Natural History Review/Series 2/Volume 1/Number 4/Report on Vegetable Parthenogenesis
Orginal Articles.
XLVI.—Report on Vegetable Parthenogenesis.
The question as to the existence of parthenogenesis in vegetables has latterly been the subject of much discussion, but it is by no means of modern origin. It has been studied at different times by numerous botanists for pretty nearly 100 years, and after the lapse of a century the point in dispute seems as far from being decided as ever. In England the subject has attracted little attention, although the most important of all the apparent instances of parthenogenesis, that namely of Cœlebogyne ilicifolia, was first noticed in this country. A short time since it would have been a work of some labour to have given any readable account of the question at issue, but the difficulties in the way of doing so have been to a great extent removed by Dr. Regel's publication in the Memoirs of the Academy of St. Petersburg.[1] The latter author has given at some length the history of all the important observations preceding his own, and in the outset of the present report we wish to acknowledge the assistance we have derived from Dr. Regel's Memoir.
For some years prior to 1767 the observations of physiological botanists had been directed to prove the existence of sexes in plants, and we find the names of Grew, Camerarius, Linnæus, and Köhlreuter conspicuous amongst the writers upon this subject. In the year above mentioned (1767) we come upon the writings of Spallanzani, with whose experiments the question of parthenogenesis, as it exists at the present day, may be considered to have originated.
In May, 1777, Spallanzani selected two young female plants of hemp (Cannabis sativa), the sex of which was only just distinguishable. These were placed in a room facing the south, twenty days before their flowers opened, and kept enclosed between two window frames. As a further test, two of the flowering branches of one of the plants were enclosed in a glass flask, the mouth of which was hermetically sealed, and all the branches of the latter plant, with the exception of those in the flask, were cut off. The window frames were kept closed, and all the plants were carefully examined from time to time, without a single male organ being detected. After all these precautions the plants in question, and the particular branches enclosed in the flask, produced seeds, which afterwards germinated.
A second experiment was made, in which the female plants were brought into flower six weeks before the time of flowering of the hemp in the open fields, and similar results were obtained.
De Marti[2] repeated the above experiment, and considered Spallanzani's observations imperfect. He was of opinion that male blossoms must have existed, which were overlooked by Spallanzani. Volta[3] also was unable to procure any perfect seeds from plants from which the anthers had been carefully removed.
In 1837, Ramisch published some observations upon Mercurialis annua, with which he had been occupied for four successive years. The results were inconclusive, for although Ramisch procured seeds, both with and without embryos, he admits that in some of the female plants upon which he experimented male blossoms were present, and he attempts to exclude the operation of the pollen in these hermaphrodite flowers by suggesting that the anthers had only been opened for a very short time.
Bernhardi's experiments with Cannabis are given in "Otto u. Dietrich's Allgemeine Gartenzeitung, 1839." These experiments were continued for six years, and each year with the same result. The plants were sown in April in the open air; the male plants which appeared were destroyed, and two female plants only allowed to stand, which were carefully examined every two days, in case any male blossoms should be overlooked. Each year seeds were ripened, from which both male and female plants were raised. Bernhardi was satisfied that at the time of the experiments no male plants were in flower near the spot where they were carried on, and consequently that accidental impregnation by pollen grains carried by the wind, or by insects, was out of the question.
Gartner's observations on Delphinium Consolida are important as showing the great care which must be taken in order to guard against deception. For some years he had taken what he considered sufficient precautions, and nevertheless had always obtained perfect seeds, apparently without any previous impregnation; but in the year 1838, when he cut off the male organs at an earlier period, and examined the plants several times daily, removing individual anthers at each examination, he found that the plants upon which he experimented produced no seeds.
