Figs. 92–94.
Figs. 92–94.
Figs. 95–99.
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, andPlagiochila 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 separationof 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 sothere 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:abeing the rudiment of a foliar organ beginning to take shape;bbeing the almost formless rudiment of the next foliar organ; andcbeing the quite-undifferentiated part whence the rudiments of subsequent foliar organs are to arise.
Figs. 100–104.
Figs. 100–104.
Figs. 105–106.
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 ahabit 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 ofMetzgeria furcata, Figs.45, 46,in which such compound prolification is partially displayed. Let us suppose, then, that the fronda, Fig.106, produces not only a single secondary frondb, but also another such secondary frondb’. Let us suppose, further, that the frondbis in like manner doubly proliferous: producing bothcandc’. Lastly, let us suppose that in the second frondb’whichaproduces, as well as in the second frondc’whichbproduces, 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.
Figs. 107–110.
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 generalprinciples 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 foliarorgan; 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 theydevelop 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.
Figs. 111–122.
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 ofthe 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; weare 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.
Figs. 123–129.
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 amuch-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 thatthe 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 theJungermanniaceæ, 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 theDrosera 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; andhence 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 growthsa,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 theAlgæ,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 ofPtilota 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]
Fig. 130.
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 processb, which ordinarily becomes a foliar appendage, differs very little from the terminal processc, 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 wemay 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 higherlevel, 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 productionof 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 leafyJungermanniaceæ, 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 isnot homologous with the liverwort plant-body, since they represent different generations. The liverwort plant-body orgametophyte,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 thesporophyteand 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 andPhanerogams, 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 thePteridophytaare 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 likeAnthoceros, 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 wellgroundedtheory 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 wasnotendogenous, 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 ofsecondary meristemortrue 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 (Dioscoraceæhave 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 thequestion 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 onVaucheria, 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.]