Several observers occupied themselves soon after 1840 with the influence of light on the growing parts of plants. Payer maintained in 1843 that the radicles of various Phanerogams turn from the light, and a controversy arose between him and Dutrochet on the point, in which Durand took part in 1845, but no definite conclusion was arrived at even as regards the certainty of the fact. The beautiful discovery of Schmitz in 1843, that the Rhizomorphs grow more slowly in the light than in the dark, and arc at the same time negatively heliotropic, might have proved much more important; but the theoretical value of this fact has till quite recently been entirely misconstrued. Sebastian Poggioli had discovered in 1817 that highly refringent rays of light were more heliotropically active, and the fact was confirmed by Payer in 1842; but Dutrochet in 1843 maintained, and incorrectly, that it is the brightness of the light, and not its refrangibility, which is the determining factor. Zantedeschi found in 1843 that red, orange, and yellow light are heliotropically inactive. Gardner on the contrary in 1844, and Guillemain in 1857, came with the help of the spectrum to the conclusion that all its rays are heliotropically active, and the question long remained hampered by these contradictory statements, till it was taken up again in 1864. This was a similar case to that of the question of the effect of variegated light on the elimination of oxygen and the formation of chlorophyll. Daubeny had given attention to the subject in 1836 and inclined to the view, that it was the brightness of the light rather than its refrangibility which was the important point; and Draper’s observation, made with the spectrum in 1844, that the elimination of oxygen reaches its maximum in yellow light and decreases on each side of it, was generally understood as though it was a question only of the brightness of the light. It is only within recent times that this view has been abandoned, and in the same way all the investigations which have just been mentioned were not settled till after 1860, and were scarcely turned to any theoretical account.

The bright point in the history of phytodynamics at this time is Brücke’s treatise in 1848 on the movements of the leaves in Mimosa, not only on account of the very important results which it records, but still more for the exactness of its method which has made it a model of research in these subjects. He first established the essential difference between the periodical nocturnal position of the leaves of Mimosa and the position which they assume when irritated, and showed that the former is connected with an increase in turgidity, the latter with relaxation; he showed further that if the upper half of the organ is removed, the periodical movements and the irritability both continue. Of great importance to the theory was the clear account given of the tension which is produced between the vascular bundle and the turgescent layer of parenchyma, and the reference of the periodic movements and of those of irritation to the movements of water in the antagonistic masses of parenchyma. The details were still imperfect, but one great advantage was secured, namely, the doing away with the mysticism associated with the idea of irritability, from which even von Mohl was not entirely free.

A full enquiry into the downward curvature of roots, published by Wigand in 1854, deserves mention, because it threw some light on the theory of the strictly mechanical questions connected with a subject which had been for some time neglected, and because, while containing other instructive matter, it refuted the theory, founded on endosmose and on the structure of tissue, which had been suggested by Dutrochet and adopted by von Mohl, since it showed that one-celled organs also exhibit geotropic curvatures. The great theoretical importance of the fact that all the various phytodynamical phenomena, with the exception of movements of irritability, are manifested in one-celled organs, was not fully understood till after 1860.

It has been already observed, that no theoretical result was obtained from the discovery of circulation in cells made by Corti in 1772, and repeated by Treviranus in 1811. The same may also be really said of the later observations of Amici, Meyen, and Schleiden, which went to show that such movements occur very generally in vegetable cells. In like manner the movements of swarmspores, of which a considerable number of instances had been observed before 1840, were rather the subject of astonishment than of scientific consideration. They could not in fact find their place in the general system until Nägeli and von Mohl discovered in 1846, that it is in the protoplasm that the so-called movement of the cell-sap takes place, and Alexander Braun made it known in 1848 that the swarmspores are naked masses of protoplasm, and indeed true vegetable cells. A new substratum for the movements in plants, and one of the simplest kind, was thus obtained; and Nägeli attempted in 1849 a mechanical explanation of the movements of swarmspores, while in 1859 De Bary exhibited in the Myxomycetes most instructive examples of such movements. If Nägeli failed in his attempt, yet it seemed possible that the protoplasm had an important share in the production of all phytodynamic phenomena, and the idea appeared capable of a very wide application when Unger pointed out in 1855 the resemblance between vegetable and animal protoplasm. It is true that not one of these later observations led to any conclusive results till after 1860; but that the whole subject of phytodynamics had made considerable advance as early as 1850 is apparent from the account given of it by von Mohl in his ‘Vegetabilische Zelle’ of 1851, and by Unger in his ‘Lehrbuch der Anatomie und Physiologie der Pflanzen’ of 1855. Von Mohl chiefly exposes the unsatisfactory nature of the attempts that had been made to explain the phenomena; Unger, on the other hand, shows how much that is fundamentally important had been already established.

