was extinguished with Hooke and Grew, and has so remained, we may
almost say, till the present day; in Holland also Leeuwenhoek found
none to follow him of equal rank with himself, and the work done in
Germany up to the year 1770 is more wretched than can well be imagined.
There was in fact no original phytotomic research in the first fifty
or sixty years of the last century; the accounts which were given of
the structure of plants were taken from Malpighi, Grew, and Leeuwenhoek
by persons, who, unable to observe themselves, did not understand
their authors and stated things not to be found in their writings.
The feebler and obscurer notions of the older writers were preserved
with a particular preference, and thus it was Grew’s complicated idea
of the web-like structure of cell-walls that made most impression on
those who reported him. This state of decline must not be ascribed to
imperfect microscopes only; these certainly were not good, and still
less conveniently fitted up; but no one saw and described clearly even
what can be seen with the naked eye or with very small magnifying
power; the worst part of the case was that no one tried fully to
understand either the little he saw himself or the observations to be
found in older works, but contented himself from want of reflection
with most misty notions of the inner structure of plants. It is not
easy to discover the causes of this decline in phytotomy in the first
half of the 18th century; but one of the most important appears to lie
in the circumstance, that botanists, following in this the example of
Malpighi and Grew, did not make the knowledge of structure the sole
aim in their anatomical investigations, but sought it chiefly for the
purpose of explaining physiological processes. The food and circulation
of the sap of plants were more and more the prominent questions, and
Hales showed how much may be done in this direction even without the
microscope; the interest therefore of the few, who like Bonnet and Du
Hamel occupied themselves almost entirely with vegetable physiology,
was concentrated on experiment.
Others who knew how to use the microscope, as the Baron von
Gleichen-Russworm and Koelreuter, were drawn away from the examination
of the structure of vegetable organs by their attention to the
processes of fertilisation and especially of propagation. The real
botanists, according to the ideas of the time, and specially those
who belonged to the Linnaean school, considered physiological and
anatomical researches generally to be of secondary importance, if
not mere trifling, with which an earnest collector had no need to
concern himself. That Linnaeus himself thought little of microscopical
phytotomy is sufficiently shown by what has been said of him in the
first book.
It is not worth while to notice each of the few small treatises on the
subject which appeared towards 1760, for they contain nothing new; a
few examples will show the truth of the opinion here expressed on the
general condition of phytotomy at this time.
We first of all encounter a writer, whom few would expect to find
among the phytotomists, the well-known philosopher Christian Baron von
Wolff, who in his two works, ‘Vernünftige Gedanken von den Wirkungen
der Natur,’ Magdeburg (1723) and ‘Allerhand nützliche Versuche,’ Halle
(1721) gives here and there descriptions of microscopes and discusses
subjects connected with phytotomy. This he does more particularly
in the latter work, where he describes a compound microscope with a
focussing lens between the objective and the ocular but without a
mirror, an instrument which must have served therefore for observing
with the light from above on an opaque surface; the objective was a
simple lens. But to magnify objects more strongly, he says that he
used a simple instead of this compound instrument, as was more the
custom at the time. Like a true amateur Wolff submitted all sorts of
small and delicate objects to his glass, without examining any of
them thoroughly and persistently. His phytotomic gains were small; he
observed for instance that starch-flour (powder) consists of grains,
but believed from the way in which they refracted light that they were
small vesicles filled with a fluid; yet he satisfied himself that these
grains are already in the grains of rye and therefore not produced in
the grinding. He laid thin sections of portions of plants on glass
which was too imperfectly polished to allow of his seeing anything
distinctly. His pupil Thümmig in his ‘Meletemata’ (1736) addressed
himself to the subject with still less skill. By the case of these
two men we may see plainly that want of success was due much less to
the imperfectness of the microscope than to unskilful management and
unsuitable preparation. But Wolff and Thümmig at least endeavoured
to see something for themselves of the structure of plants; a famous
botanist of the time, Ludwig, plainly never made a similar attempt,
for in his ‘Institutiones regni vegetabilis’ (1742) he speaks of the
inner structure of the plant in the following manner; ‘Laminae or
membranous pellicles, so connected together that they form little
cavities or small cells and often reticulated by the intervention of
fine threads, form the cell-tissue which we see pervading all parts
of plants. These are what Malpighi and others call tubes, since they
appear in different parts in the form of rows of connected vesicles!’
Boehmer’s ‘Dissertatio de celluloso contextu’ (1785) is still worse;
‘White elastic thicker or thinner fibres and threads woven together
of differing shape and size form cavities or cells or caverns, and
are usually known by the name of cell-tissue.’ We see what mischief
Grew did with his theory of the fibrous structure of the cell-walls,
and how the expression cell-tissue literally taken led the botanists
here named and others into utterly incorrect ideas. The works of Du
Hamel, Comparetti, and Senebier show that such misconceptions were not
confined to Germany, and Hill, a countryman of Grew, according to von
Mohl’s account pictured to himself cells as cups standing one above
another, closed below and open above.
Baron von Gleichen-Russworm (1717-1783), privy counsellor to the
Margrave of Anspach, gave much attention to the perfecting of the
mechanical arrangements of the microscope, but his plates themselves
show how strangely unsuitable these arrangements were. With these
instruments he made many observations, which are recorded in two
voluminous works, ‘Das Neueste aus dem Reich der Pflanzen’ (1764)
and ‘Auserlesene mikroskopische Entdeckungen’ (1777-1781). But these
works contain little or nothing about microscopic anatomy or the
structure of vegetable cells. His observations with the microscope
are chiefly devoted to processes of fertilisation and to proving that
spermatozoa are contained in the pollen[69], and in connection with
these subjects he gives magnified figures of many small flowers, some
of them beautifully executed; these figures must have made his works
very instructive to many in their time. He saw the stomata, which Grew
had already discovered, on the leaves of ferns, but took them for the
male organs of fertilisation, which at the same time showed that he was
still unacquainted with the existence of stomata in phanerogams.
CASPAR FRIEDRICH WOLFF[70] in his efforts in phytotomy stands a
solitary figure among his contemporaries, not only because he was the
first since Malpighi and Grew who devoted labour and perseverance to
the study of the anatomy of plants, but still more because at a time,
when the structure even of matured vegetable organs was almost a
forgotten subject, he endeavoured to penetrate into the history of the
development of this structure and the formation of cellular tissue.
Unfortunately he was not directed to this by an exclusive interest in
phytotomy, but by a more general question which he endeavoured to set
at rest in this manner; he wished to refute the prevailing theory of
evolution by demonstrating the development of the organs of plants,
and to obtain an inductive basis for his doctrine of epigenesis.
Though he was often diverted by these means from the pursuit of purely
phytotomic questions, yet his famous work, ‘Theoria Generationis’
(1759) is nevertheless important in the history of phytotomy; for
though it was disregarded by botanists during the succeeding forty
years, or at any rate exercised no noticeable influence, yet it was
Wolff’s doctrine of the formation of cellular structure in plants
which was in the main adopted by Mirbel at the beginning of the
present century, and the opposition which it encountered contributed
essentially to the further advance of phytotomy. This late but
lasting influence of Caspar Friedrich Wolff’s work was due not to the
actual correctness but to the thoughtfulness of his observations, and
to the earnest desire which inspired them to search out the true
nature of vegetable cell-structure and to explain it on physical and
philosophical grounds. The observations themselves on this point are
highly inexact, and influenced by preconceived opinions, and his
account of them is rendered obscure and often quite intolerable by
his eagerness to give an immediate philosophic explanation of objects
which he had only imperfectly examined. His efforts to follow the
course of development in the first beginnings of the formation of
cell-tissue were evidently not seconded by sufficient knowledge of
the structure of matured organs, and, to judge by his figures and by
his theoretical reflections, his microscope was of insufficient power
and its definition imperfect. Notwithstanding all these deficiencies,
Wolff’s treatise is doubtless the most important work on phytotomy that
appeared in the period between Grew and Mirbel, not, as has been said,
on account of any particular excellence of observation, but because its
author was able to make some use of what he saw, and to found a theory
upon it.
According to that theory all the youngest parts of plants, the punctum
vegetationis in the stem, which Wolff first distinguished, the youngest
leaves and parts of the flower, consist of a transparent gelatinous
substance; this is saturated with nutrient sap, which is secreted at
first in very small drops (we might say vacuoles), and these, as they
gradually gain in circumference, expand the intermediate substance and
so present enlarged cell-spaces. The intermediate substance therefore
answers to what we should now call the cell-walls, only these are at
first much thicker, and are constantly becoming thinner with the growth
of the cell-spaces. We may compare young vegetable tissue, formed as
Wolff imagines, with the porosity of fermenting dough, except that the
pores are not filled with gas but with a fluid. It is plain from the
above description that the vesicles or pores, as Wolff names the cells,
are connected together from the first by the intermediate substance,
and that one lamina or cell-membrane only lies between each of two
adjoining cells, a point which succeeding phytotomists were a long
time in determining. As cells are formed by the secretion of drops of
sap in the fundamental substance which is at first homogeneous, so
vessels, according to Wolff, are produced by longitudinal extension of
a drop in the mucilage and formation of a canal; consequently adjoining
vessels must be separated from one another by a single lamina of the
fundamental substance. Though Wolff expressly mentions the movement of
the sap within the firm mucilaginous substance between the cellular
cavities and the vascular canals, a movement of diffusion as it might
now be termed, he inconsistently enough thinks it necessary to assume
the existence of perforations in the bounding-walls of cells and
vessels to serve for the movement of sap from cell to cell and vessel
to vessel; yet in the single case in which he succeeded in obtaining
isolated cells, namely in ripe fruits, he was obliged to allow that the
walls were closed.
The growth of the parts of plants, according to Wolff, is effected by
expansion of existing cells and vessels, and by the formation of new
ones between them in the same way as the first vacuoles were formed
in the mucilaginous substance of very young organs; that is to say,
the sap which saturates the solid substance between the passages and
cavities of the tissue separates in the form of drops, which increase
in size and then appear as cells and vessels introduced between the
older ones. The substance between the passages and cavities, at first
soft and extensible, becomes firmer and harder with increasing age,
and at the same time a hardening substance may be deposited on it from
the sap which is stagnant in the cell-cavities and in movement in the
vascular passages, and this substance in many cases appears as their
proper membrane.
This is in all essential points Wolff’s theory. We may omit his
statements on the subject of the first formation of leaves at the
growing point and of the development of the parts of the flower, as
well as his physiological views on food and sexuality, which remained
for a long time without influence on the growth of opinion, and mention
only his doctrine of the growth of thickness of the stem. The stem is
originally the prolongation of all the leaf-stalks united together.
