Though Du Hamel, like many later writers, treated Bonnet’s investigations, uncritical as they were and without plan, with great respect, he gave himself a much better account of the various movements of plants. In the sixth chapter of the fourth book of his ‘Physique des arbres,’ 1758, he discussed all the phenomena of the kind that were known to him under the title: ‘On the direction of stem and roots, and on the nutation of the parts of plants.’ Under the head of upright or oblique direction of the stem and roots, he speaks of geotropic, heliotropic, and some other curvatures; then follows a chapter on etiolation, and under the title, ‘Movements of plants, which approximate to some extent to the voluntary movements of animals,’ he enquires into the periodical and sensitive movements of the leaves of Mimosa; he winds up with a short account of Linnaeus’ flower-clock, and of the hygroscopic movements of the valves of fruits. The movements of tendrils and climbing stems, of which Du Hamel seems to have known little, are not mentioned in this connection; but they are noticed in a former chapter with hairs, thorns and similar things,—a plan which Cesalpino also adopted. If this way of dealing with the different movements of plants is to be taken as a classification of them, it was a very unsatisfactory one; for it separates like things, and brings together things unlike; still it is an improvement on Bonnet’s arrangement, while the author gives us also some new and valuable observations. He may claim to be the first who made heliotropic curvature depend on light, and it is a significant fact that he got this conclusion from Bonnet’s experiments. After examining, like Hales, into the distribution of growth in shoots, and discovering that this ceases with the commencement of lignification, he proposed to himself the question: at what spots does the lengthening of the roots take place, and he found from suitable experiments that every root-fibre grows only at its terminal portion, which is a few lines in length, and that no other part of it increases in length. In the chapter on the direction of the parts of plants he examines the explanations which had been given of heliotropic curvatures. Astruc and de la Hire had supposed the weight of the descending sap to be the cause of the downward curvature of the roots, and the lighter vapours which ascend in the tissue to be the cause of the upward curvature of the stem; Bazin on the contrary attributed the geotropism of the roots to the moisture in the earth. Du Hamel undertook to determine whether the moisture, the low temperature, or the absence of light in the earth made the roots curve downwards, and he was obliged by the result of his experiments to deny that they do. But he was unfortunate in his own explanation of the movements which we should now call geotropic, heliotropic and periodic, for he came to the conclusion that the ‘direction of the vapours’ inside the vessels of the plant and round about the plant has more to do with producing these movements than any other causes, and that if warmth and light appear to influence them, it is perhaps only because they produce vapours or communicate a definite movement to them. As regards the movements of the leaves of Mimosa, Du Hamel repeated the experiment made by Mairan in 1729, in which the periodic movement continued even in constant darkness; he found that this was so, and concluded that the periodic movements of Mimosa do not essentially depend on temperature and changes of light; Hill had determined in 1757 that the alternation of day and night was the cause of the movements connected with the sleep of plants, because he found that darkness artificially produced in the day-time made the plants assume the nocturnal position; but Zinn in 1759 came to the same conclusion as Mairan and Du Hamel. It was not till long after that the question was to some extent cleared up by Dutrochet. Du Hamel thought it necessary to give a formal refutation of the opinion expressed by Tournefort, that the movements of plants are produced by muscles, and to show that Tournefort’s vegetable muscles are hygroscopic fibres.
We have to mention in conclusion, that Du Hamel was the first who observed that the two branches of a vine-tendril twine in opposite directions round a support that happens to be between them; he also appears to have been the first who compared the irritability of the stamens of Opuntia and Berberis with that of Mimosa-leaves; the stamens of Berberis were afterwards examined by Covolo in 1764, by Koelreuter 1788, by Smith in 1790, and by others, but without leading to any discoveries respecting the nature of the irritability. Dal Covolo’s famous essay on the stamens of the Cynareae (1764) produced no absolutely final result, but it contained some particulars which threw light on the mechanical laws of these movements of irritability. Koelreuter, who studied these objects in 1766, thought less of discovering a mechanical explanation of them, than of finding arguments in the irritability of the stamens for the necessity of insects to pollination. An entirely new kind of movement was discovered by Corti in 1772 in the cells of Chara, which is now known as the circulation of the protoplasm; this form of movement in plants appeared at first to bear no resemblance whatever to the phytodynamic processes then known, and it was not brought into connection with them till a long time after; on the contrary an erroneous idea soon began to prevail, that it was a real rotation of the sap, as understood by the early physiologists; this idea held its ground till far into the 19th century, and being combined with mistaken notions respecting the movements of latex, was developed by Schultz-Schultzenstein into the doctrine of the circulation of the vital sap. For a time indeed Corti’s discovery was forgotten, and had to be reproduced by Treviranus in 1811. A somewhat similar fortune attended the discovery of the movement of the Oscillatorieae by Adanson in 1767, which misled Vaucher into pronouncing them to be animals.
