An introduction to physiological and systematical botany/Chapter 20
OF THE PECULIAR FUNCTIONS OF THE STAMENS AND PISTILS, WITH THE EXPERIMENTS AND OBSERVATIONS OF LINNÆUS AND OTHERS ON THAT SUBJECT.
The Stamens and Pistils of flowers have, from the most remote antiquity, been considered as of great importance in perfecting the fruit. The Date Palm, from time immemorial a primary object of cultivation in the more temperate climates of the globe, bears barren and fertile flowers on separate trees. The ancient Greeks soon discovered that in order to have abundant and well-flavoured fruit, it was expedient to plant both trees near together, or to bring the barren blossoms to those which were to bear fruit; and in this chiefly consisted the culture of that valuable plant. Tournefort tells us that without such assistance dates have no kernel, and are not good food. The same has long been practised, and is continued to this very day in the Levant, upon the Pistacia, and the Fig.
The real use of the Stamens of Plants was long a subject of dispute among philosophers, till Linnæus, according to the general opinion at present, explained it beyond a possibility of doubt. Still there are not wanting persons who from time to time start objections, prompted either by a philosophical pursuit of truth, or an ambitious desire of distinguishing themselves in controverting so celebrated a doctrine, as some have written against the circulation of the animal blood. I propose to trace the history of this doctrine, and especially to review the facts and experiments upon which Linnæus founded his opinion, as well as the objections it has had to encounter. It would be endless, and altogether superfluous, to bring forward new facts in its support, nor shall I do so, except where new arguments may render such a measure necessary.
About the year l676, Sir Thomas Millington, Savilian Professor at Oxford, is recorded to have hinted to Dr. Grew that the use of the Stamens was probably to perfect and fertilize the seed. Grew adopted the idea, and the great Ray approved it. Several other botanists either followed them, or had previously conceived the same opinion, among which R. J. Camerarius, Professor at Tubingen towards the end of the seventeenth century, was one of the most able and original. Vaillant wrote an excellent oration on the subject, which being hostile to the opinions of Tournefort, lay in obscurity till published by Boerhaave. Blair and Bradley assented in England, and several continental botanists imbibed the same sentiments. Pontedera, however, at Padua, an university long famous, but then on the decline, and consequently adverse to all new inquiry and information, in 1720 published his Anthologia, quite on the other side of the question.
Linnæus, towards the year 1732, reviewed all that had been done before him, and clearly established the fact so long in dispute, in his Fundamenta and Philosophia Botanica. He determined the functions of the Stamens and Pistils, proved these organs to be essential to every plant, and thence conceived the happy idea of using them for the purpose of systematical arrangement. In the latter point his merit was altogether original; in the former he made use of the discoveries and remarks of others, but set them in so new and clear a light, as in a manner to render them his own.
We have already mentioned, p. 138, the two modes by which plants are multiplied, and have shewn the important difference between them. Propagation by seed is the only genuine reproduction of the species, and it now remains to prove that the essential organs of the flower are indispensably requisite for the perfecting of the seed.
Every ane must have observed that the flower of a plant always precedes its fruit. To this the Meadow Saffron, Engl. Bot. t. 133, seems an objection, the fruit and leaves being perfected in the spring, the blossoms not appearing till autumn; but a due examination will readily ascertain that the seed-bud formed in autumn is the very same which comes to maturity in the following spring. A Pine-apple was once very unexpectedly cited to me as an instance of fruit being formed before the flower, because the green fruit in that instance, as in many others, is almost fully grown before the flowers expand. The seeds however, the essence of the fruit, are only in embryo at this period, just as in the germen of an Apple blossom.It was very soon ascertained that flowers are invariably furnished with Stamens and Pistils, either in the same individual, or two of the same species, however defective they may be in other parts; of which Hippuris, Engl. Bot. t. 763, the most simple of blossoms, is a remarkable example. Few botanists indeed had detected them in the Lemna or Duck-weed, so abundant on the surface of still waters, and Valisneri alone for a long time engrossed the honour of having seen them. In our days however they rewarded the researches of the indefatigable Ehrhart in Germany, and on being sought with equal acuteness, were found in England. Three species have been delineated in Engl. Bot. t. 926, 1095 and 1233, from the discoveries of Mr. Turner and Mr. W. Borrer. The flowers of Mosses, long neglected and afterwards mistaken, were faithfully delineated by Micheli, carefully examined and properly understood by Linnæus as he rambled over the wilds of Lapland, and at length fully illustrated and placed out of all uncertainty by the justly celebrated Hedwig. These parts indeed are still unknown in ferns, or at least no satisfactory explanation of them has reached me, though the seeds and seed-vessels are sufficiently obvious.