As many bundles of vessels are formed in the developed stem as there
are leaves springing from the vegetative axis; each leaf has a single
vascular bundle belonging to it in the stem, in modern phraseology
an inner leaf-trace. The union of these bundles from the different
leaves forms the rind of the stem; but if the leaves are very numerous,
their descending bundles form a closed cylinder, and if the stem is
perennial, the fresh production of leaves every year produces new zones
of wood of this kind every year, which are the yearly rings. This view
of Wolff’s on the growth of the stem in thickness bears an unmistakable
resemblance to the theory afterwards suggested by Du Petit-Thouars,
according to which the roots which descend from the buds are supposed
to effect the thickening of the stem.
The contests between Mirbel and his German antagonists at the beginning
of the present century will bring us back again to the more important
points in Wolff’s theory of the cell. Contemporary botanists paid less
attention to the ‘Theoria Generationis’ than they did to Hedwig’s[71]
phytotomic views, not on the formation of cells, but on the structure
of mature tissue. HEDWIG had given various figures and descriptions of
phytotomic subjects in his ‘Fundamentum Historiae Muscorum’ (1782) and
afterwards in his ‘Theoria Generationis’ (1784); but he treats these
topics at greater length in his treatise ‘De fibrae vegetabilis et
animalis ortu,’ published in 1789, and known to the author of this work
only imperfectly from quotations in later writers. Hedwig’s figures
of histological objects appear to be better than those of any of his
predecessors; they show that he used strong magnifying powers, and that
his glass had a clear field of sight. His defect lay in preconceived
opinions and hasty interpretation of what he observed. In order to
refute Gleichen’s view of the stomata in ferns, he demonstrated the
existence of these organs in many phanerogams, and observed the opening
of the slits, which he named ‘spiracula.’ On the epidermis which he
had removed for the purpose of these observations he saw plainly the
double contour lines bounding the epidermis-cells, and therefore the
cell-walls, which are at right angles to the surface. These he took
for a particular form of vessel, and called them ‘vasa reducentia’ or
‘lymphatica,’ and afterwards ‘vasa exhalantia,’ and he thought that
he had found them again in the interior of parenchymatous tissue,
evidently taking the places where three wall-surfaces meet for vessels;
such vessels he also saw in the milk-cells of Asclepias, described in
1779 by the elder Moldenhawer, who seems himself to have regarded even
the intercellular spaces in the pith of the rose as equivalent to these
milk-cells. The word vessel even in the 18th century was used in such
an indefinite manner, that the broad air-tubes of the wood and the
finest fibres were called vessels. Hedwig’s idea of the construction
of spiral vessels was strange enough; he took the spiral band itself
for the vessel, and supposed it to be hollow because it is coloured by
absorption of coloured fluids; in those spiral vessels in which the
turns of the spiral band are distant he saw, it is true, the delicate
original membrane which lies between the turns, but he supposed that it
lay inside the spiral band, which was wound round it on the outside. On
the second plate of the first part of the ‘Historia Muscorum’ he even
figures the network of ridges which the adjoining cells have left on
the wall of the spiral vessel, but explains it as wrinkles caused by
desiccation.
Hedwig was without doubt a very practised microscopist, and he
constantly recommended the extremest care in the interpretation of
all that the instrument reveals; but if an observer so careful and
practised, who moreover was provided with a glass of tolerably strong
magnifying power, fell into such gross mistakes, it cannot surprise us
if others, as P. Schrank, Medicus, Brunn, and Senebier, accomplished
still less. These highly unimportant achievements are all that mark the
close of the 18th century.
CHAPTER III.
EXAMINATION OF THE MATURED FRAMEWORK OF CELL-MEMBRANE IN PLANTS.
1800-1840.
There is no sharp line of division between the 18th and the 19th
centuries; the phytotomists who appear on the scene during the first
years of the new century are scarcely more successful than Hedwig and
Wolff; careful and judicious interpretation of their own and others’
observations is still rare, and they are often misled by preconceived
opinions.
In one respect indeed a very great improvement appeared with the
commencement of the 19th century; the number of phytotomists working
contemporaneously, checking and criticising one another, became all at
once much larger. Hitherto ten or twenty years had intervened between
every two works on phytotomy; but in the course of the twelve years
after 1800 nearly as many publications followed one another, and
scientific discussion enlivened enquiry. Now we meet with a Frenchman
for the first time in the field of phytotomy, Brisseau Mirbel, who
brought out his ‘Traité d’Anatomie et de Physiologie Végétale’ in 1802,
and raised a series of questions in the discussion of which several
German botanists, Kurt Sprengel (1802), Bernhardi (1805), Treviranus
(1806), Link and Rudolphi (1807), at once took part. It was a step
in advance and one affecting all botanical studies, that with the
exception of Rudolphi all these men, like Hedwig before them, were
botanists by profession; it was at last felt that the examination
of the internal structure of plants, as well as the describing them
according to Linnaean patterns, was a part of botanical enquiry; it is
at the same time true that the botanical knowledge of these observers
was often of service to them in their phytotomical investigations, and
directed their work decidedly and from the first towards that which
was worth knowing, and towards the objects which claimed the first
attention. This remark applies to the younger Moldenhawer even more
than to the botanists above-named; his ‘Beiträge,’ published in 1812,
may be taken as closing the first section of this century, during
which time he improved the methods of observation, compared his own
observations and those of others with great acuteness of judgment, and
did all that could be expected with the microscopes of the time.
The period of sixteen years after Moldenhawer, from 1812 to 1828, has
nothing of material importance to show in phytotomy. On the other
hand, it produced a series of the most important improvements that the
compound microscope has undergone since its invention.
As early as 1784 Aepinus had produced objectives of flint and crown
glass, and in 1807 Van Deyl[72] made similar ones with two achromatic
lenses, and still the phytotomists complained of the condition of their
instruments. Their figures show that they could not see clearly with
them, though the magnifying powers were not high; Link says expressly
in the preface to his prize-essay of 1807, that he usually observed
with a lens that magnified a hundred and eighty times. Moldenhawer
in 1812 gives the preference over all the microscopes he had used
to one by Wright, which was serviceable with a magnifying power of
four hundred times, while the German instruments, especially those by
Weickert, could not be used with higher powers than from one hundred
and seventy to three hundred.
A certain interval elapsed each time between an improvement in the
instrument and the appearance of the advantages which phytotomy derived
from it; thus in 1824, Selligue exhibited to the Academy of Paris an
excellent microscope with double lenses, several of which could be
screwed on one over the other, and which could be used with ordinary
daylight and a magnifying power of five hundred times; in 1827 Amici
made the first achromatic and aplanatic objectives with three double
lenses screwed on one over the other, the flat sides being turned
to the object. And yet still in 1836 a practised phytotomist like
Meyen spoke with disapproval of the instruments of his time, and gave
the preference to an old English microscope by James Man, though he
allowed that the newest instruments by Ploessl were a little better.
In his work on phytotomy, which appeared in 1830, all the figures were
magnified two hundred and twenty times, as were the very beautiful
figures in his prize essay of 1836; but in his ‘Neues System’ (1837),
he had already adopted powers that magnified to over five hundred
times. How rapid the progress was in the years before and after 1830 is
shown by comparing von Mohl’s work on climbing plants of 1827 and its
antiquated illustrations, with his publications of 1831 and 1833, where
the figures have a thoroughly modern appearance.
The art also of preparing anatomical objects rose by degrees with the
improvement of the microscope. It was not in a very advanced state at
the beginning of the century, if we judge by the language of writers
and by their figures. It was a great step in advance when the younger
Moldenhawer in 1812 isolated cells by maceration and decay in water,
and was thus enabled to view cells and vessels on every side and in a
perfect condition, to see their real shape, and to survey the manner of
their combination more exactly than had hitherto been done. But even
Moldenhawer still made the mistake of submitting delicate microscopic
objects to observation in a dry state, though Rudolphi and Link in 1807
had urged the advisability of keeping every part of the preparations
moist, especially the surface towards the object-glass, which shows
that they did not then use covering glasses. Nor was sufficient
attention shown to the use of sharp knives of suitable form, such as
the razor, which is now almost exclusively employed, or to practice
in making transverse and longitudinal sections of the utmost possible
delicacy,—two things which, through the example of Meyen’s and von
Mohl’s practice, were afterwards recognised as indispensable helps to
phytotomy; even in their time observers were satisfied with crushing
and picking their preparations to pieces.
Drawing from the microscope kept even pace on the whole with increasing
skill in making preparations, and with the improvement of the
instrument. If we compare together the drawings of Mirbel and Kurt
Sprengel in the beginning of the century, those of Link and Treviranus
in 1807, Moldenhawer’s in 1812, and Meyen’s and von Mohl’s from
1827 to 1840, we shall obtain a rapid and instructive survey of the
history of phytotomy during this period of forty years. The figures
testify at once to constant increase in the magnifying powers, to
the greater clearness of the field of sight, and still more to the
constant improvement in the arts of preparing and observing objects.
But a curious misconception crept in among the phytotomists at this
time; they believed that more correct and trustworthy figures would be
obtained, if the observer and writer did not himself make them, but
employed other eyes and other hands for that purpose; they imagined
that in this way every kind of prejudice, of preconceived opinion would
be eliminated from the drawings. Thus both Mirbel and Moldenhawer had
their figures drawn by a woman, and many later phytotomists entrusted
the execution of their drawings to hired draughtsmen, as Leeuwenhoek
had done before them. A drawing from the microscope, like every
other copy of an object in natural history, cannot pretend to take
the place of the object itself, but is intended to give an exact and
clear rendering of what the observer has perceived, and by so doing
illustrate the verbal description. The drawing will be perfect in
proportion to the practised skill of the eye that observes and of the
mind that interprets the forms. The copy should only show to another
person what has passed through the mind of the observer, for then
only can it serve the purpose of a mutual understanding. There is
also another point to be considered; it is exactly in the process of
drawing a microscopic object that the eye is compelled to dwell on
the individual lines and points and to grasp their true connection in
all dimensions of space; it will often happen that in this process
relations will be perceived, which previous careful observation
had disregarded, and which may be decisive of the question under
examination or even open up new ones. As the microscope trains the
eye to scientific sight, so the careful drawing of objects makes the
educated eye become the watchful adviser of the investigating mind;
but this advantage is lost to the observer who has his drawings made
by another hand. It is not one of the least of von Mohl’s merits, that
he practised microscopic drawing under the influence of the views here
indicated, and sought to make his figures no mere undigested copies of
the objects, but an expression of his own opinions about them.
