Chapter I: History of the Sexual Theory (3)

Gärtner’s work on hybridisation had been preceded by other enquiries into the same subject, those namely of Knight mentioned above at the beginning of the century, and Herbert’s more ample investigations published in his work on Amaryllideae in 1837. Gärtner did not neglect to compare his observations at all points with the results of his predecessors, especially those of Koelreuter, and he deduced from the astonishing mass of material a number of general propositions respecting the conditions under which the production of hybrids is possible, the results of crossing, and the causes of failure. A special interest attaches to his mixed and compound hybrids, to his experiments on the various degrees of influence which foreign pollen exercises on the behaviour of the female organ, and the connection of this point with the formation of varieties. It is impossible to give a more distinct account of Gärtner’s results without entering into discussions which would exceed the limits of a historical survey. It is the less necessary to do so, since Nägeli undertook in 1865 to give a summary view of all the important results to be found in the wealth of material supplied by Koelreuter, Herbert and Gärtner[112]. Gärtner’s experiments in hybridisation were conducted at Calw in Würtemberg, the place where Koelreuter had made his in 1762 and 1763. And thus it was in two small cities of Würtemberg that the foundations of the sexual theory were laid and the theory itself perfected, as far as it could be by experiment only, by three of the most eminent among observers. Camerarius in Tübingen, Koelreuter and K. F. Gärtner in Calw contributed so largely to the empirical establishment of the theory, that all that was done by others would seem of small importance, if artificial pollination only were in question. But Koelreuter was imperfectly acquainted with the methods by which pollination is usually effected in nature; Sprengel was the first who saw into all their more important relations, and the fact must not be concealed, that Gärtner in regarding Konrad Sprengel’s observations as unworthy of serious consideration, neglected the most fruitful source of new and magnificent results. His careful study of the secreting of nectar and of the sensitiveness of the organs of fertilisation, and his many observations on other biological relations in flowers, would have found their natural termination, if he had connected them at all points with Sprengel’s general conclusions respecting the relation of the structure of the flower to the insect world. This Gärtner entirely failed to do, and hence in this case also it was reserved for Darwin’s wonderful talent for combination to sum up the product of the investigations of a hundred years, and to blend Koelreuter’s, Knight’s, Herbert’s, and Gärtner’s results with Sprengel’s theory of flowers into a living whole in such a manner, that now all the physiological arrangements in the flower have become intelligible both in their relations to fertilisation, and in their dependence on the natural conditions under which pollination takes place without the aid of man. Here, as in morphology and systematic botany, Darwin found the premisses given and drew the conclusion from them; here too the certainty of his theory rests on the results of the best observers, on investigations which find in that theory their necessary logical and historical consummation.

7. MICROSCOPIC INVESTIGATION INTO THE PROCESSES OF FERTILISATION IN THE PHANEROGAMS; POLLEN-TUBE AND EGG-CELLS[113]. 1830-1850.

Those who were convinced of the sexuality of plants had endeavoured as early as the previous century to form some idea with the help of the microscope of the way in which the pollen effects the formation of the embryo in the ovule. We may pass over Morland’s and Geoffroy’s very rude attempts in this direction: Needham (1750), Jussieu, Linnaeus, Gleichen, and Hedwig imagined that the pollen-grain bursts upon the stigma, and that the granules it contains make their way downwards through the style to the ovules, and are there either hatched into embryos or assist in their production. This way of conceiving the matter was closely connected with the theory of evolution which then prevailed, and seemed to find some countenance in the seed-corpuscles of animals; it was also supported by the observation that pollen-grains placed under the microscope in water often burst and discharge their contents in the form of a granular mucilage. It has been already mentioned that Koelreuter rejected this view; he declared the bursting of the pollen-grains to be contrary to nature, and considered the oil which exudes from the grains to be the fertilising substance. This view was adopted by Joseph Gärtner and Sprengel, but it fell into disesteem, while that of Needham and Gleichen commanded some assent some years longer. The next question was, how the granular contents of the pollen-grain reach the ovules. Accident supplied a starting-point for further consideration. Amici, who was examining the hairs on the stigma of Portulaca for another purpose, saw on that occasion (1823) the pollen-tube emerge from the pollen-grain, and the granular contents of the latter, commonly known as the fovilla, execute streaming movements like the well-known movement in Chara. The desire to verify this remarkable fact, and to discover how the fertilising substance is absorbed by the stigma, led Brongniart in 1826 to examine a great number of pollinated stigmas. He succeeded in establishing the fact that the formation of pollen-tubes is a very frequent occurrence. The want of perseverance in following out his observation and a prepossession in favour of Needham’s old theory prevented him from discovering the course of the pollen-tubes all the way to the ovules; he supposed, indeed, that after penetrating into the stigma they open and discharge their granular contents, and he maintained distinctly that these are analogous to the spermatozoids in animals, and are the active part of the pollen. But now Amici addressed himself more earnestly to the question, and in 1830 he not only followed the pollen-tubes into the ovary, but also observed that one finds its way into the micropyle of each ovule.