Plants indeed have occasionally abortive stamens in one flower and barren pistils in another, and the Plantain-tree, Musa, is described by Linnæus as having five out of its six stamens perfected in such blossoms as ripen no fruit, while those with a fertile germen contain only a single ripe stamen, five being ineffective. This only shews the resources, the wisdom, and the infinite variety of the creation. When the roots are luxuriantly prolific, the flowers are in some measure defective, Nature, relaxing as it were from her usual solicitude, and allowing her children to repose, and indulge in the abundance of good things about them. But when want threatens, she instantly takes the alarm; all her energies are exerted to secure the future progeny, even at the hazard of the parent stock, and to send them abroad to colonise more favourable situations. The existence of the parts under consideration is so incontrovertible in every flower around us, that Pontedera was reduced to seek plants without stamens among the figures of the Hortus Malabaricus, but the plates in which he confided are now known to be faulty in that very particular.
Most generally the access of the pollen is not trusted to any accidental modes of conveyance, however numerous, elaborate, and, if we may so express it, ingenious, such modes may be; but the Stamens are for greater security lodged in the same flower, under the protection of the same silken veils, or more substantial guards, which shelter their appropriate pistils. This is the case with the majority of our herbs and shrubs, and even with the trees of hot countries, whose leaves being always present might impede the passage of the pollen. On the contrary, the trees of cold climates have generally separated flowers, blossoming before the leaves come forth, and in a windy season of the year; while those which blossom later, as the Oak, are either peculiarly frequented by insects, or, like the numerous kinds of Fir, have leaves so little in the way, and pollen so excessively abundant, that impregnation can scarcely fail.
The pollen and the stigma are always in perfection at the same time, the latter commonly withering and falling off a little after the anthers, though the style may remain to become an useful appendage to the fruit. The Viola tricolor or Pansy, the Gratiola, the Martynia, and many plants besides, have been observed to be furnished with a stigma gaping only at the time the pollen is ripe. The beautiful Jacobæan Lily, Amaryllis formosissima, Curt. Mag. t. 47, is justly described by Linnæus as provided with a drop of clear liquid, which protrudes every morning from the stigma, and about noon seems almost ready to fall to the ground. It is however reabsorbed in the afternoon, having received the pollen whose vapour renders it turbid, and whose minute husks afterwards remain upon the stigma. The same phenomenon takes place several successive days.
In opposition to similar facts, proving the synchronous operation of these organs, Pontedera has, with more observation than usual, remarked that in the umbelliferous tribe the style frequently does not appear till the anthers are fallen. But he ought to have perceived that the stigma is previously perfected, and that the style seems to grow out afterwards, in a recurved and divaricated form, for the purpose of providing hooks to the seeds. It is also observable that in this family the several organs are sometimes brought to perfection in different flowers at different times, so that the anthers of one may impregnate the stigmas of another, whose stamens were abortive, or long since withered. The same thing happens in other instances. Linnæus mentions the Jatropha urens as producing flowers with stamens some weeks in general before or after the others. Hence he obtained no seed till he preserved the pollen a month or more in paper, and scattered it on a few stigmas then in perfection. There can be no doubt that, in a wild state, some or other of the two kinds of blossoms are ripe together, throughout the flowering season, on different trees.