Enough has been said to show that an important portion of the history
of phytotomy lies between the beginning and the end of the period under
consideration. The distance between the knowledge of the structure of
vegetable tissue which existed at the beginning of the century, and
that of Meyen and von Mohl on the same subject in 1840, is wonderfully
great; in the one case an uncertain groping about among obscure
ideas, in the other a complete exposition of the inner architecture
of the mature plant. But in spite of this great difference between
beginning and end, it is better to review the efforts of this period
of forty years as a connected process of historical development, and,
notwithstanding the interval between the appearance of Moldenhawer’s
contributions in 1812 and Meyen’s and von Mohl’s labours about 1840, to
consider the latter as the settlement of the questions taken up at the
commencement of the century. Moreover after 1840, with the appearance
of Schleiden and Nägeli on the scene, new points of view were suddenly
disclosed, and new aims were proposed in phytotomic investigation; it
is no objection to this view of the subject, that the most productive
portion of von Mohl’s labours falls in the succeeding twenty years, and
that during this later period his position is one of equal authority
with the new tendency and of participation in it. Up to 1845 his
discoveries were the culminating point of the older phytotomy; they put
the finishing stroke to the work which Mirbel, Link, Treviranus, and
Moldenhawer had begun. The object almost exclusively pursued during
all this period was to frame as true a scheme as possible of the inner
structure of the mature organs of the plant; it was requisite to
gain a right understanding of the diversities of cells and forms of
tissues, to classify them and supply them with names, and to secure
well-conceived definitions of these names. Hence almost exclusive
attention was paid to the configuration of the solid framework of
cell-membrane, and of this chiefly in the matured state, to the form
of the several elementary organs and their combination in the tissue,
to the sculpture of the wall-surfaces, and to the connection of
cell-spaces by pores or their separation by closed walls. There was
much discussion, especially at first, on the contents of vessels and
cells, and on supposed movements of sap in connection with anatomical
research, but there was no careful connected investigation of the
cell-contents; it was not yet recognised that the true living body of
the vegetable cell is only a definite part of the contents inclosed by
the cell-wall; the solid walls, the framework of the whole building,
were regarded as of primary importance in the structure of the cell.
It was not till the following period that in the light of historical
development another view asserted itself, namely, that the solid
framework of vegetable tissue with all its importance is yet in the
genetic sense only a secondary product of the phenomena of vegetative
life, that the true cell body, the cell-protoplasm is prior in time and
in conception, and can claim the higher position.
* * * * *
Mirbel, to whom we now return, had in 1801 laid down a theory of
cell-formation which agreed in the main with that of Caspar Friedrich
Wolff; he supposed with Wolff that each cell-space was separated from
its neighbour by a single wall, and relying on fresh observations
asserted the existence of visible pores in the dividing walls of
parenchyma and of vessels, and also maintained some new views on
the nature and formation of vessels. The essential points of this
theory found an opponent in Germany in the person of KURT SPRENGEL,
the well-known historian of botany, and one of the most variously
accomplished botanists of his time, who had published in 1802 a work
written in diffuse and familiar style under the title of ‘Anleitung
zur Kenntniss der Gewächse.’ He relied on his own observations, but
these were evidently made with small magnifying powers, an obscure
field of sight, and indifferent preparations. The cell-tissue, says
Sprengel, consists of cavities of very various shape communicating with
one another, the dividing walls being in some places broken through
and in others wanting. He took the starch-granules which he saw in the
seed-leaves of beans and other plants for vesicles, which increase in
size by absorption of water and so form new tissue; but he did not
explain how we are to conceive of the growth of organs with such a mode
of cell-formation. His account of the vessels is extremely obscure,
even more obscure than Hedwig’s, though he has the merit of refuting
the latter’s strange theory of reconducting vessels in the epidermis;
he also suggested, though only incidentally, the happy idea that spiral
passages and even vessels might arise from cell-tissue, since the
youngest parts of plants have only the latter; but he did not attempt
to explain how or where the process takes place. Like Malpighi and
Grew he supposed that the spiral vessels had no wall of their own,
but thought that the closely-rolled spiral threads formed a wall; the
constrictions in broad short-membered vessels he regarded as real
contractions in their substance, caused by the increased tightening of
the spiral threads through a sort of peristaltic movement,—a mistaken
notion often entertained at the beginning of the century, by Goethe
among others, and connected with ideas of vital power prevalent at the
time. In the stomata, to which he gave the name still in use, Sprengel
like Grew, Gleichen, and Hedwig, saw a circular cushion instead of
the two guard-cells; but he notices the observation first made by
Comparetti, that the orifice closes and opens alternately, being wide
open in the morning and closed in the evening. But he considered the
stomata to be organs of absorption.
Sprengel in enunciating his own theory of cell-formation accused
Mirbel of mistaking the starch-grains in the cells for the pores of
the cell-walls. On this point, so important in the doctrine of the
cell and in physiology, he was followed by the three candidates for
the Göttingen prize, though Bernhardi had in 1805 defended Mirbel’s
view, and had pointed out how little likely it was, that so skilful
an observer as Mirbel should fall into so gross an error. BERNHARDI’S
short treatise, ‘Beobachtungen über Pflanzengefässe,’ Erfurt
(1805[73]), was in general distinguished by a variety of new and
correct observations, and was the work of a simple and straightforward
understanding, which takes things as they are presented to the eye
without allowing itself to be led astray by preconceived opinions. His
observations are certainly the best in the whole period from Malpighi
and Grew to the younger Moldenhawer; his method of dealing with
questions of phytotomy is much more to the purpose than that of the
three rivals for the Göttingen prize.
In the work just mentioned Bernhardi treats of other forms of tissue as
well as vessels, and endeavours to distinguish and classify them more
exactly than had hitherto been done. He contrasts favourably with his
contemporaries in the fact, that he sought to define the histological
terms employed as precisely as possible,—a great step in advance at a
time when phytotomic conceptions were in a very misty condition. He
distinguishes three chief forms of vegetable tissue, pith, bast, and
vessels.
By pith he means the tissue which Grew had named parenchyma, and
which is still so called; it remained a question with him whether the
cells of the pith are pierced by visible pores. By the word bast he
understood not only the fibrous elements of the rind, but those of the
wood also, and in general what is now known as prosenchyma; this agrees
very well with Malpighi’s view, which was adopted by Bernhardi and by
all his contemporaries, that the inner layers of the bast are changed
into the exterior layers of wood to make the increase in thickness of
the woody stem; but he did not admit the same origin in the case of the
innermost portion of the wood, for this is formed from the first in the
young shoots, which alone contain true spiral vessels with threads that
may be wound off.
Bernhardi distinguishes vessels into two main groups, air-vessels and
vessels properly so called. He calls the first group air-vessels for
the same reason that led Grew to give them that name, namely, that they
are filled with air during a part at least of the period of vegetation;
they are found in the wood, and, where there is no closed woody body,
there the woody bundles are formed both of vessels and also of bast
strands which enclose vascular canals. These latter he next divides
into three chief kinds; annular vessels, which he was the first to
discover, true spiral vessels with a band which can be unwound, and
scalariform vessels, by which term he understood not only those with
broad slits, such as are found in Ferns, but also the pitted vessels in
secondary wood. His idea of annular and spiral vessels was perfectly
correct, and he mentions Hedwig’s notion already described, and shows
that its exact opposite is true, namely, that the spiral band is
surrounded by a membrane on the outside,—a fact which was afterwards
denied by Link, Sprengel, and Moldenhawer. On the other hand he did
not understand the sculpturing on the scalariform vessels; he took the
pits in the dotted vessels for thickenings of the wall, such as are
seen in the transverse ridges between the slits in true scalariform
vessels, and the slits he thought were closed. If there was much that
was erroneous in these views, yet Bernhardi contributed essentially
to the clearing up of the subject by his effort to distinguish the
different forms of air-vessels, and especially by pointing attention to
the fact that neither spiral nor annular vessels are found in secondary
wood. The resemblance between different forms of vessels misled many of
his contemporaries into supposing that they are due to metamorphosis
of true spiral vessels. Bernhardi showed that different forms of wall
are found inside one vascular tube, but that this does not depend on
modification with age; observation rather teaches that every kind of
vessel receives its character in its young state, and especially that
the youngest scalariform vessels do not present the form of spiral
vessels.
Under the head of vessels proper he reckoned all tubular forms filled
with a peculiar juice, milk-cells and true milk-vessels, and also
resin-ducts and the like, and he made many good and still valuable
observations on their distribution and sap-contents. He could not see
the differences of structure in these various fluid-conveying vessels
with the low magnifying power of his glass, and therefore attended
chiefly to the structure of the large resin-ducts, which on the whole
he rightly understood.
The question whether there are any other forms of vessels in the plant
beside those already named gave him occasion to define a vessel better
than it had yet been defined, namely as an uninterrupted tube or canal,
and at the same time he found himself obliged to consider whether his
bast-threads are vessels; but he did not give a decided answer to the
question. He declared however distinctly against Hedwig’s reconducting
vessels in the epidermis, as Sprengel had done, and it is worthy of
recognition that he understood the true nature of the corners where
three longitudinal walls of the parenchyma meet, while later observers
found difficulties in them.
Before the appearance of Bernhardi’s work the Royal scientific Society
of Göttingen proposed a subject for a prize in the year 1804, which
shows very plainly what uncertainty was felt at that time on all
points of phytotomy. For this reason it will be well to give it at
length from the preface to Rudolphi’s ‘Anatomie der Pflanzen’ (1807):
‘Since some modern physiologists deny the peculiar construction of
vessels in plants which is attributed to them by other and especially
the older observers, it would be well to institute new microscopical
investigations, which shall either confirm the observations of
Malpighi, Grew, Du Hamel, Mustel, and Hedwig, or prove that plants
have a special organisation of their own which is more simple than
that of animals, whether that organisation is supposed to originate
in simple peculiar fibres and threads (Medicus) or with cellular and
tubular tissue (tissu tubulaire of Mirbel). Attention should also be
given to the following subordinate questions: 1. How many kinds of
vessels may certainly be distinguished from the first period of their
development? The existence of certain forms having been established; 2.
Are the twisted fibres which are called spiral vessels (vasa spiralia)
themselves hollow, and do they therefore form vessels, or do they
serve by their convolutions for the formation of closed cavities,
and how? 3. Do fluids as well as gases move in these cavities? 4. Do
the scalariform ducts arise from adherence of the twisted threads
(Sprengel), or do the threads owe their origin to the ducts (Mirbel)?
Do alburnum and woody fibres originate in the scalariform ducts, or in
true vessels, or in tubular tissue?’