3. Imperfect as were the theoretical efforts of the 18th century in this branch of botanical study, yet they aimed at tracing the various movements back to the play of physical forces. But in the closing years of the century another order of ideas, injurious to the healthy progress of science, made its appearance in this, as in other parts of botany and zoology. Even the majority of those who had no sympathy with the nature-philosophy and its phraseology, believed that there was in organised bodies something of a special and peculiar nature; because the attempts made to explain the phenomena of life by mechanical laws were on the whole unsatisfactory, all such explanations were looked upon as impossible and even absurd, while it was forgotten that the vital force, which was to explain everything, was a mere word for everything that could not be explained in the life of organisms. This vital force was personified, and seemed to assume a really tangible form in the movements of plants. But the moment that a phenomenon was handed over to this force, all further investigation was abandoned; the idea with regard to phytodynamical phenomena especially was that of the peasant, who could only explain the movement of the locomotive by supposing that there was a horse shut up in it. Moreover the knowledge of the inner structure of plants was at its lowest point at the end of the 18th century; the spiral threads which could be unwound were the only structural element whose form was to some extent understood, and their hygroscopic movements were supposed to be due to a combination of the pulsations of the vital force with the spiral tendency of the plant. At the same time whole bundles of vessels were taken for spiral fibres, or were supposed to consist of them, and these were thought to be vegetable muscles, which contract under the influence of various kinds of irritation, and so cause the movements in the organs of plants; but it was forgotten that in the organs which exhibit the most striking movements, such as sensitive leaves and leaves that suffer periodical changes of position, these ‘muscles’ occupy a central position which unfits them for the function ascribed to them. It would be unprofitable and wearisome to give many examples of what is here stated, though many might easily be collected; it will suffice to quote some sentences only from Link’s ‘Grundlehren der Anatomie und Physiologie’ of 1807; they are particularly instructive, because Link declared against the nature-philosophy and professed to be on the side of inductive science. Under the head of movements of plants, he discussed geotropic curvatures and other movements in the superficial manner of the time and only to come to the conclusion, that the direction of growth of stems and roots is caused by a polarity of a definite kind in every plant, from which we may argue, he says, ‘to higher connections of our planet in the world of space.’ He says again, ‘that it is natural to conjecture that light is the cause of the sleep of plants,’ and then gives the contradictory statements of Hill, Zinn, and De Candolle, all jumbled together into an inextricable tangle in a fashion which sets all maxims of reasonable discussion at defiance. He then puts aside all attempts at mechanical explanation with the remark, that plants observe their regular times of sleep even when kept in the dark and at a low temperature, for this evident habituation is one of the most important marks of vitality. He is led to similar results by Desfontaine’s observation, that a Mimosa, exposed to the shaking of a wheeled vehicle, closes at first but then opens again. Speaking of the rapid oscillations of the leaves of Hedysarum gyrans and similar movements, he rejects Percival’s idea of a will in plants, but says that the attempts to derive them from mechanical or chemical causes has only led to solemn trifling.