We now come to the case of Cœlebogyne ilicifolia, the famous Euphorbiaceous plant, the great stumbling block of the opponents of parthenogenesis. This plant is dioecious, and the female one forms a low evergreen shrub, with pale green oval leaves, toothed like the holly. The female flowers are situated at the apex and on the side of small branches, and form short spikes of five or more flowers. The ovary is trilocular, and the stigma three-lobed. Each flower has on its calyx and bracts large wart-like glands, which at the time of flowering secrete a watery fluid. Three female plants were sent by Cunningham from Moreton Bay to Kew, where they flowered for the first time in 1839. In 1844 Mr. John Smith, of the Royal Gardens at Kew, published, in the Transactions of the Linnæan Society, an account of his observations on this plant, He staled that he had never been able to find male flowers or pollen of any Sort, but that nevertheless perfect seeds were produced end) year, from which young plants were raised resembling the parent plant in every respect. Mr. Smith suggested the possibility of the existence of a fertilizing power in the fluid secreted by the glands above mentioned. We shall have to return to the case of Cœlebogyne in a later part of this report, but there are some other intermediate observations which first require attention.
In the "Annales des Sc. Nat." Ser. III. Vol. V. Gasparini asserts that the cultivated fig produces seeds without the intervention of pollen. It bears (he says) two kinds of fruit, the one kind appears in spring and ripens early, the other appears in summer and ripens in autumn. In the former, male flowers are seldom found, and those which exist cannot serve for impregnation, as they do not appear until the stigma has withered. In these early fruits Gasparini never found perfect seeds. In the summer fruit he never found male flowers, and yet most of the ovaries produced seeds capable of germination. In order to prevent impregnation from without, Gasparini closed the opening of the young fruit of the cultivated fig with gum, or some other glutinous matter, and yet procured numerous perfect seeds. He never found in the fruits thus experimented upon any anthers or pollen-bearing organs. To these observations of Gasparini it has been objected—1st. That from time immemorial the cultivated fig has been impregnated artificially by the wild fig, an operation which would have been a waste of trouble if perfect seeds were produced without such process; 2ndly, that the impregnation cannot be watched with the necessary care, inasmuch as it takes place within the receptacle of the fruit; and 3rdly, that some observers have noticed peculiar organs in the ovule of the cultivated fig, which are called pollinidia, the nature of which is not yet understood.
The next observations at which we arrive are those of M. Naudin.[4] He experimented with Hemp, Mercuialis, Ricinus, Bryonia, and Ecbalium. He found that female plants of Hemp planted in a place surrounded by high walls, and others cultivated in pots and placed in a greenhouse in a garden, also surrounded by high walls, produced a quantity of perfect seeds, although no male plants were near, and although the females were subjected to careful examination with a view to the detection of possible male organs. Female plants raised from these unimpregnated seeds were set apart in the house of M. Decaisne, and so protected that M. Xaudin considers it altogether impossible that any pollen could have reached them; and although they were carefully examined by himself and M. Decaisne, no single male flower was ever discovered amongst the females. His observations on Mercurialis were conducted in a similar manner and with similar results. The Bryonia was kept in a room in the Museum at Paris, entirely isolated from all male plants, and yet for three years successively it produced a few perfect seeds. A young plant raised from one of these seeds also produced perfect seeds without apparent impregnation, and the number of them when counted, was found to be about the same as that produced by a female plant exposed to the influence of pollen. This result M. Naudin considers to be opposed to the supposition of impregnation by the aid of insects, which however he thinks may possibly have been the case with the Bryonia.
The plants of Ricinus and Ecbalium produced no perfect fruit, and M. Naudin is of opinion that diæcious plants are more apt to produce fruit without impregnation than monæcious ones. In 1857, Radlkofer published some remarks upon the present subject in Siebold and Kölliker's "Zeitschrift fur. wiss. Zoologie." He assumes the certainty of the absence of male organs in the female plants of Cœlebogyne. He examined the young embryo-sacs, in which he found three germinal vesicles, of which sometimes one, sometimes two, or even all three, became true embryos. He concludes that a true parthenogenesis exists in Cœlebogyne; and he considers this conclusion fortified by the fact (previously noticed by Smith) that the stigma remains fresh until just before the ripening of the seeds, whilst in ordinary cases it withers shortly after impregnation. He states that, although the stigma in Hemp and in Mercurialis withers soon after impregnation, he had noticed its persistence in one of the female Hemp plants experimented upon by Naudin, and in a female plant of Mercurialis annua which had been kept by M. Thuret apart from the male.