We see in this case as in many similar ones, that the subject was
proposed by persons who understood little of it, and who were unable
to judge of what had been written about it; how else could they have
placed the opinions of a Mustel and a Medicus side by side with those
of Malpighi and Grew? Had Bernhardi or Mirbel set the question, it
would certainly have been better conceived. It was in keeping that
the three essays sent in, all inferior to Bernhardi’s work already
mentioned, though they contradicted one another on the most important
points, were nevertheless all accepted; not less so that Treviranus’
essay obtained only the second place, though it was decidedly better
than the other two, and very much better than Rudolphi’s. The best
result of the whole affair was that it stirred up the phytotomists
of the day, and led Mirbel to submit the three prize treatises to a
searching criticism, especially that of Treviranus, which Mirbel with
professional acumen recognised as the best. Link’s essay appeared
in 1807 under the title ‘Grundlehren der Anatomie und Physiologie
der Pflanzen,’ that of Rudolphi as ‘Anatomie der Pflanzen,’ also in
1807, each forming a handsome octavo volume. The work of Treviranus
had already appeared in 1806 with the title, ‘Vom inwendigen Bau der
Gewächse.’
If we compare the works of Link and Rudolphi[74], which both received
a prize, and which had all the appearance of text-books of general
vegetable phytotomy and physiology, we miss in both any clear
exposition of the conceptions connected with the words used, and the
train of thought therefore is constantly obscure and vacillating.
Yet it is easy to see that they are opposed to one another in all
essential points, Link[75] generally hitting on the correct, or at
least the correcter view. For instance, Rudolphi denies altogether the
vegetable nature of Fungi and Lichens, because he finds no resemblance
between their hyphae and vegetable cell-tissue, and he supposes them
to arise by spontaneous generation; even of the Confervae he says that
the microscope has shown him nothing that agrees with the structure
of plants,—evidently a sign of bad observation or of incapacity to
understand what he saw. Link on the other hand regards all Thallophytes
as plants, and sees that the filaments of Lichens and Fungi consist of
cells, and that cells occur at least in many Algae. Rudolphi praises
in the same breath the views of Wolff and Sprengel on cell-tissue,
although they are directly opposed to one another, and although he
adopts Sprengel’s strange theory of cell-formation without alteration.
Link on the contrary declares against Sprengel’s theory, and on good
grounds, and shows that the vesicles which Sprengel took for young
cells are starch-grains; at the same time he makes new cells be formed
between the old ones. Rudolphi is of opinion that cells open into one
another, as is plainly shown by the passage of coloured fluids. Link
maintains that cells are closed bodies, and proves it well by the
occurrence of cells with coloured juice in the middle of colourless
tissue. Rudolphi represents the orifices of the stomata as encircled
by a roundish rim, which he takes without hesitation for a closing
muscle because the apertures enlarge and diminish. Link is more happy
in taking the part that surrounds the aperture for a cell, or a group
of cells. Rudolphi considers the great cavities in hollow stems and in
the tissue of water-plants as the only air-passages in plants; Link
explains these cavities as gaps caused by the irregular growth of
cellular tissue. With Rudolphi the word vessel means not only vascular
forms in wood, but milk-vessels and resin-ducts also, and to the former
he even transfers Malpighi’s view of the structure of spiral vessels.
Link designates the tubes of the wood only as vessels, combining the
most various forms of them under the term spiral vessels; he excludes
milk-vessels, resin-ducts, and the like from the conception of a
vessel, and in this he is somewhat inconsistent, since he assumes with
Rudolphi that a vessel, in plants as in animals, is a canal for the
conveyance of nutrient sap.
With all these contradictions, the two essays agree in adopting the
old Malpighian view of the growth in thickness of stems, according to
which the new layers of wood are formed from the inner layers of bast,
while between the bast-cells, which are here taken to be identical with
woody fibre, new spiral vessels arise contemporaneously, and, as Link
expressly says, from juices which pour out between the bast-cells.
It is hard to understand how two treatises, so contradictory as
they have been shown to be, could have both received a prize at the
same time, or how the great difference could have been overlooked
between Link’s sensible and well-arranged account of his subject, and
Rudolphi’s uncritical statements, which everywhere rely more on old
authority than on his own observation. It is however certain that
Link’s better production is inferior to Bernhardi’s treatise, unless
we choose to consider the greater copiousness of detail in Link, the
number of his observations, and his acquaintance with the literature
of the subject, as giving him the advantage. His figures, as well as
Rudolphi’s, are not so good as those of Bernhardi.
The work of TREVIRANUS[76], to which the judges at Göttingen awarded
the second place, is much less comprehensive than those of his
competitors; the style is inferior to Link’s, and may even be called
clumsy. But the much better figures show at once that Treviranus was
the more accurate observer, and his work, in spite of the inferiority
of its style, is of far higher value on account of the attention paid
in it to the history of development; Treviranus laid greater stress on
this method than either Link or Rudolphi, and it led him to form views
on some of the fundamental questions of phytotomy, in which we see the
germs of theories afterwards perfected by von Mohl. His account of the
formation of cell-tissue is mainly that of Sprengel, and therefore an
unfortunate one; but nevertheless his observations on the composition
of wood and the nature of vessels were as good and correct as could
be expected from the condition of the microscope at the time. He made
one discovery of considerable value, that of the intercellular spaces
in parenchyma, but he lessened its merit by filling these passages
with sap, and even describing its movement. Woody fibres are due, he
thinks, to strong longitudinal extension of vesicles. He supported
Bernhardi’s view of the nature of vessels, that the separable spiral
threads of spiral vessels are not wound round a membranous tube but
are surrounded by one. He maintains against Bernhardi the distinctness
of punctated vessels or porous woody tubes from false tracheae or
scalariform vessels, while he gave a more correct description of the
latter as they occur in Ferns. He rejected Mirbel’s view that the pits
in dotted vessels are depressions surrounded by a raised glandular
edge, and explained them as grains or little spheres. Against this
mistake we may set off the very important step which he made in
advance, when he not only conjectured that the pitted vessels of the
wood are formed from cells previously divided off from one another,
but proved by observation that the members composing such vessels are
at first actually separated by oblique cross-walls, which afterwards
disappear. But this correct observation was impaired by the mistaken
idea, which Treviranus shared with his predecessors, that the wood
is the result of transformation of the bast, and consequently that
the vessels of the wood are bast-fibres, which elongate considerably
after they are arranged in a direct chain one after the other; the
unevennesses caused by the oblique junctions of the tissue gradually
disappear, the boundaries of each member of a vessel being still for
some time indicated by oblique transverse markings. The dividing
walls originally existing at these points disappear by widening of
the cavities, so that the different parts come to form a continuous
canal. To illustrate the disappearance of a parting wall between two
adjoining cells Treviranus aptly points, somewhat to our surprise, to
the formation of the conjugating tube in Spirogyra. He rejects with
Bernhardi the view represented by Sprengel, Link, and Rudolphi, that
the different kinds of vessels are formed from true spiral vessels;
he says that he had found the scalariform ducts in Ferns so formed in
their earliest stage and not as spiral vessels; he thinks it highly
probable that the distinct transverse bands on false spiral vessels
(scalariform ducts) and the pits of dotted vessels are formed on
the walls of membranous fibre-tubes; in like manner he derives true
spiral vessels from long thin-walled cells, on whose inner surface the
spiral band is formed, and well compares the members of young spiral
vessels with the elaters of the Jungermannieae. Here then we find the
first more definite indications of a theory of growth in thickness
of cell-walls, which, like the theory of the origin of vessels from
rows of cells, was afterwards developed by von Mohl and laid on better
foundations. At the close of the essay the histology of the Cryptogams,
Monocotyledons and Dicotyledons is compared, and the subject is better
and more perspicuously handled than in the corresponding chapters of
his competitors.
Though Treviranus’ account of vegetable tissues was on the whole
weak as far as concerns the history of development, yet MIRBEL[77]
recognised in him the most dangerous opponent of his own theory,
and addressed a public letter to him and not to his other German
antagonists, Sprengel, Link and Rudolphi, in defence of the views he
had formerly expressed. This letter is the first part of a larger
work which appeared in 1808, ‘Exposition et défense de ma théorie
de l’organisation végétale,’ in which Mirbel endeavours to meet the
objections of his opponents with great adroitness of style and with
the results of varied rather than profound observation, and to find
new arguments for his theory of vegetable tissue; he admits that his
former treatises were in many respects faulty, but demands that his
critics should discuss his system as a whole and not take offence at
single expressions. Mirbel’s idea of the inner structure of plants
is essentially the same as that broached by Caspar Friedrich Wolff.
The first and fundamental idea is that all vegetable organisation
is formed from one and the same tissue differently modified. The
cell-cavities are only hollow spaces of varying form and extension in
homogeneous original matter, and have no need therefore of a system of
filaments, as Grew supposed, to hold them together. The tracheae only
are an exception, which Mirbel, in striking opposition to the much
more correct view of Treviranus, considers to be narrow spirally wound
laminae, inserted into the tissue and connected with it only at the
two ends. If it is asked how interchange of sap is effected in such
a cellular tissue as this, it becomes necessary to assume that the
membranous substance of plants is pierced by countless invisible pores,
through which fluids find their way. But nature has a speedier and more
powerful instrument in the larger pores, which the microscope reveals.
Mirbel does not discuss the question how the fluids are set in motion,
easily disregarding such mechanical difficulties, as was usual in those
days, when vital power was always in reserve to be the moving agent. He
warmly repels the imputation, which Sprengel had made against him, of
having confounded pores and granules, by appealing to his figures; he
says that he has depicted prominences on the outside of the walls of
the dotted vessels, and an orifice in each of them, which his opponents
simply never saw. The question whether these prominences lie on the
inside or the outside of the walls of the vessel has no meaning, if
we suppose with Mirbel that the dividing wall is single; he is only
concerned to enquire whether the perforated projections lie on the one
or on the other side of the wall. He refers Treviranus, who had denied
the presence of the pores, to his description of scalariform vessels,
in which he had himself seen the slits which correspond to the pores.