Thus the question was suddenly brought near to its solution, when observers began to wander from the right path in different directions. Robert Brown showed in 1831 and 1833 that the grains in the pollen-masses of Orchids and Asclepiads put forth pollen-tubes as in other plants, and that fine tubes are found in the ovary of Orchids in which pollination has taken place; but he was in doubt about the connection of these tubes with the pollen-grains, and rather inclined to think that they were formed in the ovary, though possibly in consequence of the pollination of the stigma. Schleiden went wrong in a very different way, and by so doing made the question as prominent in botanical research, as was that of the origin of cells at this time. He published in 1837 some excellent investigations into the origin and development of the ovule before fertilisation, certainly the best and most thorough of the day. He at the same time showed that Brongniart’s and Brown’s doubts were unfounded, and confirmed the statement of Amici, that the pollen-tubes make their way from the stigma to the ovule, which they enter through the micropyle. But he made them push forward a little too far, for he asserted positively that ‘the pollen-tube pushes the membrane of the embryo-sac before it, making an indentation, and its extremity then appears to lie in the embryo-sac. The extremity of the tube now swells out into a round or oval shape, and cell-tissue forms from its contents; the lateral organs, one or two cotyledons, are then produced, the original apical point remaining more or less free and forming the plumule. The portion of the tube underneath the embryo and the fold of the embryo-sac which envelopes it are divided off sooner or later and disappear, so that the embryo now really lies in the embryo-sac.’ This view, which appears to rest on direct observation and is illustrated by figures which answer to the description, corresponds with the old theory of evolution and has a striking approximation to the ideas of Morland and Geoffroy; and if it were correct, it would like these imply the necessity of pollination to the formation of seeds that should contain embryos, but at the same time it would do away with that which is the essential point in the sexuality of plants, for the ovule would merely be the spot adapted to the hatching of the embryo formed from the pollen. Schleiden’s idea was at once adopted by Wydler, Gelesnow and various other botanists, and especially by Schacht, but the most eminent microscopists withheld their assent. Amici was the first who openly opposed the new doctrine; before the Italian congress of savants at Padua in 1842 he endeavoured to prove that the embryo is not formed at the end of the pollen-tube, but from a portion of the ovule which was already in existence before fertilisation, and that this part is fertilised by the fluid contained in the pollen-tube. But the choice of a gourd, a plant eminently unsuitable for his purpose, prevented his discovering the exact details of the process, and Schleiden did not hesitate to denounce his assertions in 1845 in the plainest terms. But in the next year (1846) Amici produced decisive proof for the views which he had maintained; he showed from the Orchidaceae, which were peculiarly well adapted for such investigations, not only that Robert Brown’s doubts above mentioned were without foundation, but, which is the main point, that a body, the egg-cell, is present in the embryo-sac of the ovule before the arrival of the pollen-tube, and that this body is excited by the presence of the pollen-tube to further development, the formation of the embryo. He gave a connected account on this occasion for the first time of the whole course of these processes from the pollination of the stigma to the perfecting of the embryo.