It is plain that men who could print such remarks as these and still worse than these, could not possibly effect anything in the province of botany which we are considering. The broad and shallow stream of such opinions as these flowed on till later even than 1830, but it ran dry at last when its supplies were cut off by the effect of new discoveries, and scientific investigation again gained the upper hand. Some calmer thinkers, who could not rest content with empty words, had meanwhile been pursuing the path trodden by Ray, Dodart, Hales, and Du Hamel, and by experiment and earnest reflection had brought new facts to light, which were at least calculated to pave the way for the mechanical explanation of phytodynamical phenomena. Senebier in his ‘Physiologic végétale’ (1700) had described some minute researches which he had made into the subject of etiolation; and though he made the great mistake of attributing the want of colour in the leaves and the excessive elongation of the stems to the decomposition of carbon dioxide which does not take place in the dark, yet he gave an example of genuine scientific investigation and again expressed its true spirit, when he said that the Linnaean phrase, ‘the sleep of plants,’ was unsuitable, because the sleeping leaves are not relaxed, but continue as stiff as in the day-time. De Candolle also, like Senebier, experimented in 1806 on the influence of light on vegetation, and succeeded in proving that the daily period of leaves may be reversed by artificial illumination; he was, as we have said above, an adherent of the theory of a vital force, but only made use of it when physical explanations failed him. The same year, (1806) is the date of a brilliant discovery, which was extremely inconvenient to the thorough-going adherents of the nature-philosophy and the vital force, and did much to bring the scientific study of the movements of plants back to the right path. Andrew Knight[137] showed by experiment that the vertical growth of stems and primary roots is due to gravitation; he attached germinating plants to a rapidly revolving wheel, and thus exposed them to the centrifugal force, either alone or combined with gravitation; the radicles, which normally follow gravitation, here took the direction of the centrifugal force, while the stems assumed the opposite direction. The next question was, why gravitation makes the roots and stems take exactly opposite directions, why, that is, in a plant placed in a horizontal direction, the root-end curves downwards and the stem upwards. Knight supposed that the root, being of a semi-fluid consistence, is bent downwards by its own weight, while the nutrient sap in the stem moves to the underside and causes stronger growth there, until by means of the curvature so produced the stem assumes the upright position. Here too, as in Dodart’s case, it was no great misfortune that the explanation proved afterwards to be insufficient; it served at the time to explain as much as was then known of the matter. The spirit of true scientific research displayed in Knight’s explanation of geotropism was expressed in many other contributions which he made to vegetable physiology; two only must be mentioned here. He showed in 1811 that under suitable conditions roots are diverted from the vertical direction by moist earth, an observation which was confirmed by Johnson in 1828 and afterwards forgotten. More attention was excited by his discovery in 1812, that the tendrils of Vitis and Ampelopsis are negatively heliotropical, that is, that they turn away from the source of light. A few other cases of this kind of heliotropism have since been discovered, and they are highly interesting, because they teach that there is the same opposition in the relations of plants to light as in their relations to gravitation. Knight possessed some of the direct and bold reasoning power of his countryman Hales; he defied the vital force, and was always ready with a mechanical explanation, if it was at all possible to find one. Thus he explained the twining of tendrils by supposing that the pressure of the support drives the juices to the opposite side, which consequently grows more vigorously and causes the curvature, which makes the tendril wind round the support. This theory was at all events better than the one which von Mohl sought to put in its place in 1827, and no better one was suggested till very recently. Much the same may be said of Knight’s explanation of geotropic curvatures; it is true that Johnson showed in 1828 that the ends of roots as they curve downwards set in motion a heavier weight than themselves, and therefore do not simply sink down, and Pinot in 1829, that they force their way even into quicksilver, and that consequently Knight’s theory, at least as regards the roots, is unsatisfactory; but no better theory has yet been found, and his view also of the progress in the upward curvature of the stem has not given place to any one that can be said to be more generally accepted.
It was the commonly received opinion till after 1820 that the movements of the parts of plants are produced by the spiral vessels, or, which meant the same thing in those days, by the vascular bundles. It was an important event therefore when Dutrochet proved in 1822, that the movements of the leaves of Mimosa were due to the alternate expansion of the antagonistic masses of parenchyma in the pulvinus or cushion of succulent tissue found at the articulation, and that the central vascular bundle follows passively their curvatures. Lindsay had indeed arrived at the same conclusion from similar experiments as early as 1790, but his unprinted essay on the subject was first produced by Burnett and Mayo in 1827. Meanwhile Dutrochet had also found that light influences the movements of the leaves in different ways; alternation of light and darkness excites them to motion, while leaves which have become rigid in continued darkness are restored by light to their normal condition of sensitiveness.