In comparison with these fundamental questions Mirbel’s further account
of matters of detail does not concern us here. He gave a connected
view of the whole of his doctrine of tissues in the form of aphorisms,
which occupy the second part of his book. Of all that he says on the
five kinds into which he distinguishes vessels the most interesting
is the statement, that diaphragms pierced like a sieve separate the
different members of his ‘beaded’ vessels. We find that the weakest
part of Mirbel’s phytotomy, as of that of his opponents, is his
description of the true vessels (vasa propria), with which he classes
the milk-cells of the Euphorbiae and the resin-ducts of Coniferae, but
he saw clearly enough that the latter were canals inclosed in a layer
of tissue of their own. The third part of the book is devoted to these
forms of tissue, and we learn that he classes not only many kinds of
sieve-cell-bundles, but also true bast-fibres, as those of nettle and
hemp, with his bundles of true vessels. Like his opponents he makes
the growth in thickness in woody stems to be due to transformation of
the inner layer of bast; but he gives a new turn to this view, which
brings it nearer to the modern theory; during the period of vegetation
a delicate tissue with large vessels is developed in Dicotyledons on
the confines of the wood and the bark, and these augment the mass of
the woody body, while a loose cellular tissue is formed on the other
side, destined to replace the constant losses of the outer rind. To
later phytotomists, who understood by the word cambium a thin layer
of tissue constantly engaged in producing wood and rind, Mirbel’s
otherwise indistinct conception of growth in thickness must have become
more indistinct from his using the word cambium not for the layer of
tissue afterwards so called, but for a highly ‘elaborated and purified
sap’ which is intended for the food of the plant and makes its way
through all membranes; we see this cambium-sap appear at the spots
where it produces new tubes and cells after the manner of the Wolffian
theory. The cells appear at first as minute spheres, the tubes are very
fine lines; both enlarge and gradually show pores, clefts, etc. This
is essentially Wolff’s doctrine, which Mirbel afterwards endeavoured
to confirm against his German opponents from the germination of the
date-palm with the help of a more powerful microscope.
Mirbel insisted more than the German phytotomists of his day on the
idea, that all forms of vegetable tissue are developed originally
from young cell-tissue, an idea suggested by Sprengel and following
naturally with Mirbel from Wolff’s theory. Both Mirbel and Wolff were
hasty in observation and too much under the influence of theory in
giving reasons for what they observed, and therefore too ready with
far-reaching explanations of phenomena which only long-continued
observation could decide.
Treviranus replied, though after some delay, to Mirbel’s polemics
by incorporating into his ‘Beiträge zur Pflanzenphysiologie,’
Göttingen (1811), an essay entitled ‘Beobachtungen im Betreff einiger
streitigen Puncte der Pflanzenphysiologie,’ in which he again took up
the questions in dispute between himself, Mirbel, Link and others,
and supported his own views by fresh investigations. It cannot be
denied that in this short treatise Treviranus brought some important
questions nearer to a decision; he added materially to the knowledge
of bordered pits, on which subject his views now approximated more
nearly to those of Mirbel; he drew attention to the vesicular nature
of vegetable cells, which are often separable from one another, and
to the occurrence of true spiral vessels in the neighbourhood of the
pith in Conifers also, and among other things discovered the stomata on
the capsule of Mosses. On the subject of the theory of cell-formation
which he had borrowed from Sprengel, he endeavoured to extricate
himself from his difficulty by ingeniously pointing out that though
the starch-grains in the seed-leaves of the bean disappear without
producing new cells in them, they are dissolved and then serve as fluid
material for new cell-formation in other parts of the germinating
plant, which however was giving up Sprengel’s theory; yet he cited as a
direct proof of that theory the origination of gonidia in the cells of
Hydrodictyon, and their development into new nets.
Mirbel and his German opponents moved for the most part in a circle
of ideas which had been formed by the speculations of Malpighi, Grew,
Hedwig and Wolff, though it must be allowed that the observations
of Treviranus did open new points of view. But JOHANN JAKOB PAUL
MOLDENHAWER[78] travelled far beyond these older views as early as
1812 in his important work, ‘Beiträge zur Anatomie der Pflanzen.’ He
took up from the first a more independent position as regards former
opinions than either of the writers hitherto considered. He relied
on very detailed, varied, and systematic observations evidently made
with a better instrument, abided by what he himself saw, and chose his
point of view in accordance with it, while he criticised the views
of his predecessors in detail with an unmistakable superiority, and
in so doing displayed minute acquaintance with the literature of the
subject and varied phytotomical experience. He fixed his eye firmly
on the points in question, and made each one the subject of earnest
investigation and copious and perspicuous discussion. His figures prove
the carefulness of his examination and the greater excellence of his
instrument; they are undoubtedly the best that were produced up to
1812. His mode of dealing with his subject and his figures, though they
were not executed by himself, remind us in many respects of von Mohl,
though it would be more correct to say that von Mohl’s manner reminds
us of Moldenhawer, for from the great respect which von Mohl displays
for him, especially in his earlier writings, it can scarcely be doubted
that he formed himself on Moldenhawer’s ‘Beiträge,’ and first learnt
from them the earnestness and carefulness demanded by phytotomic work.
It has been already mentioned that the study of vegetable physiology
is indebted to Moldenhawer for one important practical improvement.
He was the first who isolated cells and vessels by allowing parts of
plants to decay in water and afterwards crushing and dissecting them, a
process not much used in modern times, though it may still be applied
with advantage in conjunction with what is known as Schulze’s solution,
especially if it is carried out with Moldenhawer’s carefulness and
circumspection. The isolation of the elementary organs of plants
by maceration in water necessarily brought Moldenhawer into direct
antagonism with Mirbel, who with Wolff assumed that the partition
between any two cells was a single wall; whereas Moldenhawer found
that the cells and vessels were closed tubes and sacs after isolation,
and must necessarily, as it would seem, so lie one against another
in the living plant, that the wall between every two cell-spaces is
formed of two membranous laminae, and he expressly says that this is
the case even in very thin-walled parenchyma. This result remained
unassailable, so long as no one was in a position to conclude from
the history of the development of cell-tissue that the partitions
are originally single, or by aid of strong magnifying power to prove
the true structure of the walls and their later separation, and the
differentiation of the once single wall into two separable laminae.
If the view based on the results of maceration was still not the
true view, yet it was nearer the truth as regards the matured state
of the cell-wall than that of Wolff and Mirbel, and the important
advantage was gained of being able to study the form of elementary
organs and the sculpture on their walls more accurately than before.
It is true that Link had occasionally isolated cells by boiling in
1809, and Treviranus had drawn attention in 1811 to the fact that it
was possible to isolate parenchyma-cells in their natural condition;
but neither of them made systematic use of these observations, and
to Moldenhawer belongs the exclusive merit of having first isolated
vessels and woody cells; but as usually happens, he did not himself
obtain all the possible results from his method of preparation. In his
work which indeed embraces the whole of phytotomy, he is continually
recurring to one species, maize. This supplies the starting-point in
every question to be discussed. The results obtained there are the
firm supports on which he leans in the examination of a great variety
of plants, which he then compares together at length. This mode of
treatment was well chosen both for investigation and instruction in the
existing state of the science; it was a particularly happy idea that
of choosing the maize-plant for his purpose; former phytotomists had
generally had recourse to dicotyledonous stems, and preferred those
that had compact wood and complex rind, but the examination of these
plants presents difficulties at the present day to a practised observer
with a good microscope. Occasionally observers had taken the stem of
the gourd, where the large cells and vessels suited small magnifying
power, but where many abnormal conditions occurred to interfere with
their conclusions. The Monocotyledons, like the Vascular Cryptogams,
had hitherto been comparatively neglected. When then Moldenhawer made
a monocotyledonous and rapidly growing plant, with very large-celled
tissue and comparatively very simple structure, the chief subject of
his investigations, he was sure to succeed in making out many things
more clearly than his predecessors. It was an important point that
he found the fibrous elementary organs in this plant united with
the vessels into bundles, which are separated by a strict line of
demarcation from the large-celled parenchyma that surrounds them. Thus
the peculiar character, the idea, of the vascular bundle was brought
prominently into contrast with that of other forms of tissue. This took
the place of the distinction between rind, wood, and pith, which had
served former phytotomists as the basis of their histological survey,
but which is in itself only a secondary result of the later elaboration
of certain parts of the plant. Moldenhawer, in laying the chief stress
from the first on the contrast between vascular bundles and parenchyma,
hit upon a histological fact of more fundamental importance, the right
appreciation of which has since enabled the phytotomist to find his way
through the histology of the higher plants. For while the construction
of Monocotyledons and Ferns must seem abnormal and quite peculiar to
any one who starts with examining the rind, wood, and pith of old
dicotyledonous stems, those on the contrary who, with Moldenhawer, have
recognised a special histological system in the vascular bundles of
Monocotyledons, have the way opened to them to seek for a similar one
in the Dicotyledons, and to refer the secondary phenomenon of wood and
rind to the primary existence of vascular bundles. Moldenhawer did in
fact open this way, when he showed how the growth of a dicotyledonous
stem may be understood from the structure and position of the
originally isolated vascular bundles (Beiträge, p. 49, etc.). But he
was thus of necessity led to the rejection of Malpighi’s theory of the
growth in thickness of woody stems, which all vegetable anatomists from
Grew to Mirbel had adopted. Though Bernhardi and Treviranus made weak
attempts to discredit it, Moldenhawer was the first who distinctly
rejected the origin of the external layers of wood from the inner bast,
and proposed the first really practical basis for the later and correct
theory of secondary growth in thickness (p. 35). The removal of this
ancient error is in itself a very important result, and one which,
apart from all other services, must secure him an honourable place in
the history of botany.
But the light must have its attendant shadow, and all his carefulness
in observation and cautiousness in judgment did not protect him from
one prejudice and its evil consequences. After Moldenhawer had isolated
the elementary organs by maceration, he had to answer the question how
we are to conceive of their firm coherence in the living plant. He came
to the conclusion, as did von Mohl, Schacht, and others after him,
that there must be some special connecting medium; but he did not hit
upon their idea of a matrix, in which the cells are imbedded, or of a
cement which holds them together, but on a much stranger theory, which
reminds us at once of Grew’s thread-tissue, and like that rests partly
on incorrect observations. These were too hastily accepted as the basis
of a theory which in its turn interfered with after observations. He
thought that the cells and vessels were surrounded and held together
by an extremely delicate network of fine fibres; in some cases he
really believed that he saw these fibres, and interpreted in this way
the thickened bands in the well-known cells of Sphagnum, and still
more strangely he appears to have taken the thickened longitudinal
and transverse edges of cells and vessels for such threads. The
unfavourable impression produced by this theory is necessarily
heightened by the fact that he gave the name of cell-tissue, a term
long used in a different sense, to his fancy-structure of reticulated
threads which were to hold the cells and vessels together, while he
called the parenchyma itself cellular substance, an expression which
fortunately no one copied, and which certainly contributed at a later
time to discredit the great services which Moldenhawer rendered to
phytotomy.
His ‘Beiträge zur Anatomie der Pflanzen’ are divided into two portions;
the first treats of the parts surrounding the spiral vessels; the
second of the spiral vessels themselves.
