character of the vascular bundles as compared with other tissue-masses,
so he perceived that the structure of the epidermis and of the
different forms of exterior tissue was thoroughly characteristic, and
he succeeded in arriving at a clearer understanding of the matter
in this case than in the other. Very confused ideas had prevailed
on the subject before he took it up, and we owe to him the best and
most important knowledge which we at present possess. Especially
important were his researches into the origination and true form of
stomata (1838 and 1856), and into the cuticle and its relation to the
epidermis (1842 and 1845). He brought entirely new facts to light
by his study of the development of cork and the outer bark in 1836;
these tissues had scarcely been examined with care till then, and
their formation and relation to the epidermis and the cortical tissue
were quite unknown. In the latter treatise, one of his best, the
difference between the suberous periderm and the true epidermis was
first shown, the various forms of the periderm were described, and the
remarkable fact established that the scaling of the bark was due to
the formation of fine laminae of cork, which, penetrating gradually
into the substance of the cortex, withdraw more and more of it from
its connection with the rest of the living tissue, and as they die off
form by their accumulation a rugged crust, which is the outer bark
surrounding most thick-stemmed trees. The investigation was so thorough
and comprehensive, that later observers, Sanio especially in 1860,
could only add to it some more delicate features in the history of the
process. In the same year appeared his enquiry into the lenticels,
where von Mohl however overlooked what Unger discovered at the same
time (‘Flora,’ 1836), namely, that these forms arise beneath the
stomata; but he at once corrected Unger’s hazardous supposition that
the lenticels are similar forms to the heaps of gemmae on the leaves of
the Jungermannieae. Unger, for his part, was not long in adopting von
Mohl’s explanation of the lenticels as local cork-formations.
Since von Mohl thus distinctly brought out the special character of
the vascular bundles and of the different forms of epidermal tissues,
it must excite surprise that he, like former phytotomists, did not
find himself under the necessity of framing some conception of the
rest of the tissue-masses in their peculiar grouping as a whole, as a
special system, and of classifying and suitably naming the different
forms that compose them, though his examination of Tree-ferns would
seem to have offered him an occasion for doing so. Von Mohl, like his
contemporaries, was satisfied with calling everything that is neither
epidermis, cork or vascular bundle, parenchyma, without distinctly
defining the expression.
Here we leave von Mohl and his labours for the present, to return once
more in the following chapter to the share which he took in the further
progress of phytotomy. We shall perhaps best realise his importance
in the history of the science, if we try to think of all that we have
now seen him doing for it as still undone. There would then be a huge
gap in modern phytotomic literature, which must have been filled up by
others before there could be any further addition to the knowledge of
cells and tissues founded on the history of their development; for it
can hardly be conceived that the advance to which we owe the present
condition of vegetable anatomy, could have been based upon ideas such
as those of Meyen, Link, and Treviranus, without von Mohl’s preliminary
discoveries.
CHAPTER IV.
HISTORY OF DEVELOPMENT OF THE CELL, FORMATION OF TISSUES, MOLECULAR
STRUCTURE OF ORGANISED FORMS.
1840-1860.
In the period between 1830 and 1840 it had come to be understood, that
the old theories of cell-formation of Wolff, Sprengel, Mirbel, and
others, resting on indistinct perceptions and not on direct and exact
observation, could only give an approximate idea of the formation
of cells. But in the course of that time really different cases of
formation of new cells were accurately observed by Mirbel, and more
especially by von Mohl, who described different modes of formation
of spores, and in 1835 the first case of vegetative cell-division.
Unfortunately these observations, excellent in themselves, applied
to cases of cell-formation which do not occur in the ordinary
multiplication of cells in growing organs, and von Mohl guarded himself
from founding a general theory of cell-formation on his observations
on cells of reproduction and on a growing filamentous Alga. Mirbel
also cautiously regarded the formation of pollen-cells and that which
he supposed to be the process in the germination of spores as cases of
a peculiar kind, adhering to his old theory of the origin of ordinary
tissue-cells.
Schleiden’s behaviour was different. Having somewhat hastily observed
the free cell-formation in the embryo-sac of Phanerogams in 1838, he
proceeded at once to frame a theory upon it which was to apply to all
cases of cell-formation, and especially to that in growing organs.