A similar experiment to that just mentioned was made in 1749 upon a Palm-tree at Berlin, which for want of pollen had never brought any fruit to perfection. A branch of barren flowers was sent by the post from Leipsic, twenty German miles distant, and suspended over the pistils. Consequently abundance of fruit was ripened, and many young plants raised from the seeds
Tournefort and Pontedera supposed the pollen to be of an excrementitious nature, and thrown off as superfluous. But its being so curiously and distinctly organized in every plant, and producing a peculiar vapour on the accession of moisture, shows, beyond contradiction, that it has functions to perform after it has left the anther. The same writers conceived that the stamens might possibly secrete something to circulate from them to the young seeds; an hypothesis totally subverted by every flower with separated organs, whose stamens could circulate nothing to germens on a different branch or root; a difficulty which the judicious Tournefort perceived, and was candid enough to allow.
t. 1433, stripping off the rest of that day's blossoms. Another morning he repeated the same practice, only sprinkling the stigma of that blossom, which he had last deprived of its own stamen's, with the pollen from another. The flower first mutilated produced no fruit, but the second afforded very perfect seed. "My design," says Linnæus, "was to prevent any one in future from believing that the removal of the anthers from a flower was in itself capable of rendering the germen abortive."Both the conjectures just mentioned vanish before one luminous experiment of Linnæus, of all others the most easy to repeat and to understand. He removed the anthers from a flower of Glaucium phœnicium; Engl. Bot.
The usual proportion and situation of stamens with respect to pistils is well worthy of notice. The former are generally shortest in drooping flowers, longest in erect ones. The barren blossoms stand above the fertile ones in Carex, Coix, Ricinus, Arum, &c., that the pollen may fall on the stigmas. This is the more remarkable, as the usual order of Nature seems in such plants, as well indeed as in compound, and even umbelliferous flowers, to be reversed, for the pistils are invariably central, or internal, in every simple flower, and would therefore, if drawn out into a monoecious spike, be above the stamens.
Many curious contrivances of Nature serve to bring the anthers and stigmas together. In Gloriosa, Andr. Repos. t. 129, the style is bent, at a right angle from the very base, for this evident purpose. In Saxifraga, and Parnassia, Engl. Bot. t. 82, the stamens lean one or two at a time over the stigma, retiring after they have shed their pollen, and giving place to others; which wonderful œconomy is very striking in the garden Rue, Ruta gravæolens, whose stout and firm filaments cannot be disturbed from the posture in which they may happen to be, and evince a spontaneous movement unaffected by external causes. The five filaments of the Celosia, Cock's-comb, are connected at their lower part by a membranous web, which in moist weather is relaxed, and the stamens spread for shelter under the concave lobes of the corolla. When the air is dry the contraction of the membrane brings them together, to scatter their pollen in the centre of the flower. The elastic filaments of Parietaria, Engl. Bot. t. 879, for a while restrained by the calyx, as those of the lovely Kalmiæ, Curt. Mag. t. 175, 177, are by the minute pouches in the corolla, relieve themselves by an elastic spring, which in both instances serves to dash the pollen with great force upon the stigma. The same end is accomplished by the curved germen of Medicago falcata, Engl. Bot. t. 1016, releasing itself by a spring from the closed keel of the flower.
