example of allied kind.[3] Here, though the size attained is much greater than that of many organisms which are morphologically compound, we are compelled to consider the morphological composition as simple; since the whole can no more be separated into minor wholes, than can the branched vascular system of an animal. In these cases we have considerable bulk attained, not by a number of aggregates of the first order being united into an aggregate of the second order, but by the continuous growth of an aggregate of the first order.

§ 182. The transition to higher forms begins in a very unobtrusive manner. Among these aggregates of the first order, an approach towards that union by which aggregates of the second order are produced, is indicated by mere juxtaposition. Protophytes multiply rapidly; and their rapid multiplication sometimes causes crowding. When, instead of floating free in the water, they form a thin film on a moist surface, or are imbedded in a common matrix of mucilage; the mechanical obstacles to dispersion result in a kind of feeble integration, vaguely shadowing forth a combined group. Somewhat more definite combination is shown us by such plants as Palmella botryoides. Here the members of a family of cells, arising by the spontaneous fission of a parent-cell, remain united by slender threads of that jelly-like substance which envelops their surfaces. In some Diatomaceæ several individuals, instead of completely separating, hold together by their angles; and in other Diatomaceæ, as the Bacillaria, a variable number of units cohere so slightly, that they are continually moving in relation to one another.

This formation of aggregates of the second order, faintly indicated in feeble and variable unions like the above, may be traced through phases of increasing permanence and definiteness, as well as increasing extent. In the yeast-plant, Fig. 7, we have cells which may exist singly, or joined into groups of several; and which have their shapes scarcely at all modified by their connexion. Among the Desmidiaceæ, it happens in many cases that the two individuals produced by division of a parent-individual, part as soon as they are fully formed; but in other cases, instead of parting they compose a group of two. Allied kinds show us how, by subsequent fissions of the adherent individuals and their progeny, there result longer groups; and in some species, a continuous thread of them is thus produced. Figs. 8, 9, 11, exhibit these several stages. Fig. 10 represents a Scenedesmus in which the individuation of the group is manifest. Instead of linear aggregation, many protophytes illustrate central aggregation; as shown in Figs. 12, 13, 14, 15. Other instances are furnished by such forms as the Gonium pectorale, Fig. 16 (a being the front view, and b the edge view), and the Sarcina ventriculi, Fig. 17. Further, we have that spherical mode of aggregation of which the Volvox globator furnishes a familiar instance.

Thus far, however, the individuality of the secondary aggregate is feebly pronounced: not simply in the sense that it is small; but also in the sense that the individualities of the primary aggregates are very little subordinated. But on seeking further, we find transitions towards forms in which the compound individuality is more dominant, while the simple individualities are more obscured. Obscuration of one kind accompanies mere increase of size in the secondary aggregate. In proportion to the greater number of the morphological units held together in one mass, becomes their relative insignificance as individuals. We see this in the irregularly-spreading lichens that form patches on rocks; and in such creeping fungi as grow in films or laminæ on decaying wood and the bark of trees. In these cases, however, the integration of the component cells is of an almost mechanical kind. The aggregate of them is scarcely more individuated than a lump of inorganic matter: as witness the way in which the lichen extends its curved edges in this or that direction, as the surface favours; or the way in which the fungus grows round and imbeds the shoots and leaves that lie in its way, just as so much plastic clay might do. Though here, in the augmentation of mass, we see a progress towards the evolution of a higher type, we have as yet none of that definiteness required to constitute a compound unit, or true aggregate of the second order. Another kind of obscuration of the morphological units, is brought about by their more complete coalescence into the form of some structure made by their union. This is well exemplified among the Confervoideæ and Conjugatæ. In Fig. 18, there are represented the stages of a growing Mougeotia genuflexa, in which this merging of the simple individualities into the compound individuality, is shown in the history of a single plant; and in Figs. 19, 20, 21, 22, 23, are represented a series of species from this group, and that of Cladophora,[4] in which we see a progressing integration. While, in the lower types, the primitive spheroidal forms of the cells are scarcely altered, in the higher types the cells are so fused together as to constitute cylinders divided by septa. Here, however, the indefiniteness is still great. There are no specific limits to the length of any thread thus produced, and there is none of that differentiation of parts required to give a decided individuality to the whole.