The correctness of the account given by Amici was confirmed in the following year by von Mohl and Hofmeister, the latter of whom described in detail the points which were decisive of the question from a variety of plants, and illustrated them by very beautiful figures in a more copious work, ‘Die Enstehung des Embryo der Phanerogamen,’ Leipzig, 1849. Tulasne also came forward in opposition to Schleiden’s theory, being thoroughly convinced that there was no actual contact of the pollen-tube with the egg-cell, denying indeed the existence of the egg-cell before fertilisation. Thus a vehement controversy arose on the subject; a prize offered by the Institute of the Netherlands at Amsterdam was awarded to an essay of Schacht’s in 1850, which defended Schleiden’s theory, and illustrated it by a great number of drawings giving incorrect and indeed inconceivable representations of the decisive points. Von Mohl says very admirably on this occasion (‘Botanische Zeitung,’ 1863, Beilage, p. 7): ‘Now that we know that Schleiden’s doctrine was an illusion, it is instructive, but at the same time sad, to see how ready men were to accept the false for the true; some renouncing all observation of their own dressed up the phantom in theoretical principles; others with the microscope in hand, but led astray by their preconceptions, believed that they saw what they could not have seen, and endeavoured to exhibit the correctness of Schleiden’s notions as raised above all doubt by the aid of hundreds of figures, which had every thing but truth to recommend them; and how an academy by rewarding such a work gave fresh confirmation to an experience which has been repeatedly made good especially in our own subject during many years past, namely that prize-essays are little adapted to contribute to the solution of a doubtful question in science.’ In this case the prize-essay had been refuted before it appeared by von Mohl, Hofmeister and Tulasne. Schacht adhered all the more firmly to Schleiden’s theory; after further controversy, in which other writers of less authority took part, Radlkofer published in 1856 a complete review of the question, which fully confirmed Hofmeister’s observations, and gave incidentally an account of Schleiden’s views in the altered form which they had by that time assumed; this account showed in fact that Schleiden had completely retracted his former opinions, and in this retractation Schacht was soon after compelled to follow him, having become acquainted with facts observed in the ovule of Gladiolus, which were obviously irreconcilable with Schleiden’s theory.

Hofmeister had from the first directed special attention to the questions, whether any bodies are found in the pollen-tube which answer in any way to spermatozoids, and whether any opening can be perceived at the end of the tube. He found indeed forms in Coniferae in 1851, which reminded him of the male organs of fertilisation in the higher Cryptogams; but the pollen-tube was closed both in them and in the rest of the Phanerogams, in which moreover its outer coat attains to a considerable thickness. There remained therefore only the hypothesis, that a fluid substance passes through the walls of the pollen-tube and of the embryo-sac and effects the fertilisation of the egg-cell; thus it was not the theory of preformation of the last century, to which Brongniart still adhered, but the view represented by Koelreuter, which ultimately proved to be nearer the truth, though it may be said that all that remained of that view was, that the fertilising substance in the Phanerogams is a fluid. The granular contents of the pollen-grains, which were supposed to be spermatozoids, have since been partly found to be only innocent starch-grains and drops of oil.

8. DISCOVERY OF SEXUALITY IN THE CRYPTOGAMS. 1837-1860.

By the year 1845 no one capable of forming a judgment on the question any longer doubted the existence of different sexes in Phanerogams. But it was not so with the Cryptogams, though a number of facts were acknowledged at this time which seemed to point to the conclusion, that a moment arrives sooner or later in the course of their development also, when a sexual act is accomplished. But the question had not as yet been systematically studied; no experimental investigations had been made, or observations of such a kind as to demonstrate the necessity of sexual union.

