When seeking in mechanical actions and reactions the cause of that indurated structure which forms the vertebrate axis ([§§ 254–7]), it was pointed out that in a transversely-strained mass, the greatest pressures and tensions are thrown on the molecules of the concave and convex surfaces. Hence, supposing the transversely-strained mass to be a cylinder, bent backwards and forwards not in one plane but now in this plane and now in that, its peripheral layers will be those on which the greatest stress falls. An ordinary dicotyledonous axis is such a cylinder so strained. The maintenance of its attitude either as a lateral shoot or a vertical shoot, implies subjection to the bendings caused by its own weight and by the ever-varying wind. These bendings imply tensions and pressures falling most severely first on one side of its outer layers and then on another. And if the dense substance able to resist these tensions and pressures is deposited most where they are greatest, we ought to find it taking the shape of a cylindrical casing. This is just what we do find. On cutting across a shoot in course of formation, we see its central space either unoccupied or occupied only by soft tissue. That the layer of hard tissue surrounding this is not the outermost layer, is true: there lies beyond it the cambium layer, from which it is formed, the phloëm, and the cortex. But outside of the soft phloëm there is frequently another layer of dense tissue now known as the pericyclic fibres, having frequently a tenacity greater even than that of the wood—a layer which, while it protects the cambium and offers additional resistance to the transverse strain, admits of being fissured as fast as the cylinder of wood thickens. That is to say, the deposit of resisting substance is as completely peripheral as the exogenous mode of growth permits. So, too, in general arrangement is it with the ordinary monocotyledonous stem. Different as is here the internal structure, there yet holds the same general distribution of tissues, answering to the same mechanical conditions. The vascular woody bundles, more abundant towards the outside of the stem than near the centre, produce a harder casing surrounding a softer core. In the supporting structures of leaves we find significant deviations from this arrangement. While axes are on the average exposed to equal strains on all sides, most leaves, spreading out their surfaces horizontally, have their petioles subject to strains that are not alike in all directions; and in them the hard tissue is differently arranged. Its transverse section is not ring-shaped but crescent-shaped: the two horns being directed towards the upper surface of the petiole. That this arrangement is one which answers to the mechanical conditions, is not easy to demonstrate: we must satisfy ourselves by noting that here, where the distribution of forces is different, the distribution of resisting tissue is different. And then, showing conclusively the connexion between these differences, we have the fact that in petioles growing vertically and supporting peltate leaves—petioles which are therefore subject to equal transverse strains on all sides—the vascular bundles are arranged cylindrically, as in axes.
Such, then, are some of the reasons for concluding that the development of the supporting tissue in plants, is caused by the incident forces which this tissue has to resist. The individuals in which this direct balancing of inner and outer actions progresses most favourably, are those which, other things equal, are most likely to prosper; and, by habitual survival of the fittest, there is established a systematic and constant distribution of a deposit adapted to the circumstances of each type.
§ 280. The function of circulation may now be dealt with. We have to consider here by what structures this is discharged; and what connexion exists between the demand for them and the genesis of them.
The contrast between the rates at which a dye passes through simple cellular tissue and cellular tissue of which the units have been elongated, indicates one of the structural changes required to facilitate circulation. If placed with its cut surface in a coloured liquid, the parenchyma of a potato or the medullary mass of a cabbage-stalk, will absorb the liquid with extreme slowness; but if the stalk of a fungus be similarly placed, the liquid runs up it, and especially up its loose central substance, very quickly. On comparing the tissues which thus behave so differently, we find that whereas in the one case the component cells, packed close together, have deviated from their primitive sphericity only as much as mutual pressure necessitates, in the other case they are drawn out into long tubules with narrow spaces among them—the greatest dimensions of the tubules and the spaces being in the direction which the dye takes so rapidly. That which we should infer, then, from the laws of capillary action, is experimentally shown: liquid moving through tissues follows the lines in which the elements of the tissues are most elongated. It does this for two reasons. That narrowing of the cells and intercellular spaces which accompanies their elongation, facilitates capillarity; and at the same time fewer of the septa formed by the joined ends of the cells have to be passed through in a given distance. Hence the general fact that the establishment of a rudimentary vascular system, is the formation of bundles of cells lengthened in the direction which the liquid is to take. This we see very obviously among the lower Cormophytes. In one of the lichen-like Liverworts, the veins which, branching through its frond, serve as communications with its scattered rootlets, are formed of cells longer than those composing the general tissue of the frond: the lengths of these cells corresponding in their directions with the lengths of the veins. So, too, is it with the mid-ribs of such fronds as assume more definite shapes; and so, too, is it with the creeping stems which unite many such fronds. That is to say, the current which sets towards the growing part from the part which supplies certain materials for growth, sets through a portion of the tissues composed of units that are longer in the line of the current than at right angles to that line. The like is true of Phænogams. Omitting all other characteristics of those parts of them through which chiefly the currents of sap flow, we find the uniform fact to be that they consist of cells and intercellular spaces distinguished from others by their lengths. It is thus with veins, and mid-ribs, and petioles; and if we wish proof that it is thus with stems, we have but to observe the course taken by a coloured solution into which a stem is inserted.
