The peculiar vessels, known as ‘vasa propria,’ were a stone of stumbling to Moldenhawer, as they were to his predecessors and to many of his successors, because misled by the resemblance in their contents he included under this name forms of very different kinds. A very good description of the soft bast in the vascular bundle of the maize-plant is followed by a notice of the milk-tubes of Musa, the milk-cells of Asclepias which he explains incorrectly, and the milk-vessels of Chelidonium which he understood better. All these ‘vasa propria’ he took for cellular vessels, formed of tubes opening into one another; but he clearly distinguished the turpentine-ducts from them, and has given a correct figure of such a duct from the pine, though he assumes the existence of a special membrane lying inside the cell-rows which surround it, and lining the passage. Finally he passes on to the intercellular spaces, which he considers to be gaps in the cellular substance, and illustrates by Musa and Nymphaea. He does not notice particularly the narrow interstices which Treviranus had observed traversing the parenchyma.
In the second portion of his work he includes all the vessels found in the vascular bundle of the maize-plant under the term spiral vessels, but he distinguishes the different forms of them well, and especially points out that rings and spirals appear on one and the same vascular tube in different parts of its course, as Bernhardi had already shown. The isolating of the vessels gave him a better opportunity of seeing how they are made up of portions of different lengths than his predecessors had enjoyed, and he proves at some length the existence of a thin closed membrane forming the vessel, but like Hedwig he places the thickenings on the outside. He as little overcame the difficulties of bordered pits as did von Mohl and Schleiden after him. In this case as in others, it was the history of development which first taught the true nature of these formations (Schacht, 1860).
It was mentioned in the Introduction that Moldenhawer may be said to close the first portion of the period from 1800 to 1840, not only because the majority of the questions ventilated up to that time were to a certain extent settled by him, but also because there is no material advance in phytotomy to be recorded for several years after the publication of his work in 1812. It is true that Kieser in his ‘Grundzüge der Anatomie der Pflanzen’ (1815) attempted a connected exposition of the whole subject, but his book offers nothing really new, being merely a playing with the unmeaning phrases of the current nature-philosophy, while it revived gross errors like Hedwig’s doctrine of the presence of lymphatic vessels in the tissue of the epidermis, and made the Mosses consist of conferva-threads. Phytotomy was on the contrary really enriched by the miscellaneous works of Treviranus published in 1821, especially in respect to questions connected with the epidermis, and by Amici’s discovery in 1823, that the intercellular spaces in plants contain not sap but air, and that the vessels too chiefly convey air. We may quietly pass over the later writings of Mirbel, Schulze, Link, Turpin and others, which appeared after 1812 and before 1830, as our business is not so much with an account of the literature of the subject as with evidence of real advance.
Meyen and von Mohl may be said to have commenced their labours with 1830, and in the course of the succeeding ten years they became the chief authorities on phytotomy, though a highly meritorious work of Mirbel’s on Marchantia polymorpha and the formation of pollen in Cucurbita falls as late as 1835. We may even pass over so elaborate a work as the ‘Physiologie der Gewächse’ of Treviranus (1835-1838), which embraces also the whole of phytotomy, because though its treatment of some of the details is good, it presents its subject virtually from the points of view opened before 1812. This work, though it neglects no part of its subject and contains much useful reference to the works of other observers, was unfortunately out of date at the time of its appearance, for owing to von Mohl’s labours an entirely new spirit had entered since 1828 into the treatment of phytotomy.
Though Meyen and von Mohl must be regarded as the chief representatives of phytotomy from 1830 to 1840, yet they are men of very different importance in the science. The essential difference between them cannot perhaps be better shown than by pointing to the fact, that Meyen’s labours cannot at present claim more than a historical interest, while von Mohl’s earliest investigations between 1828 and 1840, so far from being obsolete, are the sources of our present knowledge, and from them every one must still draw who proposes to cultivate any portion of phytotomy. Meyen’s views, in spite of the many investigations which he made himself, are entirely confined within the circle of thought represented by the Göttingen essayists, though in his observations he went beyond them, and even beyond Moldenhawer; but the phytotomical views of these men were from the first no law to von Mohl; he took up an entirely independent position at once with respect even to Moldenhawer and Treviranus, though a longer time certainly elapsed, before he succeeded in freeing himself wholly from Mirbel’s authority. For these reasons, and because Meyen’s work was interrupted by his death so early as 1840, while von Mohl aided to advance phytotomy for another thirty years, we will speak first of Meyen’s labours in that department.