But of all flowers that of the Barberry-bush, t. 49, is most worthy the attention of a curious physiologist. In this the six stamens, spreading moderately, are sheltered under the concave tips of the petals, till some extraneous body, as the feet or trunk of an insect in search of honey, touches the inner part of each filament near the bottom. The irritability of that part is such, that the filament immediately contracts there, and consequently strikes its anther, full of pollen, against the stigma. Any other part of the filament may be touched without this effect, provided no concussion be given to the whole. After a while the filament retires gradually, and may again be stimulated; and when each petal, with its annexed filament, is fallen to the ground, the latter on being touched shows as much sensibility as ever. See Tracts on Nat. History, 165. I have never detected any sympathy between the filaments, nor is any thing of the kind expressed in the paper just mentioned, though Dr. Darwin, from some unaccountable misapprehension, has quoted me to that effect. It is still more wonderful that the celebrated Bonnet, as mentioned in Senebier's Physiologie Végétale, v. 5. 105, should have observed this phænomenon in the Barberry so very inaccurately as to compare it to the relaxation of a spring, and that the ingenious Senebier himself, in quoting me, p. 103, for having ascertained the lower part only of each filament to be irritable, should express himself as follows:—"It has not yet been proved that the movement of the stamens is attended with the contraction of the filaments; which nevertheless was the first proof necessary to have been given in order to ascertain their irritability; it is not even yet well known which is the irritable part of the filaments, and whether it be only their base, as Smith has had the address to discover." In answer to which I need only request any one to read the above account, or the more ample detail in my original paper, and above all, to examine a Barberry-blossom for himself; and if any doubts remain concerning the existence of vegetable irritability, let him read Senebier's whole chapter intended to disprove it, where that candid philosopher, while he expresses his own doubts, has brought together every thing in its favour. Among the whole of his facts nothing is more decisive than the remarks of Coulomb and Van Marum on the Euphorbia, whose milky juices flow so copiously from a wound, in consequence of the evident irritability of their vessels; but when the life of the plant is destroyed by electricity, all the flowing is at an end. It is superfluous to add any thing on this subject, and I return to that of the impregnation of flowers.
I have already mentioned that any moisture causes the pollen to explode, consequently its purpose is liable to be frustrated by rain or heavy dews. Linnæus observes that husbandmen find their crops of rye to suffer more from this cause than barley, because in the latter the anthers are more protected by the husks; and that Juniper berries are sparingly, or not at all, produced in Sweden when the flowering season has been wet. The same great observer also remarks, what yearly experience confirms, that Cherry-trees are more certainly fruitful than Pear-trees, because in the former the opening of the anthers is, in each blossom, much more progressive, so that a longer period elapses for the accomplishment of the fertilization of the germen, and there is consequently less chance of its being hindered by a few showers.
To guard against the hurtful influence of nocturnal dews or drenching rains, most flowers either fold their petals together, or hang down their heads, when the sun does not shine; by which, their internal organs are sheltered. In some which always droop, as the Snowdrops Galanthus and Leucojum, Engl. Bot. t. 19 and 621, the Fritillary, t. 622, the Crown Imperial, various species of Campanula, and others, while the over-shadowing corolla keeps off rain, the air has free access underneath to blow the pollen to the stigma. Nor is this drooping caused by the weight of the flowers, for the fruit in most of them is much heavier, and yet stands erept on the very same stalk. The papilionaceous flowers in general spread their wings in fine weather, admitting the sun and air to the parts within; whereas many of them not only close their petals at night, but also derive additional protection from the green leaves of the plant folding closely about them. Convolvulus arvensis, t. 312, Anagallis arvensis, t. 529, Calendula pluvialis, and many others, are well known to shut up their flowers against the approach of rain; whence the Anagallis has been called the Poor Man's Weather-glass. It has been observed by Linnæus that flowers lose this fine sensibility, either after the anthers have performed their office, or when deprived of them artificially; nor do I doubt the fact. I have had reason to think that, during a long continuance of wet, the sensibility of the Anagallis is sometimes exhausted; and it is evident that very sudden thunder-showers often take such flowers by surprise, the previous state of the atmosphere not having been such as to give them due warning.