What is the original cause of this differentiation? Is it possible that this modification of cell-structure which favours the transfer of liquid towards each place of demand, is itself caused by the current which the demand sets up? Does the stream make its own channel? There are various reasons for thinking that it does. In the first place, the simplest and earliest channels, such as we see in the Liverworts, do not develop in any systematic way, but branch out irregularly, following everywhere the irregular lobes of the fronds as these spread; and on examining under a magnifier the places at which the veins are lost in the cellular tissue, it will be seen that the cells are there slightly longer than those around: suggesting that the lengthening of them which produces an extension of the veins, takes place as fast as the growth of the tissue beyond causes a current to pass through them. In the second place, a disappearance of the granular contents of these cells accompanies their union into a vein—a result which the transmission of a current may not improbably bring about. But be the special causes of this differentiation what they may, the evidence favours very much the conclusion that the general cause is the setting up of a current towards a place where the sap is being consumed. In the histological development of the higher plants we find confirmation. The more finished distributing canals in Phænogams are formed of cells previously lengthened. At parts of which the typical structure is fixed, and the development direct, this fact is not easy to trace; the cells rapidly take their elongated structures in anticipation of their predetermined functions. But in places where new vessels are required in adaptation to a modifying growth, we may clearly trace this succession. The swelling root of a turnip, continually having its vascular system further developed, and the component vessels lengthened as well as multiplied, gives us an opportunity of watching the process. In it we see that the reticulated cells which unite to form ducts, arise in the midst of bundles of cells that have previously become elongated, and that they arise by transformation of such elongated cells; and we also see that these bundles of elongated cells have an arrangement suggestive of their formation by passing currents.
Are there grounds for thinking that these further transformations by which strings of elongated cells pass into vessels lined with spiral, annular, reticulated, or other frameworks, are also in any way determined by the currents of sap carried? There are some such grounds.
As just indicated, the only places where we may look for evidence with any rational hope of finding it, are places where some local requirement for vessels has arisen in consequence of some local development which the type does not involve. In these cases we find such evidence. Good illustrations occur in those genera of the Cactaceæ, which simulate leaves, like Epiphyllum and Phyllocactus. A branch of one of these is outlined in Fig. [256]. As before explained this is a flattened axis; and the notches along its edges are the seats of the axillary buds. Most of these axillary buds are arrested; but occasionally one of them grows. Now if, taking an Epiphyllum-shoot which bears a lateral shoot, we compare the parts of it that are near the aborted axillary buds with the part that is near the developed axillary bud, we find a conspicuous difference. In the neighbourhood of an aborted axillary bud there is no external sign of any internal differentiation; and on holding up the branch against the light, the uniform translucency shows that there is no greater amount of dense tissue near it than in other parts of the succulent mass. But where an axillary bud has developed, a prominent rounded ridge joins the mid-rib of the lateral branch with the mid-rib of the parent branch. In the midst of this rounded ridge an opaque core may be seen. And on cutting through it, this opaque core proves to be full of vascular bundles imbedded in woody deposits. Clearly, these clusters of vessels imply transformations of the tissues, caused by the passage of increased currents of sap. The vessels were not there when the axillary bud was formed; they would not have developed had the axillary bud proved abortive; but they arise as fast as growth of the axillary bud draws the sap along the lines in which they lie. Verification is obtained by examining the internal structures. If longitudinal sections be made through a growing bud of Opuntia or Cereus, it will be found that the vessels in course of formation converge towards the point of growth, as they would do if the sap-currents determined their formation; that they are most developed near their place of convergence, which they would be if so produced; and that their terminations in the tissue of the parent shoot are partially-formed lines of irregular elongated cells, like those out of which the vessels of a leaf or bud are developed.