The position and collective form of the component parts of the vascular
bundle in the stem of the maize-plant are well described in the first
section of the work. It is correctly stated that there is a sheath to
the whole bundle composed of strongly thickened fibrous cells, that
each of these cells has its own membrane and is entirely closed, and
that they resemble the bast and the fibrous elements of the wood of
Dicotyledons. The segmented wood-cells and the parenchyma-cells of
the wood arranged in rows are incidentally noticed. Under the name of
fibrous tubes he included the cells of the sclerenchyma-sheath of many
vascular bundles and the true bast and wood-fibres, which latter he
says are wanting in the Coniferae. He explained the secondary growth in
thickness of rind and bast by the example of the shoot of the vine, in
which he correctly distinguished the medullary sheath and the spiral
vessels. In herbaceous Dicotyledons he found the bundles of vessels
to consist of a bast portion and a woody portion, and he attributed
the formation of the compact wood of true woody plants to the blending
together of the woody portions of these separate bundles.
In discussing the parenchymatous cell-tissue he rejects emphatically
and on good grounds the origin of new cells from the granular contents
of older ones, which had been the view of Sprengel and Treviranus, as
also the theory of Wolff and Mirbel, while he maintains against Mirbel
especially, that the separation of fibrous tubes is possible even
where no dividing line can be seen between them in the cross section.
He considers that both in thin-walled and thick-walled parenchyma
the dividing wall is double and the cell-membrane entirely closed.
‘It appears,’ he continues on p. 86, ‘from these observations that
cellular substance consists of separate closed tubes, which may be
round or oval, or more or less elongated, or almost cylindrical in
shape, and these by mutual pressure assume an angular and flattened
form, which is either regular like the cells of the comb of bees or
more or less irregular. Such an aggregate of separate cells (and here
he is certainly quite right) has nothing in common with a tissue,
and the word cell-tissue seems therefore less suitable than the term
cellular substance, composed of cell-like tubes.’ Further on he
rejects Mirbel’s idea of the existence of visible holes in the walls
of cells, and points out that they are not necessary for the movement
of sap. The dispute between Mirbel and his opponents respecting the
porousness of cell-walls was extended at the same time to the stomata
of the epidermis[79], the slits in them being supposed to be apertures
in the epidermis regarded as a simple membrane. Moldenhawer took
occasion to examine the anatomy of stomata more closely, and produced
the first accurate descriptions and figures of these organs, showing
especially that the apertures are not surrounded by a simple border,
as most previous observers believed, but lie between two cells, and
that therefore they are not examples of the existence of pores in
cell-walls, as Mirbel imagined. It may be observed here by the way,
that Mirbel afterwards considered stomata to be short broad hairs;
Amici in 1824, Treviranus in 1821, demonstrated their true structure
by cross sections, and von Mohl at a later period investigated it
thoroughly. Moldenhawer on the present occasion also enquired into
the faculty attributed to stomata of opening and closing alternately,
which, first observed by Comparetti, was then much discussed by
the German phytotomists, and has been made the subject of repeated
investigation in modern times. The whole of this discussion was in
connection with the question of the pitting of cell-walls, the true
nature of which Moldenhawer however never clearly understood.
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.
The different forms of laticiferous organs are examined under the head
of the ‘system of circulation in plants.’ Meyen sees in this system the
highest product of the plant, being fully persuaded with Schulz, that
the latex (milk), or as he also terms it the life-sap, is in constant
circulation, like the blood in the veins. He gives a more summary
account than is his wont of the course of the laticiferous organs, but
bestows more care on the nature of the latex, and on the structure of
the receptacles that contain it. That some of these are produced by
cell-fusion, that others represent intercellular spaces, while others
again are long branched cells, was not known to Meyen or even to later
phytotomists before 1860.
This condensed account of the contents of Meyen’s ‘Phytotomie’ shows a
striking mixture of advance and retrogression, when compared with what
had been achieved before his time; by the side of the fact established
by Treviranus that the epidermis does not consist of a single membrane
but of a layer of cells, to which Meyen assents, we find the gross
mistake of taking the guard-cells of stomata for cuticular glands,
the apertures in which he considers as of secondary importance. It is
still more striking that Meyen expressly rejects on page 120 the fact
established two years before by von Mohl that the pits of parenchyma
are thinner spots, and treats the various pit-formations of the
cell-wall as raised portions of the surface.
In the first volume of his later work the ‘Neues System,’ Meyen gives
a detailed account of phytotomy, which accords on the whole with the
scheme developed in the book we have been examining, and as might
be expected he corrects many errors, adduces many new observations,
and introduces us to many steps in advance of former knowledge; we
shall recur to some of his later views in ensuing portions of this
history with which they are more in connection, remarking only here,
that Meyen paid more attention to the contents of the cell than his
contemporaries, and especially made a number of observations on the
streaming movement, without however recognising the peculiar nature
of the protoplasm which is its substratum. The cell-wall, which he
had once considered to be homogeneous, he afterwards believed to be
composed of fine fibres, a view resting on correct but insufficient
observation and afterwards set right by von Mohl and Nägeli.
It is scarcely possible to imagine a more striking contrast between
two men pursuing the same science than that between Meyen and his much
more important contemporary Hugo von Mohl; Meyen was more a writer
than an investigator; von Mohl wrote comparatively little in a long
time, and only after most careful investigation; Meyen attended more
to the habit, the collective impression produced by objects seen with
the microscope, von Mohl troubled himself little about this, and
always went back to the foundation and true inner connection of the
structural relations; Meyen quickly formed his judgment, von Mohl often
delayed his even after long investigation; Meyen was not critical,
though always prone to opposition, in von Mohl the critical power
much overweighed that of constructive thought. Meyen has not so much
contributed to the definitive settlement of important questions,
as brought to light manifold phenomena, and so to speak accumulated
the raw material; von Mohl on the other hand aimed from the first at
penetrating as deeply as possible into vegetable cell-structure, and
employing all the anatomical facts in framing a coherent scheme.
We have already called attention to HUGO VON MOHL’S[81] pre-eminent
position in the history both of this and also of the succeeding period.
Occupying himself for the most part with phytotomical questions which
had been already investigated, he made the solid framework of cellulose
the object of special and searching examination, and completed the work
of his predecessors on this subject; he thus laid a firm foundation for
the researches into the history of development afterwards undertaken
by Nägeli. Von Mohl, like former phytotomists, generally connected his
researches into structural relations with physiological questions;
but there was one great and unmistakable difference; he never forgot
that the interpretation of visible structure must not be disturbed
by physiological views; he used therefore his thorough physiological
knowledge chiefly to give a more definite direction to his anatomical
researches, and to illustrate the connection between structure and
function in organs. By scarcely any other phytotomist was the true
relation between physiological and anatomical research so well
understood and turned to such practical account as by von Mohl, who was
equally averse to the entire separation of phytotomy from physiology,
and to the undue mixing up of the one with the other, which has led his
predecessors, Meyen especially, into misconceptions.
His anatomical researches profited by his extraordinary technical
knowledge of the microscope; he could himself polish and set lenses,
which would bear comparison with the best of their time. As the
majority of botanists from 1830 to 1850 had little knowledge of
the kind, there was no one so well qualified as von Mohl to give
instruction in short treatises on the practical advantages of a
particular instrument, to remove prejudices and finally as in his
‘Mikrographie’ (1846) to give detailed directions for the management of
the instrument.
But his mental endowments were of course of the higher importance,
and it is difficult to imagine any more happily suited to the
requirements of vegetable anatomy during the period from 1830 to
1850. At a time when men were building fanciful theories on inexact
observations, when Gaudichaud was once more explaining the growth in
thickness of the woody portions of the plant after the manner of Wolff
and Du Petit-Thouars, when Desfontaines’ account of the endogenous
and exogenous growth of stems was still accepted, when Mirbel was
endeavouring to support his old theory of the formation of cells by new
observations and beautiful figures, when Schulz Schulzenstein’s wildest
notions respecting laticiferous vessels were being rewarded with a
prize by the Paris Academy, when Schleiden’s hastily adopted views
respecting cells and fertilisation appeared on the scene with great
external success, von Mohl, for ever going back to exact observation,
was cutting away the ground from under ill-considered theories in
careful monographs, and at the same time bringing to light a mass of
well-established facts leading to further and serious investigation.
These theories have now only a certain historical interest, while von
Mohl’s contemporaneous works are still a rich repertory of useful
observations, and true models of clear exposition.
His written productions were preceded by a careful study of all
branches of botanical knowledge and the auxiliary sciences. That he not
merely acquired knowledge in this way, but trained the powers of his
understanding also, is shown by the striking precision and clearness
of his account of his first investigations. At a time when the
nature-philosophy and Goethe’s doctrine of metamorphosis in a distorted
form were still flourishing, von Mohl in spite of his youth approached
the subjects of his investigation with a calmness and a freedom from
prepossessions, which are the more remarkable when we observe that his
friend Unger was at first quite carried away by the stream, and only
slowly managed to reach the firm ground of genuine inductive enquiry.
Owing to the extravagances and aberrations with which he made
acquaintance as a young man in the nature-philosophy, von Mohl
contracted an aversion to all philosophy, evidently taking the
formless outgrowths from the doctrines of Schelling and Hegel for
something inseparable from it, as we may gather from his address at
the opening of the faculty of natural history in Tübingen, which had
been separated at his instance from that of philosophy. His dislike
to the abstractions of philosophy was evidently connected with his
distaste for far-reaching combinations and comprehensive theories,
even where they are the result of careful conclusions from exact
observations. Von Mohl was usually satisfied with the establishment of
separate facts, and in his speculative conclusions he kept as closely
as possible to what he had actually seen, for instance, in his theory
of the thickening of cell-walls; and where new views opened before
him as a result of his exact observation, he cautiously restrained
himself and was generally content to hint at matters which bolder
thinkers afterwards proceeded to investigate; such a case occurred in
his examination of cell-membranes by polarised light. Hence we miss
to some extent the freer flight of imaginative genius in von Mohl’s
scientific labours; but there is more than sufficient compensation for
this want in the sure and firm footing which he offers to the reader of
his works; if we pass from the study of the writings of phytotomists
before 1844 to those of von Mohl, we are sensible of one predominant
impression, that of security; we have the feeling that the observer
must have seen correctly because the account which he gives of the
matter before us seems so thoroughly natural and almost necessarily
true, and all the more because he himself notices all possible doubts,
and lets those which he cannot remove remain as doubts. In these points
von Mohl’s style resembles that of Moldenhawer, but in von Mohl it
attains to a mastery which is wanting in the other.
There is an evident connection between von Mohl’s dislike of
far-reaching abstractions and philosophic speculation on the results of
observation and the fact, that in the course of more than forty years’
unintermitted application to phytotomy he never composed a connected
general account of his subject. His efforts as a writer were confined
to monographs usually connected with questions of the day or suggested
by the state of the literature. In these he collected all that had been
published on some point, examined it critically, and ended by getting
at the heart of the question, which he then endeavoured to answer from
his own observations.