That parts of vegetables not only lose their irritability, but even their vital principle, in consequence of having accomplished the ends of their being, appears from an experiment of Linnæus upon Hemp. This is a diœcious plant, see p. 306, and Linnæus kept several fertile-flowered individuals in separate apartments from the barren ones, in order to try whether they could perfect their seeds without the aid of pollen. Some few however remained with the barren-flowered plants, and these ripened seed in due time, their stigmas having faded and withered soon after they had received the pollen. On the contrary, the stigmas which had been out of its reach continued green and vigorous, as if in vain expectation, nor did they begin to fade till they had thus lasted for a very long while. Since I read the history of this experiment, I have found it easy in many plants to tell by the appearance of the stigma whether the seed be fertilized or not. The above experiment is the more important, as the abbé Spallanzani has recorded one made by himself upon the same species of plant, with a contrary result. But as he has said nothing of the appearance of the stigmas, his experiment must yield to that of Linnæus in point of accuracy; and even if his account be otherwise correct, the result is easily explained. Hemp, Spinach, some Nettles, &c., naturally diœcious, are occasionally not completely so, a few latent barren or fertile flowers being frequently found among those of the other sort, by which provision is made against accidents, and the perfecting of a few seeds, at any rate, secured.
In general, germens whose stigmas have not received the pollen wither away without swelling at all, but some grow to a considerable size, and in such the substance of the seed, its skin, and even its cotyledons, are often to be found, the embryo only being wanting. In a Melon or Cucumber it is common to find, among numerous perfect seeds, many mere unimpregnated husks. In the magnificent Cycas revoluta which bore fruit at the bishop of Winchester's, and of which a history with plates is given in the sixth volume of the Linnæan Society's Transactions, I found the drupa and all its contents apparently perfect, except that there was only a minute cavity where the embryo shuld have been, in consequence of the want of another tree with stamens, which was not to be found perhaps nearer than Japan. Gardeners formerly attempted to assist Nature by stripping off the barren flowers of Melons and Cucumbers, which, having no germen, they found could not come to fruit, and were therefore, as they supposed, an unnecessary encumbrance to the constitution of the parent plant. But finding they thus obtained no fruit at all, they soon learned the wiser practice of admitting air as often as possible to the flowering plants, for the purpose of blowing the pollen from one blossom to the other, and even to gather the barren kind and place it over that destined to bear fruit.
The œconomy of various aquatic plants throws great light upon the subject before us. Different species of Potamogeton, Engl. Bot. t. 168, 297, 376, &c., Ruppia maritima, t. 136, and others, float entirely under water, often at some considerable depth, till the flowering season arrives, when they rise near the surface, and, throw up their flowering spikes above it, sinking afterwards to ripen and sow their seeds at the bottom. Nymphæa alba, t. 160, is very truly described by Linnæus in his Flora Suecica, as closing its flowers in the afternoon and laying them down upon the surface of the water till morning, when it raises and expands them, often, in a bright day, to several inches above the water. To this I can speak from my own knowledge, and it is confirmed by the history given by Theophrastus of his Lotus, which, according to all appearance, is the Nymphæa Lotus of Linnæus. "This," says he, "as well as the Cyamus, bears its fruit in a head. The flower is white, consisting of many crowded leaves about as broad as those of a lily. These leaves at sunset fold themselves together, covering the head (or seed-vessel). At sun-rise they expand, and rise above the water. This they continue till the head is perfected, and the flowers fall off." So far Theophrastus writes as of his own knowledge; he continues as follows: "It is reported that in the Euphrates the head and flowers keep sinking till midnight, when they are so deep in the water as to be out of reach of the hand, but towards morning they return, and still more as the day advances. At sun-rise they are already above the surface, with the flower expanded; afterwards they rise high above the water." Pliny repeats the same account, and Prosper Alpinus, whose purpose is to prove the Lotus of Theophrastus not different from the common Nymphæa, in which, as far as genus is concerned, he is correct, has the following remarkable passage: "The celebrated stories of the Lotus turning to the sun, closing its flowers and sinking under water at night, and rising again in the morning, are conformable to what every body has observed in the Nymphæa."
I have been the more particular in the above quotations, because the veracity of Theophrastus has lately been somewhat rudely impeached, on very questionable authority. For my own part, I think what we see of the Nymphæa in England is sufficient to render the above account highly probable in a country where the sun has so much more power, even if it did not come from the most faithful and philosophical botanist of antiquity, and I have always with confidence cited it on his authority. The reader, however, will perceive that the only important circumstance for our purpose is the closing of the flowers at night, which is sufficiently well established.
