The Principles of Biology, Volume 2 (of 2)

by Herbert Spencer

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 general principles 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 foliar organ; 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 they develop 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.

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 of the 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.