The very positive way in which he announced this theory and set
aside every objection that was made to it, combined with his great
reputation at the time, at once procured for it the consideration
of botanists generally; and the most important representatives of
phytotomy, von Mohl himself at first not excepted, allowed that there
was a certain amount of justification for it. It was a question in
which theoretical considerations were not of primary importance; direct
and varied observation of careful preparations with strong magnifying
powers could alone form the basis for further investigation. Unger
showed in this way that the processes at the growing point of the stem
could scarcely be reconciled with Schleiden’s theory, and in this
view he was supported by the English botanist Henfrey; but Nägeli
was the first who addressed himself with energy and sound reasoning
to the important and difficult question, how cells are formed in
reproductive and growing vegetative organs, and how far the processes
are the same in the lower Cryptogams and in the Phanerogams. He set
out by assuming that Schleiden’s theory was in the main correct, but
his long-continued investigations led him finally to the conviction
that it must be entirely abandoned, and he proposed the outlines
of the theory of cell-formation which is accepted at the present
time. In this case, as before in questions of morphology, he applied
himself first, and with great success, to the investigation of the
lower Cryptogams, while Alexander Braun’s observations on some very
simple Algae contributed materially to the further development of the
cell-theory, and especially to extending and correcting the idea of
the cell; Hofmeister’s researches also in embryology not only produced
great results for morphology, but at the same time supplied a variety
of facts which served to complete Nägeli’s view. The further this was
worked out, the more apparent it became that the external circumstances
in the processes of cell-formation might be very various, and that von
Mohl’s earlier observations especially gave a correct representation
of individual and typical cases; but more important than this result
was the fact declared by Nägeli in 1846, that in all these different
kinds of cell-formation it was only the external and secondary matters
that varied, while the essential part of the process was in all cases
the same, and it was soon perceived that cell-formation in the animal
kingdom, which was now being more thoroughly examined, agreed in the
main with that of the vegetable kingdom, as Schwann and Kölliker had
intimated in 1839 and 1845.
It is unnecessary to give any account here of the totally different
theories which Theodor Hartig and Karsten proposed about the same time.
They do not rest on careful observation, and we may omit them not
merely because they are rejected by the unanimous judgment of better
observers, but because they had no influence upon the development of
the doctrine of cell-formation, and are therefore without historical
interest.
It lies in the nature of the case, that investigations into the origin
and multiplication of cells should turn the attention of observers
more and more to their living contents, for these are actively and
immediately concerned with the formation of new cells. The various
granular, crystalline, and mucilaginous portions of the contents of
cells had been repeatedly observed before 1840, and Schleiden and
Meyen had specially studied the ‘movements of cell-sap’; but it was
in the course of observations on the history of development between
1840 and 1850 that attention was first called to a substance which
plays a regular part in the formation of new cells, which envelopes
the cell-nucleus discovered by Robert Brown, which undergoes the most
important changes as the cell grows, which forms the entire substance
of swarmspores, and the disappearance of which leaves behind it a
dead framework of cell-membrane. This substance, which is much more
immediately concerned with sustaining the processes of life than is
the cell-wall, was seen by Schleiden in 1838 and taken for gum. It was
more carefully studied by Nägeli between 1842 and 1846, and perceived
by him to be nitrogenous matter. Von Mohl described it in 1844 and
1846 from new points of view, gave it the name of protoplasm which it
still bears, and showed that it is this substance, and not the proper
cell-sap, which carries out the movement of rotation and circulation in
cells discovered by Corti in the previous century, and again observed
by Treviranus in 1811. The Algae proved highly instructive in the
study of this remarkable substance also. The swarmspores of Algae and
Fungi observed by Alexander Braun, Thuret, Nägeli, Pringsheim, and De
Bary showed that protoplasm is not dependent on the cell-membrane for
its vitality, that by virtue of its own internal powers it can alter
its form, and even move in space. In 1855 Unger in his ‘Lehrbuch’
pointed out the resemblance of this substance to the matter known as
sarcode in the lower forms of animals, a resemblance brought out more
plainly in 1859, when De Bary’s studies of the Myxomycetes proved that
the substance of these forms was protoplasm, which continues to live
for a considerable time, and often in large masses, before it forms
cell-membranes. Zootomists now began to take an interest in these
results of botanical research; Max Schulze (1863), Brücke, and Kühne
studied animal and vegetable protoplasm, and the conviction gained
ground more and more in the years from 1860 to 1870 that protoplasm is
the immediate principle of vegetable and animal life. This discovery is
one of the most important results of research in modern natural science.
