|March 16, 2012||Filled under All text of Paley Natural Theology, Chapter 20||
I think a designed and studied mechanism to be, in general, more evident in animals than in plants; and it is unnecessary to dwell upon a weaker argument, where a stronger is at hand. There are, however, a few observations upon the vegetable kingdom, which lie so directly in our way, that it would be improper to pass by them without notice.
The one great intention of nature in the structure of plants, seems to be the perfecting of the seed; and, what is part of the same intention, the preserving of it until it be perfected. This intention shows itself, in the first place, by the care which appears to be taken, to protect and ripen, by every advantage which can be given to them of situation in the plant, those parts which most immediately contribute to fructification, viz. the anthræ, the stamina, and the stigmata. These parts are usually lodged in the centre, the recesses, or the labyrinths of the flower; during their tender and immature state, are shut up in the stalk, or sheltered in the bud: as soon as they have acquired firmness of texture sufficient to bear exposure, and are ready to perform the important office which is assigned to them, they are disclosed to the light and air, by the bursting of the stem, or the expansion of the petals; after which, they have, in many cases, by the very form of the flower during its blow, the light and warmth reflected upon them from the concave side of the cup. What is called also the sleep94 of plants, is the leaves or petals disposing themselves in such a manner as to shelter the young stems, buds, or fruit. They turn up, or they fall down, according as this purpose renders either change or position requisite. In the growth of corn, whenever the plant begins to shoot, the two upper leaves of the stalk join together, embrace the ear, and protect it till the pulp has acquired a certain degree of consistency. In some water plants, the flowering and fecundation are carried on within the stem, which afterwards opens to let loose the impregnated seed95. The pea or papilionaceous tribe, enclose the parts of fructification within a beautiful folding of the internal blossom, sometimes called, from its shape, the boat or keel; itself also protected under a penthouse formed by the external petals. This structure is a very artificial; and what adds to the value of it, though it may diminish the curiosity, very general. It has also this farther advantage, (and it is an advantage strictly mechanical,) that all the blossoms turn their backs to the wind, whenever the gale blows strong enough to endanger the delicate parts upon which the seed depends. I have observed this a hundred times in a field of peas in blossom. It is an aptitude which results from the figure of the flower, and, as we have said, is strictly mechanical; as much so as the turning of a weather-board or tin cap upon the top of a chimney. Of the poppy, and of many similar species of flowers, the head, while it is growing, hangs down, a rigid curvature in the upper part of the stem giving to it that position; and in that position it is impenetrable by rain or moisture. When the head has acquired its size, and is ready to open, the stalk erects itself, for the purpose, as it should seem, of presenting the flower, and with the flower, the instruments of fructification, to the genial influence of the sun’s rays. This always struck me as a curious property; and specifically as well as originally, provided for in the constitution of the plant, for, if the stem be only bent by the weight of the head, how comes it to straighten itself when the head is the heaviest? These instances show the attention of nature to this principal object, the safety and maturation of the parts upon which the seed depends.
In trees, especially in those which are natives of colder climates, this point is taken up earlier. Many of these trees (observe in particular the ash and the horse-chestnut) produce the embryos of the leaves and flowers in one year, and bring them to perfection the following. There is a winter therefore to be gotten over. Now what we are to remark is, how nature has prepared for the trials and severities of that season. These tender embryos are, in the first place, wrapped up with a compactness which no art can imitate; in which state they compose what we call the bud. This is not all. The bud itself is enclosed in scales; which scales are formed from the remains of past leaves, and the rudiments of future ones. Neither is this the whole. In the coldest climates a third preservative is added, by the bud having a coat of gum or resin, which, being congealed, resists the strongest frosts. On the approach of warm weather, this gum is softened, and ceases to be a hinderance to the expansion of the leaves and flowers. All this care is part of that system of provisions, which has for its object and consummation the production and perfecting of the seeds.
The seeds themselves are packed up in a capsule, a vessel composed of coats, [Pl. XXXIV. fig. 1,] which, compared with the rest of the flower, are strong and tough. From this vessel projects a tube, through which tube the farina, or some subtile fecundating effluvium that issues from it, is admitted to the seed. And here also occurs a mechanical variety, accommodated to the different circumstances under which the same purpose is to be accomplished. In the flowers which are erect, the pistil is shorter than the stamina; [Pl. XXXIV. fig. 2,] and the pollen, shed from the anthræ into the cup of the flower, is caught in its descent by the head of the pistil, called the stigma. But how is this managed when the flowers hang down, (as does the crown imperial, for instance,) and in which position the farina, in its fall, would be carried from the stigma, and not towards it? The relative length of the parts is now inverted. The pistil in these flowers is usually longer instead of shorter than the stamina, [Pl. XXXIV. fig. 3,] that its protruding summit may receive the pollen as it drops to the ground. In some cases (as in the nigellal,) [Pl. XXXIV. fig. 4,] where the shafts of the pistils or styles are disproportionably long, they bend down their extremities upon the anthræ, that the necessary approximation may be effected96.