Braun's elaborate essay on parthenogenesis appeared in 1857 in the "Transactions of the Berlin Academy." After referring to the accounts of previous observers, which, before Cœlebogyne was known, had rendered the existence of parthenogenesis probable, he states that the latter plant is one which fulfils the necessary conditions. The observations made at Berlin agreed with those at Kew, as to the fact of the production by female plants of perfect seeds without any process of impregnation. He considers it to be against all probability that any abnormal mode of impregnation, as by the glands, observed by Smith, should exist, and notices in detail some observations made at his request by M. Deecke, as to the mode of origin of the embryo in Cœlebogyne, the result of which was to show that the process differed in no way from ordinary embryo-formation as observed by Hofmeister, Tulasne, and Radlkofer. After noticing that Radlkofer's observations differed from Deecke's only in the fact that the former found three and the latter only two embryonic vesicles, Dr. Braun remarks, "These observations lead to the result, that in "Cœlebogyne the germs of new individuals are developed within a normally constructed female organ of generation without any previous influence of pollen, and consequently a true parthenogenesis exists."
Further on, Dr. Braun alludes to the fact of the persistence of the stigma (upon which, as wo have mentioned, Radlkofer relies) showing the absence of impregnation. In Cryptogams a remarkable instance of apparent parthenogenesis occurs in Chara crinita. In all the Characeæ, with the exception of this species, the male and female organs are equally common, sometimes on the same, sometimes on separate plants. After noticing the distinctive features of the species, its geographical distribution, and the certainty that in many localities the female plant alone exists, Dr. Braun gives it as his opinion that, at least in certain places, Chara crinita has the capacity of producing, without the operation of any male organ, normal spores capable of germination, and consequently that it affords an instance of veritable parthenogenesis.
In 'Bonplandia,' for 1857 (p. 209), Klotzsch suggested that the so-called embryo in Cœlebogyne is in fact not an embryo at all. He says that all the Euphorbiaceæ, without exception, have anatropal ovules, and a highly developed straight embryo with the radicle turned to the micropyle, whilst the large flat cotyledons winch enclose the plumule are directed to the chalaza. In Cœlebogyne, on the contrary, no freely developed embryo is perceptible, nor is there any trace of a radicle turned towards the micropyle, or of cotyledons turned towards the chalaza, Instead of the above, there is found an elliptical body within a fleshy, not albuminous, envelope, and consisting of a convoluted leaf-like mass, firmly attached on the inside of the seed to the chalaza by a discoid foot. From these facts Klotzsch arrives at the conclusion that the supposed embryo is a bud formed within the seed.
Ruprecht[5] has objected that Klotzsch has given no figure of the perfect seed, and without this he seems to consider Klotzsch's observations open to doubt, at the same time expressing no opinion either for or against parthenogenesis.
Radlkofer's second essay on parthenogenesis relates to some matters of opinion in dispute between himself and Braun. He discusses the nature of the germinal vesicle before impregnation, and considers that in that stage it must be looked upon, not as the germ of the future plant, but as a rudimentary body capable of becoming a germ; a distinction somewhat subtle, and not very easily appreciable. He also enters upon the question of the analogy between the embryo-sac and the spore of the higher cryptogams. These matters, however, have no bearing upon the practical question as to the existence or non-existence of parthenogenesis in vegetables, and we refrain therefore from any further details with regard to them.
In 1859, Hegel's paper, "Die Parthenogenesis im pflanzen-reiche," was published in the Memoirs of the Academy of St. Petersburg, (VII. Ser. Tome I. No. 2.) He considers that Spallanzani's experiments are the only ones which have been well conducted, and admits that if he (Regel) could have procured perfect seeds by operating as Spallanzani did with the Hemp and the Water-melon, the doctrine of parthenogenesis might be considered established. Regel attempts to get rid of Spallanzani's authority, by suggesting either that his observations were not carefully conducted, or that Spallanzani did not speak the truth. The latter accusation appears to have no sort of foundation; at least the grounds for it given by Regel are of the weakest description.
