Figs. 77, 78.
A thin layer of substance gains greatly in power of resisting a transverse strain, if it is bent round so as to form a tube: witness the difference between the pliability of a sheet of paper when outspread, and the rigidity of the same sheet of paper when rolled up. Engineers constantly recognize this truth, in devising appliances by which the greatest strength shall be obtained at the smallest cost of material; and among organisms, we see that natural selection habitually establishes structures conforming to the same principle, wherever lightness and stiffness are to be combined. The cylindrical bones of mammals and birds, and the hollow shafts of feathers, are examples. The lower plants, too, furnish cases where the strength needful for maintaining an upright position, is acquired by this rolling up of a flat thallus or frond. In Fig. [77] we have an Alga which approaches towards a tubular distribution of substance; and which has a consequent rigidity. Sundry common forms of lichen, having the thallus folded into a branched tube, still more decidedly display the connexion between this structural arrangement and this mechanical advantage. And from the particular class of plants we are here dealing with—the Archegoniates—a type is shown in Fig. [78], Riella helicophylla, similarly characterized by a thin frond that is made stiff enough to stand, by an incurving which, though it does not produce a hollow cylinder, produces a kindred form. If, then, as we have seen, natural selection or survival of the fittest will favour such among these recumbent Archegoniates as are enabled, by variations in their structures, to maintain raised postures; it will favour the formation of fronds that curve round upon themselves, and curve round upon the fronds growing out of them. What, now, will be the result should such a modification take place in the group of proliferous fronds represented in Fig. [76]? Clearly, the result will be a structure like that shown in Fig. [79]. And if this inrolling becomes more complete, a form like Jungermannia cordifolia, represented in Fig. [80], will be produced.
Figs. 79, 80.
Figs. 81–89.
Figs. 90, 91.
When the successive fronds are thus folded round so completely that their opposite edges meet, these opposite edges will be apt to unite: not that they will grow together after being formed, but that they will develop in connexion; or, in botanical language, will become “adnate.” That foliar surfaces which, in their embryonic state, are in close contact, often join into one, is a familiar fact. It is habitually so with sepals or divisions of the calyx. In all campanulate flowers it is so with petals. And in some tribes of plants it is so with stamens. We are therefore well warranted in inferring that, under the conditions above described, the successive fronds or leaflets will, by union of their remote edges, first at their points of origin and afterwards higher up, form sheaths inserted one within another, and including the axis. This incurving of the successive fronds, ending in the formation of sheaths, may be accompanied by different sets of modifications. Supposing Fig. [81] to be a transverse section of such type (a being the mid-rib, and b the expansion of an older frond; while c is a younger frond proliferously developed within it), there may begin two divergent kinds of changes, leading to two contrasted structures. If, while frond continues to grow out of frond, the series of united mid-ribs continues to be the channel of circulation between the uppermost fronds and the roots—if, as a consequence, the compound mid-rib, or rudimentary axis, continues to increase in size laterally; there will arise the series of transitional forms represented by the transverse sections 82, 83, 84, 85; ending in the production of a solid axis, everywhere wrapped round by the foliar surface of the frond, as an outer layer or sheath. But if, on the other hand, circumstances favour a form of plant which maintains its uprightness at the smallest cost of substance—if the vascular bundles of each succeeding mid-rib, instead of remaining concentrated, become distributed all round the tube formed by the infolded frond; then the structure eventually reached, through the transitional forms 86, 87, 88, 89, will be a hollow cylinder.[11] And now observe how the two structures thus produced, correspond with two kinds of Monocotyledons. Fig. [90] represents a species of Dendrobium, in which we see clearly how each leaf is but a continuation of the external layer of a solid axis—a sheath such as would result from the infolded edges of a frond becoming adnate; and on examining how the sheath of each leaf includes the one above it, and how the successive sheaths include the axis, it will be manifest that the relations of parts are just such as exist in the united series of fronds shown in Fig. [79]—the successive nodes answering to the successive points of origin of the fronds. Conversely, the stem of a grass, Fig. [91], displays just such relations of parts, as would result from the development of the type shown in Fig. [79], if instead of the mid-ribs thickening into a solid axis, the matter composing them became evenly distributed round the foliar surfaces, at the same time that the incurved edges of the foliar surfaces united. The arrangements of the tubular axis and its appendages, thus resulting, are still more instructive than those of the solid axis. For while, even more clearly than in the Dendrobium, we see at the point b, a continuity of structure between the substance of the axis below the node, and the substance of the sheath above the node: we see that this sheath, instead of having its edges united as in Dendrobium, has them simply overlapping, so as to form an incomplete hollow cylinder which may be taken off and unrolled; and we see that were the overlapping edges of this sheath united all the way from the node a to the node b, it would constitute a tubular axis, like that which precedes it or like that which it includes. And then, giving an unexpected conclusiveness to the argument, it turns out that in one family of grasses, the overlapping edges of the sheaths do unite: thus furnishing us with a demonstration that tubular structures are produced by the incurving and joining of foliar surfaces; and that so, hollow axes may be interpreted as above, without making any assumption unwarranted by fact. One further correspondence between the type thus ideally constructed, and the monocotyledonous type, must be noted. If, as already pointed out, the transverse growth of an axis arises when the axis comes to be a channel of circulation between all the roots at one of its extremities and all the leaves at the other; and if this lateral bulging must increase as fast as the quantity of foliage to be brought in communication with the roots increases—especially if such foliage has at the same time to be raised high above the earth’s surface; what must happen to a plant constructed in the manner just described? The elder fronds or foliar organs, ensheathing the younger ones, as well as the incipient axis serving as a bond of union, are at first of such circumference only as suffices to inclose these undeveloped parts. What, then, will take place when the inclosed parts grow—when the axis thickens while it elongates? Evidently the earliest-formed sheaths, not being large enough for the swelling axis, must burst; and evidently each of the later-formed sheaths must, in its turn, do the like. There must result a gradual exfoliation of the successive sheaths, like that indicated as beginning in the above figure of Dendrobium; which, at a, shows the bud of the undeveloped parts just visible above the enwrapping sheaths, while at b, and c, it shows the older sheaths in process of being split open. That is to say, there must result the mode of growth which helped to give the name Endogens to this class.