The mechanics of growth had not been included by former writers among the phenomena of phytodynamics, nor was it so included by either Unger or von Mohl. It seemed to be supposed that there was a fundamental difference between growth and other movements in the vegetable kingdom, and this idea was entertained even in the most recent times. From the time of Mariotte and Hales no one had made the mechanical laws of growth the subject of special investigation or theoretical consideration; yet some observations had been made on the formal relations of growth and its dependence on external influences. Ohlert (1837) was the first after Du Hamel who studied the distribution of growth in the root; Cotta in 1806, Chr. F. Meyer in 1808, Cassini in 1821, Steinheil and others made measurements in connection with the same question in the stem, but only with the result of showing that the distribution of growth at the internodes may vary very greatly, and even Münter’s measurements in growing internodes in 1841 and 1843, and Grisebach’s in 1843 led to no appreciable result, because the observers neglected to apply the figures obtained to the theory of the subject. It seemed to be generally supposed that it was enough simply to write down the measurements in figures, and that a theoretical result would spring into being of itself; on the contrary the real scientific work begins after the figures are obtained. The same cause prevented the observations which have yet to be mentioned from producing real fruit. The influence of the variability of the temperature of the air[138], and of the alternation of daylight and darkness on the longitudinal growth of internodes and leaves after they have emerged from the bud-condition, had often been investigated; Christian Jacob Trew published in 1727 long-continued daily measurements on the flowering stem of Agave Americana in conjunction with observations on temperature and weather; a hundred years later similar observations were made by Ernst Meyer in 1827, by Mulder in 1829, and by Van der Hopp and De Vriese in 1847 and 1848; but Harting in 1842 and Caspary in 1856 were the first who went at all deeply into the questions involved. These observations, some of which were carefully made, led to no further result than the discovery of the fact, which Münter indicated and Harting applied to theoretical purposes but which no one else thought worthy of attention, namely that the rate of growth increases at first and independently of external causes, till it reaches a maximum, and then decreases till at length it comes to an end; they did not even establish a really practical method of observation. Scarcely two observers arrived at the same result, because the questions respecting the relations of growth in length to temperature and light had not been clearly and distinctly put. Communications were published in the periodicals, which simply tabled long-continued measurements of the longitudinal growth of parts of plants, and gave an idea of constant irregularity of growth, without suggesting any explanation of the causes which produced it; so indistinct were the ideas of observers on these subjects even after 1850, that the majority of them proposed to themselves the question, what difference there is between growth by day and by night; it did not occur to them that day and night are not simple forces of nature, but different and very variable complications of external conditions of growth, such as temperature, light and moisture, and that such a mode of putting the question could not possibly lead to the discovery of the relations established by law, so long as the several factors were unknown which are included in the conceptions of day and night. Harting’s essay of 1842 is superior to those above mentioned, inasmuch as he distinctly endeavoured to obtain from his measurements some definite propositions that might be applied to the theory of the subject, and especially to give a mathematical expression to the dependence of growth on temperature, but his success in this particular point was not great. The idea, that there must be a simple arithmetical relation to be discovered between growth and temperature, had been suggested by Adanson in the previous century, and it found many supporters in the period between 1840 and 1860: but it should be observed that the term growth was used in a loose and popular sense to sum up all the phenomena of vegetation in one expression. Adanson had supposed that the time occupied in the unfolding of the bud was determined by the sum of the degrees of the mean daily temperature, reckoned from the beginning of the year; Senebier, and at a later time De Candolle, declared against the existence of any such relation, but a similar idea was not only very generally entertained after 1840, but it even came to be treated as a probable natural law. Boussingault had pointed out that in the case of cultivated plants in Europe and America, if the whole period of vegetation expressed in days is multiplied by the mean temperature of the same period, the products do not deviate widely from one another in the same species. It was thereupon assumed that these deviations are due to incorrect observation, and that such a constant product of the period of vegetation and the mean temperature will be found in every species. This product then received the unmeaning appellation of the sum of the temperature. If such a relation between vegetation and temperature really exists, it would necessarily follow that other things, such as light, moisture, the soil, &c., have no influence at all on the period of vegetation, not to speak of those internal causes which help to complicate the simplest processes of growth. It is unnecessary to expose in this place the absurdities involved in this idea of the sum of the temperature; the needful remarks will be found in the ‘Jahrbücher für wissenschaftliche Botanik’ of 1860, i. p. 370. It is a remarkable fact however that such monstrous reasoning should have been able to prejudice science in various ways even later than the year 1860. A new science was actually invented and called Phaenology, which accumulated thousands and thousands of figures, in order to discover the sum of the temperature for every plant, and as this crude empiricism showed that the simple multiplication of the period of vegetation by the temperature gave no constant result, the square of the temperature was tried and other tricks of arithmetic adopted. Though Alphonse de Candolle as early as 1850 expressed well-founded objections to the whole of this method of treating the subject, in which the mean temperature played much too important a part, yet he was so far unable to keep clear of the prevailing ideas, that he thought he could express the effect of light by an equivalent number of degrees of temperature, and so save the supposed law of temperature in vegetation. To this idea may be traced his work on the geography of plants, published in two volumes in 1855, which however contains a rich treasure of personal experience and knowledge of the works of other writers.

It appears then that scarcely any point of fundamental importance in phytodynamics was cleared up before the period at which this history closes; it was not till after that date that these questions began to be studied from new points of view, and they are at the present time still under discussion.

[INDEX.]

THE END.