In the course of a series of experiments on hybridization, Regel observed that the anthers of many plants are fully formed and contain perfect pollen some time before the opening of the flowers; from which he concludes that it is necessary, in all experiments on parthenogenesis, to cut off the anthers at a very early period, or otherwise impregnation may have taken place without the observer having any suspicion of it. He considers monæcious and diæcious plants to be especially likely to have led to deceptions, because sufficient care was not taken to examine each individual flower— a precaution which is necessary on account of the frequent occurrence of accidental anthers in the female flowers, and because monæcious and diæcious plants produce a greater quantity of pollen than hermaphrodites, and consequently accidental impregnation by wind or insects is highly probable. He considers the Hemp-plant, which has been so much employed for these experiments, to be especially ill-suited for them, on account of the abundance of its easily dispersible pollen, and of its universal cultivation. As therefore (he says) these precautions have not been taken by any trustworthy observer, and as his own observations have afforded only negative results, he considers it certain that the formation of a true embryo can only take place under the influence of impregnation.
Regel then proceeds to give an account of his observations upon the Cycadeæ, and especially upon Ceratozamia, and the result that he arrived at was, that no embryo can be formed without the influence of a pollen-tube, but that the growth of the embryo-sac and the production of endosperm, as also the formation of corpuscula in Gymnosperms, may take place independently of impregnation. The case of the Cycadcæ he considers very conclusive, as showing that the development in the embryo-sac can only proceed up to the point at which the stimulus to the commencement of the formation of an embryo must be given, and that where this stimulus is wanting, the germinal vesicle, even in vigorous seeds, exhibits no further development.
Regel subsequently proceeds to detail his experiments on Mercurialis annua, from which he concludes that the previous observers who have imagined that they have procured perfect seeds without impregnation, have overlooked the very frequent occurrence of anthers in the so-called female flowers; and with regard to the persistence of the stigma, upon which Kadlkofer places so much reliance, as proving the fact of non-impregnation, a figture is given (after Nees) of the young fruit still surmounted, by the perfect stigma; and it is stated that the stigma is not only persistent, but even increased in growth after impregnation—a circumstance which (it is added) has often been observed by Klotzsch in the Euphorbiaceæ.
Experiments upon Spinacia oleracea, similar to those just mentioned upon Mercurialis, led the author to the conclusion that Spinacia is, in point of fact, an hermaphrodite plant, which can produce no perfect seeds when impregnation is really prevented, but that such prevention is a most difficult task, it being next to impossible to remove the male flowers at so early a stage and with so much care, as to be certain that impregnation has not taken place.
Lastly, Regel entered upon the same investigations with female plants of Cannabis saliva. He cut the plants down to a few branches, so that he might be able to examine with a lens the numerous flowers which were daily produced, and so that the whole vegetative force of the plant might be directed to those few branches, and thus favour the formation of fruit. He kept these plants in favourable situations until the month of October, up to which time none of the ovaries produced seeds; but all of them, without exception, withered and dropped off. He then put these plants, and another female plant subsequently reared, but not cut down like the former, into a room with a male plant. The heat of the room and other circumstances he considered to be unfavourable to fructification. Nevertheless, the female plants which had been cut down produced and ripened seed, whilst the other female plant did not fructify. The results are thus recapitulated by the author.—Two plants cut down so that the whole vegetative power was directed to the formation of seeds, placed under favourable circumstances, vigorous in their growth, and having daily access to fresh air, produced no fruit so long as impregnation was withheld. The same plants under unfavourable circumstances, in a close hot room, and when the days were shortened, produced and ripened seeds as soon as they were subjected to impregnation. A plant not cut down like the above, and impregnated under the (unfavourable) circumstances just mentioned, produced no seeds.
Regel states that he has not had the opportunity of examining Cœlebogyne, and can therefore give no decided opinion as to that plant. He suggests the possibility of the future discovery of sessile anthers between the bracts or near the glands, or that individual pollen-grains may be developed in the interior of the embryo the latter suggestion arising from the fact of Deecke having seen in Cœlebogyne a pollen-tube which had penetrated to the embryo-sac, although neither he nor Radlkofer could discover pollen-grains upon the stigma.