Much attention was bestowed in the period between 1820 and 1830 on various questions connected with the movements of the organs of plants. In 1826 the faculty of medicine in Tübingen offered a prize for an essay on the peculiar nature of tendrils and climbing plants, which was intended to bring into discussion all the points which required to be cleared up before a more thorough understanding of the whole subject could be obtained. The two essays which gained the prize were published in 1827. One was by Palm, the other by von Mohl, both of very different value. Palm’s essay is a good and careful college-exercise; but there is nothing of this character in von Mohl’s. The skill of the composition, the exact knowledge of the literature of the subject, the wealth of personal experience, the searching criticism, the prominence given to all that is fundamental and important, the feeling of certainty and superiority which the book inspires, all unite to make the reader forget that it is not the work of a mature and professed naturalist, but of a student of two-and-twenty years of age. This academical prize-essay on the structure and twining of tendrils and climbing-plants was one of von Mohl’s best works, and altogether the best that appeared on the subject before Darwin wrote upon it in 1865; at the same time it must be said that von Mohl did not explain the exact mechanical processes in the tissues, for he assumed a sensitiveness in both cases which causes the winding round the support, and thought that this sensitiveness must be conceived of ‘dynamically’ and not ‘mechanically.’ Nevertheless von Mohl conducted his investigation up to this point according to strict rules of inductive science, and studied the facts which were capable of being established by observation and experiment with an exactness such as had not yet been applied to any case of movement in plants. It was a genuine production of its author, strictly inductive up to the point at which deduction became necessary. Von Mohl pointed out in it essential differences in the behaviour of tendrils and climbing plants, and the corresponding distinction between the organs which have to be considered in each case, and he made the important discovery that contact with the support acts as a stimulus on the tendril, though he was wrong in supposing that the climbing stem also is similarly affected. He at once assented to Dutrochet’s new view, that it is not the vascular bundles but the layers of parenchyma which produce the movements. He distinctly rejected the notion constantly repeated, though with some hesitation, since the time of Cesalpino, that tendrils and climbing-plants ‘seem to seek for’ their supports, as also the idea which many had adopted without reflection from Grew, that the varying direction of a climbing-stem is due to the varying influence of the course of the sun and moon, and showed that the movements of nutation in the stem are sufficient to explain the apparent seeking for the support; it is true that he did not fully explain the corresponding phenomena in tendrils, but he saw enough to set aside the old ideas. We must not here go further into his many, and for the most part excellent, observations; some of course had afterwards to be corrected, but the important point was, that his full investigation of the subject showed how such phenomena must be studied, if we are to arrive at a strictly mechanical explanation of them.
If von Mohl had attempted to give a mechanical explanation of the processes in the tissue of twining organs he must necessarily have failed from ignorance of the agency of diffusion, which must certainly be taken into consideration. This agency was not discovered by Dutrochet till the year (1826) in which von Mohl undertook his investigation, and some time elapsed before it was sufficiently understood to be successfully applied to the explanation of phenomena in vegetation. Dutrochet did indeed attempt so to apply his theory in 1828, and showed that changes in the turgidity of tissue are produced by endosmose and exosmose, and consequently that a new mechanical method of explanation had been discovered for processes which had been usually referred to a supposed vital principle; but in his later and more detailed researches into geotropism, heliotropism, periodical movements and movements of irritability, which he collected together in his ‘Mémoires’ of 1837, he fell into two different mistakes: he assumed conditions of size and stratification in cells which do not actually exist, for the purpose of explaining very various kinds of curvature by endosmose, and he was not satisfied with endosmose in the parenchyma; he postulated changes in the vascular bundles also, which were supposed to be produced by the influence of the oxygen in a way which he did not explain. Thus there were blots in his explanation of separate processes, and his mechanical theories remained unsatisfactory; but it is worthy of recognition and was most important for the development of phytodynamics, that he was thoroughly in earnest in his purpose of explaining every movement in plants by mechanical laws. Even the opponents of such explanations were obliged to go deeply into mechanical relations in order to refute him, and no one could any longer be imposed upon by the simple assertion that all depends on the vital force; so devoted a partisan of vital force as Treviranus had to deal with endosmose as an established principle. Moreover Dutrochet’s copious investigations presented such an abundance of interesting observations, delicate combinations, and suggestive considerations, that the study of them is still instructive and indeed indispensable to any one who is occupied with such researches. Comparison of his papers in the ‘Mémoires’ of 1837 with what was before known on the mechanical laws of the movements of plants leaves us in no doubt that energetic mental effort had taken the place of the old complacent absence of thought.
Still no single movement had as yet been fully explained on mechanical principles; but by the year 1840 clearer views had been attained on the whole subject; the co-operation of external agencies was in substance recognised, and the different forms of movement were better distinguished, though much still remained to be done in this direction; and as regards the mechanical changes in the tissue of the parts capable of movement, a factor had been given in endosmose which must be taken into account, though it might be necessary to seek a different mode of applying it.