The great majority of botanists in the second half of the 18th century had no longer any doubt that the stamens were organs of reproduction, and they were anxious to prove the existence of similar organs in the Cryptogams; they rested in this matter on external resemblances and analogies, which they interpreted in a more or less arbitrary manner. The obvious external resemblance between the antheridia and archegonia in Mosses and the sexual organs in the Phanerogams led Schmidel and Hedwig to consider them to be stamens and ovaries, and the conjecture was correct, though the true nature of the moss-fruit had to be learnt in another way. Micheli, Linnaeus and Dillen, trusting still more to external appearance and with slight knowledge of these plants, had before this taken the fruit for a male flower, and in the case of the rest of the Cryptogams the best botanists were only feeling their way in the dark with no certain experience to guide them. It is not necessary to give a particular account of the views which originated in this way; one or two may be mentioned by way of example. Koelreuter regarded the volva of Mushrooms, Gleditsch and Hedwig certain tube-like cells in their lamellae, as the male organs of fertilisation. Gleichen took the stomata, Koelreuter the indusium, Hedwig even the glandular hairs of Ferns for anthers. It was not yet suspected that the course of development and the whole morphology of the Cryptogams could not be so compared with that of the Phanerogams; correct and incorrect assumptions with regard to the sexual organs of the Cryptogams were alike devoid of scientific value, being mere guesses and vague conjectures. Nor was the state of things much better even in the first years of the 19th century; and if by that time a number of occasional observations had been made which could afterwards be turned to scientific account, these were as yet only isolated facts without scientific connection, and every one was at liberty to concede or to refuse sexual organs to the Cryptogams generally at his own discretion. Meanwhile observations gradually accumulated, and towards 1845 it began to be possible by critical examination of them to arrive at something like a clearer understanding of this part of botany. The majority of botanists readily accepted Schmidel’s and Hedwig’s opinion with respect to the Mosses; Vaucher had as early as 1803 maintained that the long-known conjugation of Spirogyra was a sexual act; Ehrenberg observed in 1820 the conjugation of a Mould, Syzygites; Bischoff and Mirbel explained the organisation of the antheridia of the Liverworts in 1845, while Nees von Esenbeck saw the spermatozoids of Sphagnum in 1822 and Bischoff those of Chara in 1828, though they were at first taken for Infusoria, an opinion maintained by Unger as late as 1834. But it was Unger[114], who in 1837, after careful study of the spermatozoids of the Mosses in 1837, declared them to be the male organs of fertilisation; in 1844 Nägeli discovered corresponding forms on the prothallium of Ferns, which had till then been called a cotyledon, and in 1846 the spermatozoids of Pilularia, the products of the small spores which Schleiden had explained to be the pollen-grains of that plant.

These facts were of the highest importance, but little was to be made of them as long as the female organ in the plants in question, the Mosses excepted, was unknown, and meanwhile it was only the resemblance between vegetable and animal spermatozoids which led to the conjecture, that the one had the same sexual significance as the other.

Light was suddenly thrown upon the subject, when Count Lesczyc-Suminsky discovered in 1848 on the supposed cotyledon (prothallium) of Ferns both the antheridia and the peculiar organs, inside which the embryo or young fern is formed. Though the statements respecting the structure and development of these female organs and of the embryo were inaccurate in some important points, yet the place was now indicated where it might be presumed that the fertilisation by the spermatozoids takes place; and as the history of the germination of the rest of the vascular Cryptogams was to some extent known through the earlier labours of Vaucher and Bischoff, the organs of fructification of these plants might now be sought, where they are really to be found. But an erroneous idea respecting the meaning of the small spores of the Rhizocarps propounded by Schleiden had first to be put out of the way, and this was done by an appeal to the discovery of Nägeli mentioned above and by the investigations of Mettenius. Then in 1849 Hofmeister supplied a connected description of the germination of Pilularia and Salvinia, in which the decisive points as regards the sexual act were clearly set forth, and the connection of the spermatozoids with the fertilisation of the egg-cells in the archegonium was established. He did the same for Selaginella, which is very unlike the Rhizocarps and Ferns, and in which the spermatozoids are developed from smaller spores, and fertilise the egg-cells in archegonia formed in the prothallium of the large spores. By comparing the processes of germination in these plants with those of Ferns and Mosses, he succeeded in throwing entirely new light on the whole of the morphology of these classes of plants, and thus made it possible for the first time to compare them with one another and with the Phanerogams, and to form a right estimate of the sexual act in the Muscineae and Vascular Cryptogams in its relation to the history of the development of these plants. Hofmeister arrived at the following conclusion from his observations in 1849: ‘The prothallium in the vascular Cryptogams is the morphological equivalent of the leaf-bearing Moss-plant, while the leafy plant of a Fern, of a Lycopodium and a Rhizocarp answers to the capsule of the Moss. In Mosses as in Ferns there is an interruption of the vegetative development by sexual procreation, an alternation of generations; this takes place in the Vascular Cryptogams very soon after germination, in the Mosses much later.’ The vast importance of this discovery to systematic botany has been already noticed. The conception of these relations developed by Hofmeister was not less important to the doctrine of the sexuality of plants; it swept away at one stroke all the old false analogies between Phanerogams and Cryptogams and brought to light the real agreement; Hofmeister had detected in the archegonium of the Cryptogams the body which is developed there, as in the ovule of the Phanerogams, into an embryo after fertilisation, namely the germinal vesicle or egg-cell. Here was the point of departure for all further systematic comparison in the sexual propagation of Cryptogams and Phanerogams. All beside was of secondary importance, even the fact, that the fertilisation of the egg-cell in the Cryptogams is not effected by a pollen-tube, but by spermatozoids. It was now easy to show the corresponding relations of generation in the other cases which Hofmeister had not yet observed.