Since the publication of Regel's paper, Dr. Braun has returned to the subject in a communication made to the Berlin Academy, and published in their Transactions for 1859. This essay, which has since appeared separately, is of enormous length, occupying about 150 quarto pages. Only a small portion of it, however, relates directly to the simple question of the existence, or non-existence, of fructification without impregnation. The author expressly contradicts the theory advanced by Klotzsch as to the nature of the body within the seeds of Cœlebogyne. He says: "I can confidently reaffirm, and prove at any time by sections of seeds in my possession, that the embryo-formation of Cœlebogyne which I have observed and described, fulfils all the conditions necessary to constitute a veritable embryo, and agrees in its essential features with that of the other Euphorbiaceæ." In a later part of the essay some remarks are given under the head "Weitere Zeugnisse für die Parthenogenesis," of which the following is a short account. Some observations on individual plants given by Ruprecht, Tenore, Lecoq, and Jacquemont are noticed, the author stating that he was un- willing to pass them over entirely, although (he adds), as merely special instances, little importance may be attached to them. Dr. Braun then refers (as bearing upon the question of parthenogenesis) to those cases of fructification called by Gartner Fructifcatio spuria, in which fruit is formed, and even seeds also; the latter being apparently perfect, but containing no embryo. Instances of this occur in Datisca, Adelia, the Cycadeæ, and the Conifers; and give rise to the question, whether the unimpregnated germinal vesicle disappears without undergoing any development; or whether the development progresses to a certain extent, and is then arrested. Dr. Braun considers this point deserving of investigation, as, in his opinion, it is not improbable that, besides perfect parthenogenesis, there may exist indications of parthenogenesis, as has been observed in the animal kingdom. The above question, he adds, is connected with the further ones; 1, whether or not, speaking generally, the formation of seeds and fruits depends upon the development of the germinal vesicle into an embryo; and, 2, whether, when impregnation occurs, it acts directly only upon the development of the germinal vesicle into an embryo, and thus indirectly upon the formation of the surrounding parts; or whether impregnation acts upon the entire ovule, or even upon the ovary itself. Observations, he says, are wanting as to the well-known cases of fructification without seeds which occurs in certain cultivated varieties of Citrus, Pyrus, Cydonia, Vitis, Arlocarpus, Musa, and Ananassa. It should be ascertained in what stage of development the ovules fail, and whether the formation of these seedless fruits is in all cases independent of impregnation.
Having regard to the cases in which parthenogenesis has been noticed in the animal kingdom, Dr. Braun observes that it might be expected to play a more important part amongst cryptogamic, than amongst phænogamic plants. He adds, however, that this question is surrounded by many difficulties, and that he can do no more than make a few suggestions on the subject. He alludes, in the first place, to the ferns, in which he considers that the constant reappearance of individual peculiarities, and even of monstrosities, points to the existence of zelotypical[6] reproduction, and consequently of parthenogenesis. He then refers to the mosses, and mentions that in some them male blossoms are not even known, whilst in others, although known, they occur but seldom, and yet in both cases fruit is produced plentifully, at least in some localities. For example, the male blossoms of Dicranum undulatum are entirely unknown, and yet it is a moss which forms an abundance of fruit. In Sphagnum molluscum the male plants are known, but yet the species fruits freely in places where no trace of them is to be found; and the same thing occurs in Camptothecium lutescens. Alrichum undulatum bears a male flower only in the first year; from which, in the second and subsequent years, innovations are produced bearing female flowers. Patches of this moss are often found bearing fruit, but having no first-year male plants in or near them. Fissidens incurvus bears the male flowers at the apex of a lateral innovation of the second year, but it produces fruit in the first year of its growth. Dr. Braun then refers to the Algæ, and dismisses the Vlorideæ and Fucoideæ as exhibiting no satisfactory proofs of parthenogenesis. He then mentions the Conjugate; and, adopting De Bary's theory that the process of copulation is a peculiar modification of sexual reproduction, he considers that the exceptional cases where the spores in the Conjugate are formed without copulation must be looked upon as instances of parthenogenesis; at least, if it may be assumed (what he admits is not yet proved) that such spores agree with the others in their structure and mode of germination.