For the purpose of these observations he looked about in every case
for the most suitable objects for examination, a point to which
former phytotomists, with the exception of Moldenhawer, had paid
little attention; he then studied these objects thoroughly, and thus
prepared the way for the examination of others, which presented greater
difficulties. Every monograph of this kind was a nucleus, round which
a larger number of observations might afterwards gather. In a long
series of such solid productions he treated conclusively all the more
important questions of phytotomy.
Von Mohl’s extraordinary carefulness was not however able to guard
him, calm observer though he was, from some serious mistakes, at least
in his earlier years, such as those which occur in his first theory
of intercellular substance (1836), and in his earliest views on the
nature of the cell-membrane of the pollen-grain (1834). These and some
other errors on the part of a gifted and truly inductive enquirer are
instructive, since they show that observation without any groundwork
of theory is psychologically impossible; it is a delusion to suppose
that an observer can take the phenomena into himself as photographic
paper takes the picture; the sense-perception encounters views
already formed by the observer, preconceived opinions with which the
perception involuntarily associates itself. The only means of escaping
errors thus produced lies in having a distinct consciousness of these
prepossessions, testing their logical applicability and distinctly
defining them. When von Mohl laid down his theory of intercellular
substance, there evidently floated before his mind indistinct,
half-conscious ideas of the kind that Wolff and Mirbel entertained
of the structure of the vegetable cell; and as he considered the
cell-membrane of the pollen-grain to consist of a cell-layer, he
summarised its obscure structural relations under the then very
obscure conception of the cell. As a true investigator of nature, who
adheres always and firmly to the results of further observation, and
endeavours to clear his ideas by their aid, conceding only a relative
value to every view, von Mohl soon escaped from these errors, and
himself supplied proofs of the incorrectness of his former opinion. The
number of really erroneous statements in his works is wonderfully small
considering the very large number of investigations in which he engaged.
In examining the part which von Mohl played in the general development
of phytotomy we can distinguish satisfactorily two periods in his
scientific career, the first of which extends from 1827 to about 1845.
Before 1845 he was acknowledged to be the first of phytotomists,
decidedly superior to all rivals; his authority, though often attacked
by unimportant persons, grew from year to year. This period may be said
to close with the publication of his ‘Vermischte Schriften’ in 1845.
Up to that time investigations into the form of the solid framework of
cell-membrane had chiefly attracted the interest of phytotomists, and
in this subject there was no one who could measure himself with von
Mohl. Yet he began soon after 1830 to study the history of development
in plants; in 1833 he described the development of spores in a great
variety of Cryptogams, in 1835 the multiplication of cells by division
in an alga, and the cell-division in the formation of stomata in 1838;
in this period appeared Mirbel’s first observations on the formation
of pollen-cells (1833). Von Mohl too was the first, if we disregard
Treviranus’ somewhat imperfect notices of the origin of vessels in 1806
and 1811, who explained the history of the development of those organs;
and his theory of the thickening of cell-membranes, the principles of
which are to be found in his treatise on the pores in cellular tissue
(1828), may also be regarded as a mode of conceiving the sculpture of
the cell-membrane from the point of view of the history of development.
Ever since 1838 Schleiden had raised the history of development to
the first rank in botanical investigation, but he had proposed a
thoroughly faulty theory of cell-formation, to which von Mohl at
first at least did not withhold his assent in spite of previous and
much better observations; but after 1842 Nägeli devoted himself still
more thoroughly and with more lasting results to the study of the
development both of vegetable cells and tissue-systems, and of the
external organs. He introduced new elements into phytotomic research,
and it soon became apparent that even the questions hitherto examined
must be grappled with in a different fashion. Von Mohl did not hold
aloof from the new direction, but completed a series of excellent
investigations connected with the new questions in the theory of
cell-formation. The most important of these was his enquiry into the
nature of protoplasm, to which he gave the name still in use. In
his treatise, ‘Die Vegetabilische Zelle,’ which came out in 1851 in
Wagner’s Dictionary of Physiology, he even gave an excellent account
of the modern theory of cell-formation; but notwithstanding all this,
and the great authority which he rightly continued to enjoy, he was no
longer the guide who led the way in the domain of phytotomy, as he had
been before 1845.
His zeal as an observer had at all times been chiefly attracted to the
solid framework of vegetable structure in its matured condition, though
a number of his most important works were devoted to the study of
cell-contents.
Except in his ‘Anatomic der Palmen’ (1831), where he expended much
and to some extent even unnecessary labour on figures representing
the general appearance of the tissue (histologic habit), von Mohl’s
microscopic drawings do not aim at giving the collective impression,
but at facilitating the understanding of the delicate structure of
single cells and their combinations by aid of the simplest possible
lines. He always despised pictures from the microscope, such as were
introduced at a later time by Schacht,—a kind of artistic restoration
of the originals and to some extent a playing with science; and in his
later publications he was more sparing of illustrations or omitted them
altogether, in proportion as he acquired the power of giving clear
verbal explanations of even difficult structural conditions.
Von Mohl’s scientific activity was so wonderfully productive that it
is not easy to present the reader with a clear account of it; but we
must endeavour at least to furnish such a summary of its chief results
as may serve to give a general idea of his importance in the history
of our science. We may here pass over such of his treatises as do not
bear on the main questions of phytotomy, and notice only those that
relate to the structure of the solid framework of plants, because
the historical significance of his investigations into the history of
development can only be understood in connection with the questions to
be treated in the following chapter. But we shall not limit ourselves
to publications which appeared before 1845, though we may be thus
compelled to notice researches which in succession of time belong to
the next period, and indeed almost to the present moment.
1. The view that the cell is the sole and fundamental element in
vegetable structure had been already maintained by Sprengel and Mirbel,
but not supported by exact observations. Treviranus too had shown
that the vessels in wood are formed by the union of rows of cell-like
tubes, but he had never arrived at a thoroughly clear conception of the
matter. On the one side was the theory that the plant consists entirely
of cells, on the other, and for long the old and strange view, that
the spiral thread was an independent elementary organ of vegetable
structure,—a view which Meyen still maintained in 1830. Von Mohl must
be regarded as the first who took up the all-important position, that
not only the fibrous elements of bast and wood, which had long been
considered to be elongated cells, but the vessels of the wood also
are formed from cells; and we may on this point give great weight to
his own assertion that he was the first who observed the formation
of vessels from rows of closed cells. This discovery happened in the
year 1831, and he describes distinctly, though briefly, the decisive
observations in his treatise on the structure of the palm-stem. At the
points of constriction in the vessels he saw the dividing walls, the
existence of which had been denied by all former phytotomists; ‘these
dividing walls,’ he says, ‘are entirely different from the rest of
the membranes of the plant, being formed of a network of thick fibres
with openings between them.’ He studied the history of the development
of these vessels both in palms and in dicotyledonous plants. ‘In the
young shoot,’ he says, ‘are found at the spots, where afterwards
there are large vessels, perfectly closed large cylindrical tubes
with a transparent and very delicate membrane.’ He then shows how by
degrees the sculpture peculiar to the walls of vessels is formed on the
inside of these tubes, and he takes the opportunity of saying that a
metamorphosis in time from one form of vessel into another is entirely
out of the question, as Treviranus also and Bernhardi had maintained.
‘The dividing walls (transverse septa),’ he continues, ‘are formed
in a precisely similar manner to the side-walls of vessels; only the
original tender membrane of the septa is usually lost in the meshes
of the network of fibres.’ From that time no phytotomist capable of
an independent judgment has had any doubt with regard to this view of
the formation of the vessels in wood. It is however striking enough
that von Mohl, who thought it so important to show that the cell is
the sole foundation of vegetable structure, never extended the proof
to milk-vessels and other secretion-canals in order to show whether
and how these also are formed from the cells. In his treatise on the
vegetable cell (1851) he still expressed doubt about Unger’s assertion,
that the milk-vessels are also formed from rows of cells that coalesce
with one another, and held rather to the view of an anonymous writer
in the ‘Botanische Zeitung’ of 1846, page 833, that these vessels are
membranous linings of gaps in the cell-tissue. He might well lose his
taste for the examination of these and similar organs after Schultz
Schultzenstein had by his various treatises, written after 1824, on the
so-called vital sap and the circulation which he attributed to it, made
this part of phytotomy a very quagmire of error, and had not refrained
from replying in an unbecoming manner to von Mohl, who repeatedly
opposed his views; moreover Schultz’s essay ‘Ueber die Circulation des
Lebenssafter’ (1833), which teems with absurdities, had received a
prize from the Academy of Paris.
2. The growth in thickness of the cell-membrane, and the sculpture
caused by it was a subject that is more or less connected with most
of von Mohl’s writings. He developed the chief features of his
view in 1828 in his first work, ‘Die Poren des Pflanzengewebes!’
The way in which he represented to himself the growth in thickness
of cell-membranes at a later time may be expressed as follows. All
elementary organs of a plant are originally very thin-walled perfectly
closed cells, which in the tissue are separated by walls formed of two
laminae[82]; on the inside of these primary cell-membranes, after they
have ceased to increase in circumference, new layers of membranous
substance are formed, which lying one upon another adhere closely
together, and represent the whole amount of secondary thickening
layers; on the inner side of the membrane thus thickened by apposition
there may usually[83] be perceived a tertiary layer of a different
character.
But there are certain sharply defined spots on the original cell-wall,
where this thickening does not take place; in such spots the cell is
still bounded only by the primary membrane; it is these thin spots
which bear the name of pits, and which Mirbel, and in some cases
Moldenhawer, took for holes, but von Mohl considered that it was only
in very exceptional cases that they were really changed into holes by
resorption of the thin primary wall. In accordance with this theory,
the spiral, annular, and reticulated vessels are produced by deposition
of thickening matter in the form suitable to each case on the inside
of the originally smooth thin cell-wall. But like Schleiden and other
phytotomists, von Mohl was not quite clear in his views either of the
origin or mode of formation of matured bordered pits; it was supposed
that the two laminae of the dividing wall parted from one another
at certain spots in such a manner that a lenticular hollow space
was formed between them, and that this space answered to the outer
border of the pit, while the inner border was the result of ordinary
pit-formation. This view, which could be shown to be incorrect by the
history of development, arose in fact from inexact observation,—a rare
case with von Mohl; the true course of events in the formation of
bordered pits was first described by Schacht in 1860.