Hofmeister’s statements and conclusions respecting Selaginella and Isoetes were confirmed and some additions made to them by Mettenius in 1850, and in 1851 appeared Hofmeister’s exhaustive work ‘Vergleichende Untersuchungen,’ in which the mode of production of the embryo in Coniferae was represented as an intermediate form between those of Phanerogams and Cryptogams. Further contributions were made to the knowledge of the subject; Henfrey confirmed Hofmeister’s results in the case of Ferns; Hofmeister himself and Milde observed in 1852 the history of fertilisation in Equisetaceae, and the former supplied at the same time a more complete account of the development of Isoetes; in 1855 he described the decisive points in Botrychium and Mettenius in 1856 those in Ophioglossum.

The processes of development before and after fertilisation were now cleared up by all these discoveries, but the direct observation of the act of fertilisation was still wanting. Hofmeister (‘Flora,’ 1857, p. 122) describes the state of affairs in the following terms: ‘While numerous investigations had thrown a clear light on the character of the male and female organs, and on the way in which the embryo is formed by repeated division of the egg-cell present before fertilisation, we continued quite in the dark respecting the particular nature of the fertilisation. Observation and experiment had established the fact, that the influence of the spermatozoids on the archegonia was required to produce an embryo in the latter. Female moss-plants[115] separated from the male, macrospores in the Vascular Cryptogams separated from the microspores, had in all cases proved unproductive; but it was not even certainly known to what point in the female organ the spermatozoids force their way. It is true that Lesczyc and after him Mercklin had seen the entry of moving spermatozoids into the mouth of archegonia in Ferns; but Lesczyc’s account of the part which he supposed them to play there afterwards, was proved to be an illusion. I had myself observed motionless spermatozoids halfway down the neck of archegonia of an Equisetum; but nothing was to be learnt of the manner in which the spermatozoid affects the egg-cell. Then it happened that in the spring of 1851, being engaged in observing the development of the organs of vegetation of Ferns, I repeatedly saw spermatozoids moving about in the basilar cells which enclose the egg-cell in the archegonia of Ferns, and the majority of them even playing about the egg-cell. Their movements were put an end to during the observation by the commencement of changes, which the contents of young vegetable cells which have been cut open usually experience under the prolonged influence of water.’ Later observations leave no doubt now that in the Muscineae and Ferns single spermatozoids force their way into the naked egg-cell of the archegonium.