In a note at pp. 117 and 118 of his essay, Dr. Braun refers to some experiments of Schenk, made, during the three previous years, in the botanical garden at Wurzburg, and also to some observations of De Bary made at Freiburg. Schenk directed his attention to Cannabis sativa, Mercurialis annua, Ricinus communis, Momordica elaterium, and Cucurbita Pepo. De Bary speaks of Cannabis sativa alone. Both observers obtained only negative results, and the same was the case with some later observations upon Cannabis sativa and Mercurialis made by Schenk, and reported in the "Wurzburg Nat. Zeitschrift," Bd. 1. pp. 85–89.
The last publication which we have to mention is Karsten's treatise, entitled "Das Geschlechtsleben der Pflanzen und die Parthenogenesis," published at Berlin in 1860. He asserts that anthers are not unfrequently developed at the base of the calyx of the female flowers of Cœleboqyne; thathie has himself observed this in the botanical garden at Berlin; that if Cœlebogyne be carefully examined, a succession of hermaphrodite flowers may be observed at intervals throughout the whole summer, from the beginning of May to the end of August; and that, in fact, about every fifth flower is hermaphrodite.
With, these observations of Karsten the present report comes to a close; and the readers of it, being now in possession of the evidence on both sides, may form their own opinion whether or not parthenogenesis exists in the vegetable world. It is no part of the duty of the writer of a report to give his own views upon the subject to which it relates, although we ventured a statement at the outset that the point in dispute was far from decided. Setting aside the case of Cœlebogyne, it appears to the writer that, although some of the facts might lead to a suspicion on the subject, there is really no proof whatever of the existence of parthenogenesis, at least, in phamogamic plants; and with regard to the Cryptogamia, the discovery of their sexual organs is of such comparatively recent date, and the examination of them is surrounded by so many difficulties, and is in the hands of so few observers, that it would be rash in the extreme to found any theory upon the results hitherto obtained. Cœlebogyne, however, still remains a striking instance of the apparent possibility of reproduction without impregnation; for, although the value of the evidence afforded by this latter plant is doubtless shaken by Karsten's observations, it is quite impossible to assume, with him, that parthenogenesis is yet disproved. It cannot be supposed that the observations of Smith, Radlkofer, Deecke, and Braun have been so imperfectly and carelessly conducted as must be the case if, as Karsten would lead us to suppose, every fifth flower in every plant of Coœlebogyne is hermaphrodite. We do not at all intend to deny the correctness of his observations, but we think it highly improbable that, if the stamens of Cœlebogyne were of such frequent occurrence, they would have escaped the notice of so many other equally able observers.
In conclusion, it is hardly necessary to remark, that further observations by competent botanists, as to the anatomy of the inflorescence of Cœlebogyne, are much to be desired; and that further inquiries into the reproductive process in cryptogamic plants may hereafter throw additional fight upon the subject. For the present, all that can be said is that vegetable parthenogenesis is not proven.
- ↑ Mémoires de l'Acaclémie Impériale des Sciences de St. Petersbourg, VII Série Tome I. No. 2.
- ↑ Experimento y Observaciones sobre los sexos y fecondation de los plantas. Vol. i. Barcelona, 1791.
- ↑ Memoires de l'Acad. de Mantoue, i. 226.
- ↑ Comptes rendues, Vol. 43 (1856).
- ↑ Ein Beitrag zur Frage über die Parthenogenesis bei Pflanzen, im Bulletin de l'Acad. Imp. de S. Peterebourg, 1858, p. 274; No. 378.
- ↑ "Zelotypie" and "Idiotypie" are words coined by Radlkofer in his treatise on the relation of parthenogensis to other modes of reproduction. In sexual reproduction, the new individual, although retaining the peculiarities of the species, may vary to some extent from the original type. This is called by Radlkofer "idiotypical" reproduction. In asexual reproduction, the now individual is, so to speak, a copy of the old one: this Radlkofer calls "zelotypical " reproduction.