To constitute something like a true aggregate of the second order, capable of serving as a compound unit that may be combined with others like itself into still higher aggregates, there must exist both mass and definiteness.

§ 183. An approach towards plants which unite these characters, may be traced in such forms as Bangia ciliaris, Fig. 24. The multiplication of cells here takes place, not in a longitudinal direction only, but also in a transverse direction; and the transverse multiplication being greater towards the middle of the frond, there results a difference between the middle and the two extremities—a character which, in a feeble way, unites all the parts into a whole. Even this slight individuation is, however, very indefinitely marked; since, as shown by the figures, the lateral multiplication of cells does not go on in a precise manner.

From some such type as this there appear to arise, through slight differences in the modes of growth, two closely-allied groups of plants, having individualities somewhat more pronounced. If, while the cells multiply longitudinally, their lateral multiplication goes on in one direction only, there results a flat surface, as in the genus Ulva (Sea-lettuce) or in the upper part of the thallus of Enteromorpha Linza, Fig. 25; or where the lateral multiplication is less uniform in its rate, in types like Fig. 26. But where the lateral multiplication occurs in two directions transverse to one another, a hollow frond may be produced—sometimes irregularly spheroidal, and sometimes irregularly tubular; as in Enteromorpha intestinalis, Fig. 27. And often, as in Enteromorpha compressa, Fig. 28, and other species, this tubular frond becomes branched. Figs. 29 and 30 are magnified portions of such fronds, showing the simple cellular aggregation which allies them with the preceding forms.

In the common Fuci of our coasts, other and somewhat higher stages of this integration are displayed. We have fronds preserving something like constant breadths and dividing dichotomously with approximate regularity. Though the subdivisions so produced are not to be regarded as separate fronds, but only as extensions of one frond, they foreshadow a higher degree of composition; and by the comparatively methodic way in which they are united, give to the aggregate a more definite, as well as a more complex, individuality. Many of the higher lichens exhibit an analogous advance. While in the lowest lichens, the different parts of the thallus are held together only by being all attached to the supporting surface, in the higher lichens the thallus is so far integrated that it can support itself by attachment to such surface at one point only. And then, in still more developed kinds, we find the thallus assuming a dichotomously-branched form, and so gaining a more specific character as well as greater size.

Where, as in types like these, the morphological units show an inherent tendency to arrange themselves in a manner which is so far constant as to give characteristic proportions, we may say that there is a recognizable compound individuality. Considering the Thallophytes which grow in this way apart from their kinships, and wholly with reference to their morphological composition, we might not inaptly describe them as pseudo-foliar.

§ 184. Another mode in which aggregation is so carried on as to produce a compound individuality of considerable definiteness, is variously displayed among other families of Algæ. When the cells, instead of multiplying longitudinally alone, and instead of all multiplying laterally as well as longitudinally, multiply laterally only at particular places, they produce branched structures.

Indications of this mode of aggregation occur among the Confervoideæ, as shown in Figs. 22, 23. Though, in some of the more-developed Algæ which exhibit the ramified arrangement in a higher degree, the component cells are, like those of the lower Algæ, united together end to end, in such way as but little to obscure their separate forms, as in Cladophora Hutchinsiæ, Fig. 31; they nevertheless evince greater subordination to the whole of which they are parts, by arranging themselves more methodically. Still further pronounced becomes the compound individuality when, while the component cells of the branches unite completely into jointed cylinders, the component cells of the stem form an axis distinguished by its relative thickness and complexity. Such types of structures are indicated by Figs. 32, 33—figures representing small portions of plants which are quite tree-like in their entire outlines. On examining Figs. 34, 35, 36, which show the structures of the stems in these types, it will be seen, too, that the component cells in becoming more coherent, have undergone changes of form which obscure their individualities more than before. Not only are they much elongated, but they are so compressed as to be prismatic rather than cylindrical. This structure, besides displaying integration of the morphological units carried on in two directions instead of one; and besides displaying this higher integration in the greater merging of the individualities of the morphological units in the general individuality; also displays it in the more pronounced subordination of the branches and branchlets to the main stem. This differentiation and consolidation of the stem, brings all the secondary growths into more marked dependence; and so renders the individuality of the aggregate more decided.