4. Before proceeding to give some account of the theoretical efforts that were made in this subject between 1840 and 1860, it should be mentioned that new cases of movement in plants had been discovered. Dutrochet observed that the stem in the embryo of Viscum is negatively heliotropic, and had carefully studied its behaviour; he opposed the old notion that the geotropic downward curvature is peculiar to main roots, and that that is the reason why they are in ‘polar’ opposition to the stem, by pointing to the shoots of the rhizomes of Sagittaria, Sparganium, Typha, and other plants, which at least when young curve downwards with some force; and on extending Knight’s experiment with a rotating wheel he found that the leaves also exhibit a peculiar geotropism. These observations and some new examples of periodical movement and movements of irritability were connected without difficulty with the forms of movement that had been long known in the vegetable kingdom, and contributed to correct the views that had been entertained respecting them. But this was not the case for a time with two phenomena which also fall within the province of phytodynamics, namely normal growth and the movements of the protoplasm, which exhibit the two opposite extremes, so to speak, of the facts connected with movement. Various measurements had been made of the growth of plants since the beginning of the century, and attempts had been made to establish its dependence on light and heat, but without any great success. Treviranus had rediscovered the movements of the protoplasm in 1811 in Nitella. Similar movements were repeatedly pointed out by Amici, Meyen, and Schleiden in the cells of higher plants, but they were taken for streamings of the cell-sap; it was still unknown that all these were movements of the same organised substance, which moves independently in water in the form of swarmspores. These phenomena, especially the movements of swarmspores, were noticed and studied separately between 1830 and 1840, but no one thought of bringing both these movements and the mechanical laws of normal growth into connection with the phenomena which had usually been treated together under the head of movements in the vegetable kingdom. De Candolle and Meyen did not mention them in this connection in their ‘Compendia’ of 1835 and 1839; Meyen on the contrary discussed the ‘circulation of the cell-juice’ with nutrition, and the movement of swarmspores with the propagation of Algae. The two writers just named, like Du Hamel before them, divided into two main groups the movements in the vegetable kingdom which had been long known and were usually put together, and treated of geotropic and heliotropic curvatures and the movements of tendrils and climbing plants under the head of direction of plants, and the periodical movements and movements connected with irritability under that of movements, though they gave no reasons for this classification; it rested evidently on an indistinct feeling outrunning clear perception—that in the one they were dealing with growing parts of plants, in the other with parts which had ceased to grow. Dutrochet made no such distinction, but he was the only one among the chief representatives of vegetable physiology between 1830 and 1840 who had thoroughly adopted the mechanical view of phytodynamical phenomena. We have said that Treviranus was a warm adherent of the theory of vital force. De Candolle and Meyen, it is true, endeavoured to explain each separate movement if possible by mechanical laws, but in their more general speculations they readily lapsed into antiquated views; thus De Candolle speaks of the sensitiveness of Mimosa as a case of extreme ‘excitability,’ and Roeper, in accordance with his other views, translated De Candolle’s expression, autonomous movements, by the term ‘voluntary’ movements. The movements he is speaking of are those of Hedysarum gyrans, and Meyen also terms them ‘voluntary’ movements, and ranks them with those of Oscillatoria. That he was influenced in this by a dim reminiscence of the old vegetable soul is shown by the heading, ‘Of movements and sensation in plants,’ placed over the section of his work in which the expression occurs; and in the last chapter of this section, he attributes some kind of sensation to plants on account of the evident marks of design in their movements, though he veils his meaning in obscure and tortuous phrases.
5. The mists of the nature-philosophy and the vital force disappeared from the phytodynamical province of botanical science after the year 1840. The methodical research of inductive science, which had still to contend with them up to that time, was once more acknowledged as the supreme guide and ruler. A few stray dissentients were still to be found, but the general voice was against them. There was an eager desire for exact investigation of the facts, in order to lay a firmer foundation for future theory. But no conclusive results, no such entirely new points of view were gained before 1860, as were established during the same time in phytotomy, morphology, and systematic botany. To these subjects the most eminent enquirers applied their best powers almost exclusively, while phytodynamics vanished from the field of view of the generality of botanists, and no one made them the object of the comprehensive, intense, and effectual study, which Dutrochet had previously devoted to them. At the same time his example was not without a powerful effect. The working of endosmose was further investigated and treated as a part of molecular physics. Greater freedom was thus gained in the mechanical treatment of phytodynamical questions, and a firmer basis secured by aid of the advances which were being at the same time made in phytotomy. But with the exception of Brucke’s essay on Mimosa (1848), the works produced during this period were chiefly devoted to the critical examination of the writings of previous observers, and whatever appeared that was new and positive remained incomplete till after the date at which this history ends. Under these circumstances we must be content to indicate briefly the more important of the new discoveries and of the efforts made at this time to advance the theory of the subject.