Meyen[80] is remarkable for the extraordinary number of his written productions. In 1826, at the early age of twenty-two, he wrote his treatise ‘De primis vitae phenomenis in fluidis’; two years later he published researches anatomical and physiological into the contents of vegetable cells, and in 1830 appeared his ‘Lehrbuch der Phytotomie,’ founded on his own investigations in every branch of the subject, with many figures on thirteen copper plates very beautifully executed for the time. His industry as a writer was then interrupted by a voyage round the world made in the years 1830-1832, but was again marvellously productive during the last four years of his life (1836-1840); it is difficult to conceive how he found time even for the mechanical part of his work, for in 1836 he published his treatise on the latest advances in vegetable anatomy and physiology, a quarto volume of 319 pages with twenty-two plates, which gained the prize from the Teyler society in Haarlem; the figures are well drawn, the style is that of a practised writer, but the matter of the work is somewhat superficially handled. A year later (1837) appeared the first volume of his ‘Neues System der Pflanzenphysiologie,’ and two more volumes by the year 1839,—a work also rich in new observations and figures. In the course of the same years (1836-39) he wrote detailed annual reports of the results of investigations in the field of physiological botany, which fill a portly volume, and published in 1837 a prize-essay on the organs of secretion, and in 1836 a sketch of the geography of plants; in 1840 appeared a treatise on fructification and polyembryony, and a posthumous work on vegetable pathology in 1841. The number of works thus given to the world between the years 1836 and 1840, though partly prepared before that period, is so unprecedented, that it is impossible for the composer to have maturely meditated his facts or their inner connection, and the study of his writings shows that he was often too hasty in propounding new views, and in rejecting or accepting the statements of others. The style is perspicuous and flowing, and animated by a genuine scientific spirit; but the expressions are often inexact, the ideas not unfrequently immature, and points of fundamental importance are sometimes neglected for unimportant and secondary matters. These faults are the result of hasty production; we must set against them conspicuous merits; Meyen had an eye open to every question in phytotomy and left nothing unnoticed, while he made it his constant aim to give clear general views of his subject as a connected whole, and enable his reader to see his way in every direction, in order to make phytotomy and vegetable physiology accessible to wider circles of scientific men; the like praise is due to his drawings from the microscope which are beautifully executed; they present to the reader not the small fragments of earlier phytotomic works but whole masses of tissue so connected together, that it is possible to gain some insight into the disposition of the different systems of tissue and their mutual relations. The superiority of Meyen’s drawings of 1836 as compared with those of 1830 is very striking, though he used the same microscope in both cases and the same magnifying power of two hundred and twenty times.
To learn what were Meyen’s independent contributions to the advance of phytotomy, we must turn to his ‘Phytotomie’ of 1830; for in his later works and especially in the ‘Neues System der Physiologie’ of 1837 he was able to avail himself of von Mohl’s earliest and searching investigations; these necessarily influenced his views, though he always assumed the character of a rival and opponent of von Mohl, and treated not only Treviranus and Link, but even Kieser and men of his stamp, as entitled to equal rank with him. And as in his later writings he was reluctant to acknowledge von Mohl’s services to science and overlooked their fundamental importance, so in his earlier work in 1830 he often appears as an assailant of Moldenhawer and tries to set up Link’s authority against him; we find to our astonishment in the first volume of the ‘Neues System’ a dedication to Link as the ‘founder of German vegetable physiology.’ The position of a scientific man in relation to his science as a whole is certainly most simply and clearly defined by his judgment on the merits of his contemporaries and predecessors, and we may conclude from what has now been said that Meyen moved within the circle of ideas of the Göttingen prize-essays, and did not clearly see the importance of the points of view opened by Moldenhawer and von Mohl; though it must always be allowed that Meyen working independently far outstripped Link on his own path.
