The Principles of Botany, and of Vegetable Physiology, pp. 322-332 (1805)
Karl Ludwig Willdenow
Koelreuter examined, in a very laborious manner, how many globules of pollen might be required to complete an impregnation. His chief discoveries on this point are as follow:
All the anthers of Hibiscus syriacus contained 4863 globules of pollen, 50 or 60 of which were necessary to complete impregnation. But whenever he took less than 50 globules, then not all the seeds ripened, but those, which were formed, were perfect. Ten globules were the least he could take in this flower, as less would not suffice for it. The Mirabilis Jalappa had 293 globules of pollen in one flower, Mirabilis longiflora 321. But in each of the two plants 2 or 3 globules were sufficient for impregnation. The seed did not appear more perfect, though many more globules were put upon the stigma.
To ascertain whether, in flowers with more than one style, each ought to become impregnated separately, Koelreuter in several of them cut all off but one, and the fecundation was as successful as ever. Even in flowers, in which the style was entirely separated, fecundation took place through one of them. These experiments shew, that the hollow tubes of one style communicate with all the rest, and that more styles and more pollen are formed, merely to ensure their final determination. From this circumstance philosophers have concluded, that the cellular texture of all germens fixed in the receptacle, must cohere amongst each other.
The great and wonderful process of generation has led various philosophers to form peculiar, and often very singular hypotheses of their own, which each has tried to establish by a number of arguments. To give an accurate account of all of would be transgressing the bounds of our present researches; it will suffice to mention only the chief of them. Some of the oldest philosophers thought, that an accidental commixtion of solid and liquid parts was sufficient to form, according to circumstances, animals or plants. This was called Generatio aequivoca. Others imagined, that the small animals which were observed in the semen, (animalcula spermatica), go into the ovaries of the mother, and thus form the future being. Others again, believed that in the mother a rudiment of the future animal pre-existed, to which the semen of the male imparted life. This theory was called the pre-formation system, or the Systema praeformationis, predelineationis, or the theory of evolution. Those three appellations properly denoted three different ideas; but in reality they all concur in this one point, that all three suppose a pre-existence of the future being in the mother. Lastly, philosophers alleged, that the fecundating fluids both of female and male become mixed together, and thus give existence to the future animal. This theory was styled, Epigenesis.
The generatio aequivoca, was supposed in former times chiefly to take place in insects, worms and plants, but is now entirely abandoned by all rationalmen. Harvey's principle is now well known, omne vivum ex ovo, and we daily find this truth confirmed by new and bold observations, and the important conclusions of philosophers. I would indeed no longer rest with this old theory, did not some some botanists explain the formation of Fungi, merely by the fermentation of putrifying vegetable matter. What led them to this, was their sudden rise, and the places which some of them always occupy. But there are likewise animals of the shortest duration, and others which are found on certain peculiar spots only, and no where else. To draw any conclusions from such circumstances is rather improper. And now, as the seeds and flowers of these plants have been discovered, this idea will be altogether abandoned. No organic body arises almost in any other way but from ova, (§ 296), and the Generatio aequivoca therefore is a mere nothing.
The theory of animalcula in the semen of animals being carried over to the ovarium of the mother, where the new animal is formed, has Leuwenhoeck for its author. Some therefore, in the vegetable kingdom, assumed pre-existing germs or corcles in the pollen, which in the mother's ovaries unfolded themselves into the future plant. A very zealous supporter of this opinion was Mr Gleichen. Some even went so far as to see, under the microscope, small asses in the semen of an ass, and small lime trees in the pollen of a lime. Strange things may be seen, if persons are disposed to see them. Koelreuter's observations, of which immediately, at once overthrow this doctrine.
The system of pre-formation, which in former times was much in vogue, is not, even by its most zealous admirers, much insisted on in the vegetable kingdom. Spallanzani, who in animals, by means of tedious experiments, attempted to prove the pre-existence existence of the animal, before the impregnation of the ovum in the ovaries, sincerely confesses, that there is no pre-existence of plants like that in animals.