94“The periodical change in the direction of leaves, which has been called the ‘Sleep of Plants,’ is undeniably connected with the stimulating operation of light. It is established, that during the clear light of the sun, the leaves become erect, and move their upper surface to the light, whilst, on the contrary, during the absence of light, they either hang downwards, and turn to the horizon, or they take an upright position, so that the under surface of the leaves is turned more outward. On account of this particular position of what has been called ‘Sleeping Plants,’ we cannot properly ascribe this direction to sleep, because the leaves do sometimes even raise themselves during this state with greater energy, and press upon the stem or leaf-stalk, for the purpose of turning their lower surface outwards. This change is much rather, therefore, the consequence of the contest between the activity of the plant, and the great activity of nature. This change is the more evident, and the sleep leaves the more striking, the finer and more compounded the organization of the leaves are. We hence most frequently observe it in the pinnated leave of leguminous plants, although also in some others, as in atriplex.
That an internal and self-dependent activity is to be taken into account in this sleep of plants, is plain from the fact that this sleep does not equally follow from a short withdrawing of the light, but only from its complete and long-continued removal; as also from this other circumstance, that leaves fall asleep or awake at fixed hours, whether the sky be serene or troubled, exactly as happen with regard to animals. Other stimula, too, and especially heat, have a great influence upon this phenomenon, because, in the cold, leaves awaken later, and fall more easily asleep, notwithstanding the influence of light.” Vide Elements of the Philosophy of Plants by Decandolle.—Paxton.
95Phil. Trans. pa[e.pk] 1796; p. 502
96Amongst the various means which nature has provided for the purpose of assisting the impregnation of plants, that afforded by the agency of insects is not one of the least. In the spring and summer month, numerous species of these lively little beings may be seen in almost every expanded flower; and whether they are in search of honey, which is contained in the nectaries of many flowers, or whatever may be the object of their attraction, by being continually on the move, they, no doubt, further the dispersion of the pollen, and thus, in a great measure, contribute to the fertility of the plants they visit.
In many plants, as those which belong to the Linnaean class diœcia, where the stamens and pistils are in separate flowers, and those flowers situated on two separate plants of the same species, the operation of insects, or the efficacy of winds, is indispensably necessary to the perfecting the fruit, by transporting the pollen of the one to the stigma of the other.
Some plants, indeed, that have perfect, or united flowers, have the anthers so situated that it is almost impossible the pollen can, of itself, reach the stigma; in this case insects generally become the auxiliaries to the fertilization of the seed. An instance of this may be seen in the aristolochia clematitis. “According to Professor Willdenow, the flower of this plant is so formed, that the anthers of themselves cannot impregnate the stigma; but this important affair is devolved upon a particular species of tipula, (T. pennicornis.) The throat of the flower is lined with dense hair, pointing downward so as to form a kind of funnel, or entrance like that of some kinds of mouse-traps, through which the insects may easily enter but not return: several creep in, and, uneasy at their confinement, are constantly moving to and fro, and so deposit the pollen upon the stigma: but when the work intrusted to them is completed, and impregnation has taken place, the hair which prevented their escape shrinks, and adheres closely to the sides of the flower, and these little go-betweens of Flora at length leave their prison. A writer, however, in the Annual Medical Review (ii. 400,) doubts the accuracy of this fact, on the ground that he could never find T. pennicornis, though A. clemititis has produced fruit two years at Brompton.” Introduction to Entomology, by Kirby and Spence, vol. i. p. 298.
That the tipula pennicornis does enter the flowers of aristolochia clematitis, as recorded by Professor Willdenow, I can confidently affirm, from having observed them in great plenty in the inflated base of the corolla every year, for these last fifteen years, in the Oxford Botanic Garden, where the plant generally forms fruit. The first time I found this insect in the flowers of the above species of aristolochia, was on the 12th of July, 1812, at Godstow, near Oxford, where the plant was then growing in a wild state near the ruins of the nunnery.
For the above observations the editor is indebted to an excellent botanist, Mr. W. Baxter.—Paxton.