Concluding, then, that sap-vessels arise along the lines of least resistance, through which currents are drawn or forced, the question to be asked is—What physical process produces them? Their component cells, united end to end more or less irregularly in ways determined by their original positions, form a channel much more permeable, both longitudinally and laterally, than the tissue around. How is this greater permeability caused? The idea, first propounded I believe by Wolff, that the adjoined ends of the cells are perforated or destroyed by the passing current, is one for which much is to be said. Whether these septa are dissolved by the liquids they transmit, or whether they are burst by those sudden gushes which, as we shall hereafter see, must frequently take place along these canals, need not be discussed: it is sufficient for us that the septa do, in many cases, disappear, leaving internal ridges showing their positions; and, in other cases, become extremely porous. Though it is manifest that this is not the process of vascular development in tissues that unfold after pr-determined types, since, in these, the dehiscences or perforations of septa occur before such direct actions can have come into play; yet it is still possible that the disappearances of septa which now arise by repetition of the type were established in the type by such direct actions. Be this as it may, however, a simultaneous change undergone by these longitudinally-united cells must be otherwise caused. Frame-works are formed in them—frameworks which, closely fitting their inner surfaces, may consist either of successive rings, or continuous spiral threads, or networks, or structures between spirals and networks, or networks with openings so far diminished that the cells containing them are distinguished as fenestrated. Their differences omitted, however, these structures have the common character that, while supporting the coats of the vessels, they also give special facilities for the passage of liquids, both through the sides of the transformed cells and through their united ends, where these are not destroyed.
To attempt any physical interpretation of this change is scarcely safe: the conditions are so complex. There are reasons for suspecting, however, that it arises from a vacuolation of the substance deposited on the cell-wall. If rapidly deposited, as it is likely to be along lines where sap is freely supplied, this may, in passing from the state of a soluble colloid to that of an insoluble colloid, so contract as to leave uncovered spaces on the cell-membrane; and this change, originally consequent on a physico-chemical action, may be so maintained and utilized by natural selection, as to result in structures of definite kinds, regularly formed in growing parts in anticipation of functions to be afterwards discharged. But, without alleging any special cause for this metamorphosis, we may reasonably conclude that it is in some way consequent upon the carrying of sap. If we examine tissues such as that in the interior of a growing turnip that has not yet become stringy, we may, in the first place, find bundles of elongated cells not having yet developed in them those fenestrated or reticulated structures by which the ducts are eventually characterized. Along the centres of adjacent bundles we may find incomplete lines of such cells—some that are partially or wholly transformed, with some between them that are not transformed. In other bundles, completed chains of such transformed cells are visible. And then, in still older bundles, there are several complete chains running side by side. All which facts imply a metamorphosis of the elongated cells, indirectly caused by the continued action of the currents carried.
§ 281. Here, however, presents itself a further problem. Taking it as manifest that there is a typical distribution of supporting tissue adapted to meet the mechanical strains a plant is exposed to by its typical mode of growth, and also that there goes on special adaptation of the supporting tissue to the special strains the individual plant has to bear; and taking it as tolerably evident that the sap-channels are originally determined by the passage of currents along lines of least resistance; there still remains the ultimate question—Through what physical actions are established these general and special adjustments of supporting tissue to the strains borne, and these distributions of nutritive liquid required to make possible such adjustments? Clearly, if the external actions produce internal reactions; and if this play of actions and reactions results in a balancing of the strains by the resistances; we may rationally suspect that the incident forces are directly conducive to the structural changes by which they are met. Let us consider how they must work.