We might not inappropriately call this type of structure pseud-axial. It simulates that of the higher plants in certain superficial characters. We see in it a primary axis along which development may continue indefinitely, and from which there bud out, laterally, secondary axes of like nature, bearing like tertiary axes; and this is a mode of growth with which Phænogams make us familiar.

§ 185. Some of the larger Algæ supply examples of an integration still more advanced; not simply inasmuch as they unite much greater numbers of morphological units into continuous masses, but also inasmuch as they combine the pseudo-foliar structure with the pseud-axial structure. Our own shores furnish an instance of this in the common Laminaria; and certain gigantic Laminariaceæ of the Antarctic seas, furnish yet better instances. In Necrocystis the germ develops a very long slender stem, which eventually expands into a large bladder-like or cylindrical air-vessel; and the surface of this bears numerous leaf-shaped expansions. Another kind, Lessonia fuscescens, Fig. 37, shows us a massive stem growing up through water many feet deep—a stem which, bifurcating as it approaches the surface, flattens out the ends of its subdivisions into fronds like ribands. These, however, are not true foliar appendages, since they are merely expanded continuations of the stem. In Egregia branches of the thallus not only take the form of leaves, but these are differentiated into several categories in accordance with a division of labour. In any of these Laminariaceæ the whole plant, great as may be its size, and made up though it seems to be of many groups of morphological units, united into a compound group by their marked subordination to a connecting mass, is nevertheless a single thallus, which is added to by intercalary growth at the “transition place,” at the junction of the stem-like and leaf-like portions. The aggregate is still an aggregate of the second order.

But among certain of the highest Algæ, we do find something more than this union of the pseud-axial with the pseudo-foliar structure. In addition to pseud-axes of comparative complexity; and in addition to pseudo-folia that are like leaves, not only in their general shapes but in having mid-ribs and even veins; there are the beginnings of a higher stage of integration. Figs. 38, 39, and 40, show some of the steps. In Rhodymenia palmata, Fig. 38, the parent-frond is comparatively irregular in form, and without a mid-rib; and along with this very imperfect integration, we see that the secondary fronds growing from the edges are distributed very much at random, and are by no means specific in their shapes. A considerable advance is displayed by Phyllophora rubens, Fig. 39. Here the frond, primary, secondary, or tertiary, betrays some approach towards regularity in both form and size; by which, as also by its partially-developed mid-rib, there is established a more marked individuality; and at the same time, the growth of the secondary fronds no longer occurs anywhere on the edge, in the same plane as the parent-frond, but from the surface at specific places. Delesseria sanguinea, Fig. 40, illustrates a much more definite arrangement of the same kind. The fronds of this plant, quite regularly shaped, have their parts decidedly subordinated to the whole; and from their mid-ribs grow other fronds which are just like them. Each of these fronds is an organized group of those morphological units which we distinguish as aggregates of the first order. And in this case, two or more such aggregates of the second order, well individuated by their forms and structures, are united together; and the plant composed of them is thus rendered, in so far, an aggregate of the third order.

Just noting that in certain of the most developed Algæ, as the Sargassum, or common gulf-weed, this tertiary degree of composition is far more completely displayed, so as to produce among Thallophytes a type of structure closely simulating that of the higher plants, let us now pass to the consideration of these higher plants.

§ 186. Having the surface of the soil for a support and the air for a medium, terrestrial plants are mechanically circumstanced in a manner widely different from that in which aquatic plants are circumstanced. Instead of being buoyed up by a surrounding fluid of specific gravity equal to their own, they have to erect themselves into a rare fluid which yields no appreciable support. Further, they are dissimilarly conditioned in having two sources of nutriment in place of one. Unlike the Algæ, which derive all the materials for their tissues from the water bathing their entire surfaces, and use their roots only for attachment, most of the plants which cover the Earth’s surface, absorb part of their food through their imbedded roots and part through their exposed leaves. These two marked unlikenesses in the relations to surrounding conditions, profoundly affect the respective modes of growth. We must duly bear them in mind while studying the further advance of composition.