If it was our purpose to write a biography of Meyen, we should have to go through his works, and show the steps by which his views arrived at clearness and precision; it is sufficient in this history to show what was peculiar and original in his general conception of the problems of phytotomy. This appears most plainly in the ‘Phytotomie’ of 1830; and we may base our historical survey on that work because its views are in the main those of the first volume of the ‘Neues System’ which appeared seven years later, and still more because a detailed examination of the later publication would involve us in a lengthy discussion on Meyen’s scientific relation to von Mohl. It is less important in this place to give an estimate of Meyen’s character as a man of science than to show, how in the year 1830, when Mohl was beginning to apply himself to phytotomy but as yet exercised no important influence on opinion, views on the structure of plants were formed by one who gave himself up to its study with decided ability and great zeal; in this way we shall gain a standard by which to judge of the advance made chiefly by von Mohl and in part by Mirbel during the succeeding ten years. In judging of Meyen’s book, we must not forget that it was written when he was only twenty-five or twenty-six years old, and that it is under any view of it a remarkable performance for so young a man.
Meyen adopted three fundamental forms of elementary organs in plants; cells, spiral tubes, and sap-vessels; systems, he says, are formed by union of similar elementary organs; hence there is a cell-system, a spiral tube-system, and a system of sap-vessels (vascular system). We see at once by this classification how closely he follows the ideas formed before Moldenhawer. The establishment of these three systems is a retrograde step, since Moldenhawer had already clearly distinguished between vascular bundles and cell-tissue. Meyen then discusses each system at length and shows how they are grouped together. He lays great stress, as he did also at a later period, on the difference in the characteristic forms of cell-tissue, for which he introduced the names merenchyma, parenchyma, prosenchyma and pleurenchyma. These he calls regular cell-tissue, the shapes of the cells being like geometrical bodies, in opposition to the irregular tissue of Fuci, Lichens and Fungi. It is a decided improvement on former practice, and one that marks his later works also, that in connection with the structure of the solid cell-fabric he discusses the contents of cells in a special chapter, in which first the matter in solution, then the granular bodies with organized structure are considered, though with the latter he classes not only starch-grains, chlorophyll-corpuscles and the like, but also the spermatozoa in pollen-grains and layers of thickening matter projecting on the inside of cell-walls, such as the spiral bands in the elaters of Jungermannieae and several similar formations. He describes the crystals in vegetable cells at some length, and finally discusses the movement of the cell-contents (‘sap’), not omitting that of rotation in the Characeae as observed by Corti, and in other water-plants. The chapter on intercellular spaces also shows considerable advance on the views which obtained in 1812; Meyen calls it an account of the spaces produced in cell-tissue by the union of the cells; the true intercellular passages filled with air are here distinguished from receptacles of secretions, resin-passages, gum-passages, oil-passages, and secretion-receptacles of the nature of cavities. The large air-passages and gaps, such as occur in water-plants, are a third form of intercellular space; his air-canals in the wood of oak filled with cell-tissue are obviously vessels filled with the substance known as thylosis. The form of the cells in the tissue he thinks is not due to mutual pressure, and he rejects Kieser’s view that the ideal fundamental form of cells must be a rhombododecahedron; but he thinks there is a significant resemblance between the shape of cells and that of basaltic columns.
In dealing with the spiral tube-system he first discusses the spiral fibre, which appears, he says, either detached between the cells or inside them as well,—an account of the matter decidedly inferior to those of Bernhardi and Treviranus. The spiral tubes are, he says on page 225, cylindrical or conical bodies formed of spiral fibres which are afterwards surrounded by a delicate membrane. He puts annular, reticulated, and pitted vessels together as metamorphosed spiral tubes. His explanation of these forms cannot well be understood except by supposing that he assumed an actual metamorphosis in time in accordance with the view of Rudolphi and Link; but he afterwards in his ‘Neues System,’ i. p. 140 declares this to be a misunderstanding, though his real meaning is still doubtful; the obscurity attending the doctrine of metamorphosis did not fail to cause misunderstandings in phytotomy, as it did in the morphology of organs. Meyen makes only the striated and pitted vessels in the wood convey air, the true spiral vessels sap. That vessels are formed from cells, as Mirbel had already maintained and Treviranus had partly observed, Meyen intimates indeed, but not with an air of entire conviction.