The Epigenesis, or generation by a commixtion of the fluids given out both by male and female, is what most physiologists now assume as the only true theory of generation both in the animal and vegetable kingdoms. Koelreuter confirmed it by numerous experiments, of which we shall mention one only: He took of the genus Nicotiana, the Nicotiana rustica and paniculata. The first he deprived of all its stamens, and fecundated its pistil with pollen of the last species. Nicotiana rustica has egg-shaped leaves, and a short, greenish yellow corol. Nicotiana paniculata, a stem half as long again as the former, and roundish, cordate leaves, and much longer, yellowish green corols. The bastard offspring of both, kept in all its parts the middle betwixt the two species. He tried the same with more plants, and the result accorded perfectly with the first.
Were we therefore to admit the animalcula seminalia, the hybrids could necessarily not have differed in their form from the male plant; and, on the other hand, were the evolution system founded in nature, they would have the same form as the female plant. The hybrid, however, was a medium between both, it therefore certainly adopted some parts both from father and mother, and was formed by Epigenesis.
Koelreuter, however, could only obtain hybrids by intermixing similar plants. Dissimilar plants never produced them, even though, according to our systems, they belonged to one genus. It appears that nature thus avoids unnatural mixtures.
The instance of mules not generating, as it was once believed at least, induced many philosophers to make it an axiom, that hybrids are barren. But we now know a good many instances in Zoology of hybrids being very productive, and even the instance of mules does not prove any thing, as in warm climates they are sometimes prolific.
Koelreuter likewise found hybrids of various species of tobacco and some more plants to be sterile, the pistil in them being very perfect, but the stamens not completely formed. But there are now several instances of hybrid plants which retain their original form, and propagate themselves. I shall only mention a few with their parents;
Sorbus hybrida. The mother was Sorbus aucuparia; the father, Crataegus Aria.
Pyrus hybrida. The mother was Pyrus arbutifolia; the father, Sorbus aucuparia.
Rhamnus hybridus. The mother was Rhamnus alpinus; the father, Rhamnus Alaternus.
What mixtures do not the species of Pelargonium produce in our gardens? All plants of the 21st, 22d, and 23d classes of Linné mostly generate prolific hybrids. Linné wrote a particular treatise on hybrids, in which he attempted to explain the origin of some particular plants; but unfortunately he has given nothing but hypotheses, his observations not according with experience.
Should it not, from the observations made with regard to the hybrids of the animal and vegetable world, be laid down as a rule, admitting some exceptions, that all hybrids are productive, but that some only want a warm climate, to unfold the male semen? I do not attempt to establish this rule as quite certain; I should be happy, on the contrary, would philosophers consider this subject more accurately, and attend more to the hybrids of different climates, on purpose to settle the point.
But Koelreuter made some experiments, which put the doctrine of Epigenesis beyond all doubt. I shall only mention one of his observations as an instance. He obtained, as we have seen, a hybrid from Nicotiana rustica and paniculata. Nicotiana rustica was the female plant, paniculata the male. The hybrid, like all the others which he brought up, had imperfect stamens, and kept the middle between the two species. He afterwards impregnated this hybrid with Nicotiana paniculata, and got plants, which much more resembled the last. This he continued through several generations, till in this way, by due perseverance, he actually changed the Nicotiana rustica into the Nicotiana paniculata. By those and other experiments, often repeated, and made in various ways and upon other plants, it seems clearly established, that there is no pre-formation in plants.
According to the theory of Epigenesis then, the fluids of the male and female are mixed, and an off spring is obtained from these two, which in form and properties resembles both father and mother.
But there have been philosophers, as well in former as at the present times, who in plants have altogether denied the existence of sexes. Smellie seems to favour this opinion, because he repeated an experiment of Spallanzani's, with a female plant of hemp, which he kept remote from all male plants, and notwithstanding obtained, though in a small quantity, perfect seeds, and hence he deduces his arguments. But indeed such experiments are too difficult to be free from error, and who can assert, that he has not, even with the greatest attention, been deceived? Spallanzani placed his female plant in a room, to which no insects could get, and, for the greater security, likewise covered it. But could he, before flie first flower appeared, distinctly enough distinguish the female plant of the hemp? And could not a very small, minute insect escape his eyes, and effect a fecundation? Besides, do we not find on female plants sometimes a few male flowers, which perhaps was here the case? The few seeds which he got, prove, that a few single parts were necessarily fecundated. But even supposing that in hemp the female plant produces ripe seeds without fecundation, can we draw any just conclusion from this single plant to any other vegetable? We have in the animal kingdom an instance in the Aphis, an insect which, without the aid of a pale, propagates itself till autumn. But who would, from from this isolated observation, founded as it is in truth, attempt to deny in all animals the existence of a difference of sex? Sirice Gleditsch first, in a botanic garden, impregnated the Chamaerops humilis, which is a female plant, with pollen of the male plant, which Koelreuter sent to him from Karlsruhe, and obtained ripe seeds and young plants, which before never had been possible, thousands of similar experiments have been made which put it beyond doubt, that two sexes exist in plants. Every person may indeed easily convince himself of the fact, by repeating such experiments on the species, of melon and gourd, and everywhere in the vegetable kingdom, he will find two distinct sexes.