But the most memorable of aquatic plants is the Valisneria spiralis, well figured and described by Micheli, Nov. Gen. t. 10, which grows at the bottoms of ditches in Italy. In this the fertile flowers stand on long spiral stalks, and these by uncoiling elevate them to the surface of the water, where the calyx expands in the open air. In the mean while plenty of barren flowers are produced on a distinct root, on short straight stalks, from which they rise like little separate white bubbles, suddenly expanding when they reach the surface, and floating about in such abundance as to cover it entirely. Thus their pollen is scattered over the stigmas of the first-mentioned blossoms, whose stalks soon afterwards resume their spiral figure, and the fruit comes to maturity at the bottom of the water. All this Micheli has described, without being aware of its final purpose; so different is it to observe and to reason!
Some aquatic vegetables, which blossom under water, seem to have a peculiar kind of glutinous pollen, destined to perform its office in that situation, as Chara, Engl. Bot. t. 336, &c.; as well as the Fucus and Conferva tribe: but of the real nature of the fructification of these last we can at present only form analogical conjectures.
The fertilization of the Fig is accomplished in a striking manner by insects, as is that of the real Sycomore, Ficus Sycomorus. In this genus the green fruit is a hollow common calyx, or rather receptacle, lined with various flowers, seldom both barren and fertile in the same fig. This receptacle has only a very small orifice at the summit. The seeds therefore would not in general be perfected, were it not for certain minute flies of the genus Cynips, continually fluttering from one fig to the other all covered with pollen, and depositing their eggs within the cavity.A very curious observation is recorded by Professor Willdenow concerning the Aristolochia Clematitis, Engl. Bot. t. 398. The stamens and pistils of this flower are enclosed in its globular base, the anthers being under the stigma, and by no means commodiously situated for conveying their pollen to it. This therefore is accomplished by an insect, the Tipula pennicornis, which enters the flower by the tubular part. But that part being thickly lined with inflexed hairs, though the fly enters easily, its return is totally impeded, till the corolla fades, when the hairs lie flat against the sides, and allow the captive to escape. In the mean while the insect, continually struggling for liberty, and pacing his prison round and round, has brushed the pollen about the stigma. I do not doubt the accuracy of this account, though I have never caught the imprisoned Tipula. Indeed I have never seen any fruit formed by this plant. Probably for want of some insect adapted to the same purpose in its own country, the American Aristolochia Sipho, though it flowers plentifully, never forms fruit in our gardens.
The ways in which insects serve the same purpose are innumerable. These active little beings are peculiarly busy about flowers in bright sunny weather, when every blossom is expanded, the pollen in perfection, and all the powers of vegetation in their greatest vigour. Then we see the rough sides and legs of the bee, laden with the golden dust, which it shakes off, and collects anew, in its visits to the honeyed stores which invite it on every side. All Nature is then alive, and a thousand wise ends are accomplished by innumerable means that "seeing we perceive not;" for though in the abundance of creation there seems to be a waste, yet in proportion as we understand the subject, we find the more reason to conclude that nothing is made in vain.
- This hitherto unknown fact will appear in his Tour through that country, now preparing for the press in English.
- What species of Palm was the subject of this experiment does not clearly appear. In the original communication to Dr. Watson, printed in the preface of Lee's Introduction to Botany, it is called Palma major foliis flabelliformibus, which seems appropriate to Rhapis flabelliformis, Ait. Hort. Kew. v. 3. 473; yet Linnæus, in his Dissertation on this subject, expressly calls it Phœnix dactylifera, the Date Palm, and says he had in his garden many vigorous plants raised from a portion of the seeds above mentioned. The great success of the experiment, and the "fan-shaped" leaves, make me rather take it for the Rhapis, a plant not well known to Linnæus.
- Exot. Bot. t. 31, 32.