The question was first set at rest in the Algae, where the process of fertilisation could be seen directly and without exposing the objects to destructive influences. That sexual propagation occurs in the Algae also had seemed probable, since Decaisne and Thuret in 1845 discovered organs in species of Fucus, and Nägeli in 1846 in Florideae, which scarcely admitted of any other explanation. Alexander Braun also had called attention to the formation of two kinds of spores in a large number of fresh-water Algae. But as yet there was only conjecture. Then Thuret proved by experiment in 1854, that in the genus Fucus the large egg-cells must be fertilised by very small swarming spermatozoids, in order to set up germination; both organs can be collected separately and in numbers in this genus, and be brought together at pleasure; Thuret even succeeded in obtaining hybrids. Pringsheim first observed in 1855 the formation of spermatozoids in the little horns of Vaucheria and established the fact that spores capable of germination are not formed unless the spermatozoids approach the egg-cell. To Thuret’s statements he added the very important one, that the remains of spermatozoids may be recognised on the surface of the contents of the fertilised egg-cell of Fucus, which is already surrounded by a membrane. About the same time Cohn published his observations on Sphaeroplea annulina, which confirmed the fact of the approach of the spermatozoids to the egg-cells, which consequently, as in Fucus and Vaucheria, form a cell-wall and are rendered capable of further development.

Still the decisive observation had not yet been made; no one had yet seen how the two fertilising elements behaved at the moment of fertilisation. Pringsheim had the good fortune to make this observation in one of the commonest of fresh water Algae, Oedogonium. There he saw the moving spermatozoid first come into contact with the protoplasmatic substance of the egg-cell, and then force its way into it, blend with it and dissolve. And thus the first observation was made, which proved decisively that a real intermixture takes place of the male and female elements of fertilisation; this important fact was confirmed by De Bary in the same year.

Now that it was once established, that fertilisation in Cryptogams consists in the blending together of two naked bodies of protoplasm, the spermatozoid and the egg-cell, it was reasonable to conclude that conjugation in Spirogyra and generally in Conjugatae, was an act of fertilisation, only in this case the two fertilisation-elements are not of different size and shape, but similar in appearance. To this conclusion De Bary arrived in 1858 in his monograph of the Conjugatae. This extension of the idea of fertilisation to cases in which the uniting cells are to outward appearance alike, was of special value to the theory of sexuality, as was seen in the sequel, when other forms of fertilisation were observed which made it necessary still further to extend the idea of sexuality. In 1858 Pringsheim discovered arrangements for fertilisation in another group of Algae, the Saprolegnieae, which to outward appearance at least departed widely from those hitherto known in the lower plants.

Thus between the years 1850 and 1860 a number of fundamental facts were discovered, and were afterwards confirmed and extended by fresh observations in the course of the following years. It does not fall within the limits of this work to notice the many discoveries that were made in this part of botanical science after 1860; we will only remark, that between 1860 and 1870 the processes of fructification were observed by Thuret and Bornet in Florideae, and especially by De Bary and his pupils in Fungi, in some of which very peculiar forms were brought to light. No doubt any longer exists that difference of sex prevails generally in the Thallophytes also, though it is still an open question, whether it may not be wanting in some of the very simplest and smallest kinds.

One of the most important results of these investigations is obviously the striking resemblance between many of the processes of fertilisation in Cryptogams and in the lower animals; here is another confirmation of the fact, often brought out in other ways by modern zoological and botanical research, that the points of resemblance in the vegetable and animal kingdoms appear most plainly, if we compare together the simplest forms to be found in both; we have in this fact a plain proof also, that both kingdoms have been developed from like common elements, as the theory of descent implies. With respect to the true nature of fertilisation itself, which is evidently a similar process in the main in animals and plants, we can only say at present, that it amounts in all cases to a material blending together of the contents of two cells, neither of which is capable of further development by itself, while the product of the combination is not only capable of such development, but unites in itself the characteristics of the two parent forms and transmits them to its descendants. That fertilisation is not the intimate union of two bodies possessing a definite form, but that the male fertilising substance at least may be a simple fluid, appears to be distinctly shown by the process in Phanerogams; and we may assume, that in Cryptogams also, the sexual act is not affected by the form of the fertilisation-elements, though a certain shape and power of movement is necessary for the conveyance of the fertilising substance to that which is to be fertilised.