Each seed, as we know, (§ 288), already exists in the germen during the time of blooming, before fecundation takes place, and contains a very clear liquor, called by Malpighi the Chorion. With this, most likely, the fecundating particle of the male semen become mixed, and thus produce the embryo of the new plant. Koelreuter, on the contrary, thinks that the moisture of the stigma, which he, according to his favourite idea of an oily, impregnating fluid of vegetables, supposes likewise to be of the nature of oil, becomes mixed with the fluid of the male, and that these two combined, are conveyed into the seed. However, though this may be true, many other changes take place in the seed sooner or later after fecundation. For in the neighbourhood of the navel a small vesicle appears, filled with some liquid. The first is called the sacculus colliquamenti, and the liquor in it, the amnios. This vesicle grows larger, absorbs the chorion, which at last entirely disappears, and finally becomes the membrana interna of the seed, (§ 114). The amnios grows hard, and forms the cotyledons, (§ 114). As soon as the vesicle shews itself, the embryo of the future plant likewise appears gradually, which is, properly speaking, the corcle, (l. c.). It is formed gradually, and becomes visible in the sunflower, (Helianthus annuus) three days after impregnation; in the cucumber, (Cucumis sativus), a week after; and in Colchicum autunmale, some months after. It is soft in the beginning, but in time becomes, like the vesicle which contains it, of a better size and firmness. The vesicle does not in all seeds increase in the same form, in some it grows larger in its whole circumference, in others it grows longer towards one extremity, and the sides afterwards become extended.
Thus the seed comes to maturity, and when perfectly ripe, separates in different ways from its mother plant, and begins a new life itself, passing through all the scenes, again, just now explained. This is the common way in which plants are progagated. But we have plants, which do it in another way besides evolving their seeds. At the stem, or near the angles of the leaves, nature or even accidents form sometimes knots, which become buds, and separating spontaneously from the plant itself, send out roots and leaves, and form an entirely new plant of the same species. Such plants are called, viviparous plants, (vegetabilia vivipara). Several species of garlic, (allium); the Lilium bulbiferum; Poa bulbosa and other plants, shew this phenomenon spontaneously. The garden tulip, (Tulipa gesneriana), exhibits this curious phenomenon by means of a simple manoeuvre of art, if the flower is cut off, before impregnation has taken place, and the stem with the leaves be allowed to remain, provided it be in a shaded spot. Several succulent plants, for instance, Eucomis punctata, do it when treated in the above manner. Thus gardeners produce a greater number of young plants, by grafting and inoculating with cions, and by other similar processes. The bud of a tree or shrub, when grafted into another stock, will there be unfolded, and must indeed be regarded as a different plant altogether. It is not changed in its nature, but grows as if placed in the earth; the stem only serves to convey the imbibed sap to it, which it must digest itself.
Agricola and Barnes, it appears, were more successful in these operations, for they placed buds directly in earth, and produced perfect young plants.
CybeRose: It is interesting to consider just how rapidly hybridists progressed in the early 19th century. As Willdenow explains, some "philosophers" of his time still held views we now regard as nearly Medieval. But the gardeners, in particular, charged ahead with hybridizing — even crossing plants that the academic philosophers had placed in separate genera. Beaton (1862) discussed the heady times of crossing in the early 19th century that he had experienced first hand, and revealed how the "philosophers" were obliged to re-think their tidy sysetms of classification to accommodate the achievements of the hybridists.