It was mentioned above, that Meyen in his ‘Neues System der
Physiologie’ of 1837, i. p. 45, made cell-membranes consist of
spirally wound fibres; von Mohl had described in 1836 the structural
relations of certain long fibrous cells of Vinca and Nerium, which
might be provisionally explained in this way; he was led by Meyen’s
ideas on the subject to a renewed and minute examination of the more
delicate structure of the cell-membrane in 1837; he first of all
cleared the ground round the question, by distinguishing the cases in
which real spiral thickenings lie on the inner side of the membrane,
from those in which the membrane is smooth on the outside, but shows
an inner structure of fine spiral lines; in these cases he assumed a
peculiar arrangement of the molecules of cellulose, and endeavoured
to illustrate the possibility of such a disposition by the phenomena
of cleavage in crystals (‘Vermischte Schriften,’ p. 329); but he did
not succeed in explaining these very delicate conditions of structure,
which we now call the striation of the cell-membrane, so clearly as
Nägeli afterwards did in connection with his molecular theory.
3. The question of the substance and chemical nature of cell-membranes
was intimately connected with von Mohl’s theory of its growth in
thickness; he was engaged in 1840 in minutely studying the reactions
which various cell-membranes exhibit with iodine solution under
different conditions,—a question on which Schleiden and Meyen had
recently disagreed; von Mohl arrived at the result, that iodine imparts
very various colours to vegetable cell-membrane, according to the
quantity in which it is absorbed; a small amount produces a yellow or
brown, a larger a violet, a still larger a blue tint; this depends
partly on the extent to which the membrane is capable of distention;
the blue colour especially depends on the absorption of a sufficient
quantity of iodine. Greater interest, excited at first by a very
important work by PAYEN[84] in 1844, was taken in the question of
the chemical nature of the solid framework of the vegetable body, in
which it was shown that the substance of all cell-membranes exhibits a
similar chemical composition when freed from foreign elements. Payen
considers that this material, cellulose, is present in a tolerably
pure form in the membranes of young cells, but is rendered less pure
in older ones by ‘incrusting substances,’ whose presence changes the
physical and chemical characters of cell-membranes in various ways.
These incrusting substances may be more or less removed by treating
the membranes with acids, alkalies, alcohol, and ether, while other
inorganic matters remain behind after combustion as an ash-skeleton.
This theory, which has been more perfectly worked out in modern times,
was soon afterwards met by Mulder with the assertion, that a large part
of the layers composing the walls of cells consist from the first of
other combinations and not of cellulose; he at the same time deduced
from this view certain conclusions respecting the growth in thickness
of cell-walls. He and Halting, relying on microscopic examination,
maintained that the innermost tertiary layer in thickened membranes
is the oldest, and that the other layers are deposited on the outside
of this, and are not composed of cellulose. Von Mohl opposed this
view decidedly and successfully in the Botanische Zeitung of 1847; he
likewise in his work on the vegetable cell (p. 192), refuted the view
of the varying substance of cell-membrane, which Schleiden had founded
on some obscure chemical grounds.
It would carry us much too far to enter into the details of this
scientific dispute; Payen’s view of the chemical nature of the
vegetable cell-wall, which von Mohl adopted and elaborated, has
maintained itself to the present day, and is generally considered to be
the true one; on the other hand, the foundations of von Mold’s theory
of growth in thickness were shaken in 1858 by Nägeli’s observations,
and we may say that on the whole it has been for ever superseded. It
has been nevertheless of great service in the development of our views
on cell-structure in plants; keeping closely to the facts directly
observed, it served to bring almost all the conditions of the sculpture
of cell-walls under one point of view, and to refer their formation to
one general and very simple scheme; every such theory helps to advance
science, because it facilitates mutual understanding; in this case,
when Nägeli proposed his more profound theory of intussusception,
the understanding of it was essentially assisted by a previous exact
knowledge of von Mohl’s theory in its principles and results. In
conclusion it may be mentioned here that von Mohl afterwards in his
investigation into the occurrence of silica in cell-membranes made a
large and important addition to the knowledge of their more delicate
structure, and of the way in which incrusting substances are deposited
in them (Botanische Zeitung, 1861).
4. The views of phytotomists on the so-called intercellular substance
during the twenty years from 1836 to 1856 were closely connected with
the older theories of cell-formation, but were opposed to the modern
doctrine of the cell founded by Nägeli in 1846. Von Mohl himself had
introduced this idea for the first time into the science in 1836 in
one of his earlier and inferior essays, ‘Erläuterung meiner Ansicht
von der Structur der Pflanzensubstanz,’ rather in opposition to than
in connection with his own theory of the growth and structure of
cell-walls. Setting out from modes of formation of cell-membranes
in some Algae, difficult to understand and in some respects quite
peculiar, von Mohl believed that he saw in many cases in the higher
plants also between the sharply-defined membranes, which bound the
cell-spaces and which he regarded as the entire cell-membranes, a
substance in which the cells are imbedded, for such is its appearance
when it is largely developed; when it lies in small quantity only
between cells in close apposition, it looks like a thin layer of
cement. After Meyen in his ‘Neues System,’ pp. 162, 174 had declared
against this view in 1837, von Mohl too abandoned it more and more,
and afterwards limited the occurrence of intercellular substance to
certain cases, being convinced that much that he had before taken for
it consisted only of layers of secondary thickening, between which
he still saw the primary lamina of the cell-membrane. The theory of
intercellular substance was taken up and further developed by other
phytotomists, by Unger especially in the Botanische Zeitung for 1847,
p. 289, and afterwards chiefly by Schacht; Wigand came forward as an
opponent of it in 1854 in his ‘Botanische Untersuchungen,’ p. 65, and
logically following out von Mohl’s theory of the cell-membrane declared
the thin layers of intercellular substance as well as the cuticle,
which had been first correctly distinguished by von Mohl, to be laminae
of primary cell-membrane, the substance of which had undergone profound
chemical change. These ideas also of the intercellular substance and
the cuticle assumed an entirely different aspect when Nägeli introduced
his theory of intussusception.
The limits imposed on this history render it necessary to be content
with these indications of von Mohl’s share in the working out of the
theory of cells in its connection with the structure of the solid
framework of cell-membrane; we shall return again to his observations
on the formation of individual cells.
5. Forms of tissue and comparative anatomy. Phytotomy up to 1830
had been weak in its classification of tissues, in its ideas as to
their arrangement, and consequently in its histological terminology;
the inconvenience arising from this state of things was most
distinctly felt when it became necessary to compare the structure of
different classes of plants, Cryptogams, Conifers, Monocotyledons
and Dicotyledons, and to establish their true differences and actual
agreements. How little phytotomy had advanced in this respect is
shown plainly in the account of tissues given by Meyen in his ‘Neues
System’ in 1837. To von Mohl belongs the merit of having perceived at
an early period in his scientific career, and more clearly than his
contemporaries, the value of a natural and sufficient discrimination
of the various forms of tissue, and the necessity of obtaining a
correct view of their relative disposition; he thus showed the way to
an understanding of the general structure of the higher plants, and
rendered it possible to make a scientific comparison of the structure
of different classes of plants.
Von Mohl, like Moldenhawer long before, showed from the first a correct
apprehension of the peculiar character of the vascular bundles as
compared with other masses of tissue. He, too, examined them first in
Monocotyledons, and gave an account of them in his treatise on the
structure of Palms (1831), and also in his later essays on the stems
of Tree-ferns, Cycads, and Conifers and on the peculiar form of stem
in Isoëtes and Tamus elephantipes, to be found in his ‘Vermischte
Schriften’ of 1845. His just conception of them as special systems
composed of various forms of tissue has made his account clear and
intelligible, and his whole treatment of the subject appears new in
comparison with that of every previous writer except Moldenhawer. If
these labours of von Mohl are surpassed in value by later studies of
the history of development, they served for the time as a nucleus
for further investigations, especially into the nature of stems. It
contributed in a high degree to a correct insight into the structure
of the stem, that von Mohl, agreeing in this with Moldenhawer,
distinguished the portion belonging to the wood from the portion
belonging to the bast in the vascular bundles, and regarded both as
essential constituents of a true vascular bundle. Not less important
were his enquiries into the longitudinal course of the vascular bundle
in the stem and leaf, which showed that in the Phanerogams the bundles
in the stem are only the lower extremities of the bundles, the upper
extremities of which bend outwards into the leaves, and that the
Monocotyledons and Dicotyledons agree in this particular, though the
course of the bundle differs considerably in the two cases. He obtained
an important result in this respect in his researches on palm-stems
in 1831, when he proved the incorrectness of the distinction between
endogenous and exogenous growth in thickness, which had been laid
down by Desfontaines, and even employed by De Candolle in framing his
system. According to Desfontaines, the wood of Monocotyledons appears
as a collection of scattered bundles, of which those that run out
above into the leaves come from the centre of the stem. From this very
imperfect observation he deduced the view, that the bundles of vessels
in Monocotyledons originate in the centre of the stem, and that they
continue to be formed there, until the older hardened bundles in the
circumference form so solid a sheath that they withstand the pressure
of the younger; then all further growth in thickness must cease, and
hence the columnar form of the monocotyledonous stem. This doctrine
found general acceptance, and was employed by De Candolle to divide
vascular plants into Endogens and Exogens, in accordance with the very
general inclination felt in the first half of the present century to
distinguish the great groups of the vegetable kingdom by anatomical
characters. It is true that Du Petit-Thouars had already shown that
some monocotyledonous stems have unlimited growth in thickness; neither
his nor Mirbel’s later observations succeeded in shaking the theory,
the adherents of which met such cases by assuming a peripherical as
well as a central growth. Then von Mohl in the treatise above-mentioned
demonstrated the true course of the vascular bundles in the stem
of Monocotyledons, and at once did away with the whole theory of
endogenous growth in the opinion of all who were capable of judging,
though some even eminent systematists for a long time maintained the
old error. The results which von Mohl obtained from his study of the
comparative anatomy of the stem, rested mainly on careful observation
of the mature tissue-masses, and when he studied the history of
development, he was not in the habit of going back to the very earliest
and most instructive stages. Hence he failed to explain fully the real
points of agreement and difference of structure between Tree-ferns
and other Vascular Cryptogams and Phanerogams, and in like manner he
stopped halfway when engaged in explaining the secondary growth in
thickness of dicotyledonous stems from the nature of their vascular
bundles, and the formation of cambium. The account of growth in
thickness which he still gave in 1845 (‘Vermischte Schriften,’ p. 153),
and which rests less on observation than on an ideal scheme, is highly
obscure, and even in the treatise which he published in the Botanische
Zeitung in 1858 on the cambium-layer of the stem of Phanerogams, and
in which he criticises the newer doctrines of Schleiden and Schacht,
the subject is far from being fully cleared up, though the views there
advocated are decidedly superior to his former ones. A satisfactory
conclusion with respect to growth in thickness of the woody body and of
the rind was not reached till the history of development in vegetable
histology began to be more thoroughly studied.