The class of plants to which we now turn—that of the Archegoniatæ—is nearly related by its lower members to the classes above dealt with: so much so, that some of the inferior liverworts are quite licheniform, and are often mistaken for lichens. Passing over these, let us recommence our synthesis with such members of the class as repeat those indications of progress towards a higher composition, which we have just observed among the more-developed Algæ. The Jungermanniaceæ furnish us with a series of types, clearly indicating the transition from an aggregate of the second order to an aggregate of the third order. Figs. 41 and 42, indicate the structure among the lowest of this group. Here there is but an incomplete development of the second order of aggregate. The frond grows as irregularly as the thallus of a lichen: it is indefinite in size and outline, spreading hither or thither as the conditions favour. Moreover, it lacks the differentiations required to subordinate its parts to the whole: it is uniformly cellular, having neither mid-rib nor veins; and it puts out rootlets indifferently from all parts of its under surface. In Fig. 43, Pellia epiphylla, we have an advance on this type. There is here, as shown in the transverse section, Fig. 44, a thickening of the frond along its central portion, producing something like an approach towards a mid-rib; and from this the rootlets are chiefly given off. The outline, too, is much less irregular; whence results greater distinctness of the individuality. A further step is displayed in Metzgeria furcata, Fig. 45. The frond of this plant, comparatively well integrated by the distribution of its substance around a decided mid-rib, and by its comparatively-definite outlines, produces secondary fronds. There is what is called proliferous growth; and occasionally, as shown in Fig. 46, representing an enlarged portion, the growth is doubly-proliferous. In these cases, however, the tertiary aggregate, so far as it is formed, is but very feebly integrated; and its integration is but temporary. For not only do these younger fronds that bud out from the mid-ribs of older fronds, develop rootlets of their own; but as soon as they are well grown and adequately rooted, they dissolve their connexions with the parent-fronds, and become quite independent. From these transitional forms we pass, in the higher Jungermanniaceæ, to forms composed of many fronds that are permanently united by a continuous stem. A more-developed aggregate of the third order is thus produced. But though, along with increased definiteness in the secondary aggregates, there is here an integration of them so extensive and so regular, that they are visibly subordinated to the whole they form; yet the subordination is really very incomplete. In some instances, as in Radula complanata, Fig. 47, the leaflets develop roots from their under surfaces, just as the primitive frond does; and in the majority of the group, as in J. capitata, Fig. 48, roots are given off all along the connecting stem, at the spots where the leaflets or frondlets join it: the result being that though the connected frondlets form a physical whole, they do not form, in any decided manner, a physiological whole; since successive portions of the united series, carry on their functions independently of the rest. Finally, the most developed members of the group, whether lineally descended from the less developed or from an early type common to the two, present us with tertiary aggregates which are physiologically as well as physically integrated.[5] Not lying prone like the kinds thus far described, but growing erect, the stem and attached leaflets become dependent upon a single root or group of roots; and being so prevented from carrying on their functions separately, are made members of a compound individual: there arises a definitely-established aggregate of the third degree of composition.

The facts as arranged in the above order are suggestive. Minute aggregates, or cells, the grouping of which we traced in § 182, showed us analogous phases of indefinite union, which appeared to lead the way towards definite union. We see here among compound aggregates, as we saw there among simple aggregates, the establishment of a specific form, and a size that falls within moderate limits of variation. This passage from less definite extension to more definite extension, seems in the one case, as the other, to be accompanied by the result, that growth exceeding a certain rate, ends in the formation of a new aggregate, rather than an enlargement of the old. And on the higher stage, as on the lower, this process, irregularly carried out in the simpler types, produces in them unions that are but temporary; while in the more-developed types, it proceeds in a systematic way, and ends in the production of a permanent aggregate that is doubly compound.

Must we then conclude that as cells, or morphological units, are integrated into a unit of a higher order, which we call a thallus or frond; so, by the integration of fronds, there is evolved a structure such as the above-delineated species possess? Whether this is the interpretation to be given of these plants, we shall best see when considering whether it is the interpretation to be given of plants which rank above them. Thus far we have dealt only with the Cryptogamia. We have now to deal with the Phanerogamia or Phænogamia.