Not less important were the results obtained from the study of
the rest of the organised contents of cells; von Mohl proved that
chlorophyll-corpuscles, the most considerable organs of nutrition
in the plant, are formed of protoplasm, and Theodor Hartig, though
his cell-theory was a mistake, did good service by his discovery of
aleurone-grains in seeds and of the crystalloids which sometimes occur
in the grains, and which are also formed of protoplasm and renewed
from protoplasm. Radikofer, Nägeli, and others added to our knowledge
of the form and chemical composition of these aleurone-grains. To
starch-grains, which had been frequently examined, by Payen especially,
Nägeli devoted an investigation at once comprehensive and profound,
and obtained results of extraordinary value; these were given to the
world in an exhaustive work published in 1858 under the title ‘Die
Stärkekörner,’ and form an epoch not in phytotomy only, but in the
general knowledge of organised bodies. By the application of methods of
research unknown before in microscopy, Nägeli arrived at clear ideas
of the molecular structure of the grains, and of their growth by the
introduction of new molecules between the old ones. This theory of
intussusception founded on the observation of starch-grains derives
its great importance from the fact that it served directly to explain
the growth of cell-membrane, could be applied generally to molecular
processes in the formation and alteration of organic structures, and
accounted for a long series of remarkable phenomena, especially the
behaviour of organised bodies in polarised light. Nägeli’s molecular
theory is the first successful attempt to apply mechanico-physical
considerations to the explanation of the phenomena of organic life.
While men of the highest powers of mind were devoting themselves to
the solution of these difficult problems, the study of tissues was
not neglected in the years after 1840, and here too it was Nägeli who
gave the chief impulse and the direction to further development. In
the periodical which he published in conjunction with Schleiden he had
already (1844-46) given an account of some searching enquiries which he
had made into the first processes in the formation of vascular bundles
from uniform fundamental tissue; in the Cryptogams he observed the
production of the tissue of the whole plant from the apical cell of the
growing stem, and this discovery, still further pursued by Hofmeister
especially, has given rise during the last twenty years to a copious
literature, which has been of service to the theory of the formation of
tissues, to morphology, and consequently also to systematic botany.
The researches of Hofmeister, Nägeli, Hanstein, Sanio, and others into
the first formation of vascular bundles from the fundamental tissue of
young organs led to important results for morphology, in so far as it
was now for the first time possible to judge of the morphological value
of anatomical and histological relations. The growth in thickness of
woody plants, a question of primary importance to vegetable physiology,
was first made intelligible by the discovery of the mode of formation
of vascular bundles and their true relation to cambium; Hanstein and
Nägeli, and afterwards Sanio especially, cleared up the questions
connected with growth in thickness in their main features before and
after 1860.
* * * * *
When we pass on to show how the great results above-mentioned were
attained, we encounter some difficulties. After 1840 botanical
literature multiplied to an extent before unknown; it is from elaborate
monographs on single subjects in phytotomy, from some text-books, and
especially from smaller essays in botanical periodicals that we must
gather an account of the further development of scientific thought.
Much as the founding of scientific periodicals has facilitated
communication between professed botanists, yet this form of literature
makes it more difficult to see the way clearly through the work of
earlier periods and to discover the historical connection in the
science, not to speak of the harm that usually results from it to young
and inexperienced students.
Such being the nature of the sources from which we must draw our
information, we shall obtain a better general view of the whole subject
if we depart from the practice of former chapters, and follow out the
more important questions in their historical development instead of
connecting them directly with leading persons. Such a treatment of the
subject is indeed suggested by the fact that we are now no longer on
pure historic ground; for the majority of the men who have developed
modern doctrines since 1840 are still alive, and it must be uncertain
whether the account here attempted may not be impugned on some ground
or other. Owing to the extraordinary diversity of opinion that exists
among botanists even on the most general questions in the science, it
is extremely difficult to ascertain what can be considered as a common
possession,—an unfortunate condition of things, from which no science
perhaps suffers so much as botany.
The extent to which individual botanists have contributed to the
advance of phytotomy during the period under consideration will
appear of itself from the following narrative; and if we speak almost
exclusively of Germans, it is for the simple reason that Englishmen
from Grew’s time till now can scarcely be said to have added anything
to our knowledge of phytotomy; the Italians also, once so gloriously
represented by Malpighi, scarcely come under consideration in the
questions now to be dealt with, while French botanists, represented by
Mirbel in the preceding period, though they have produced many works on
phytotomy since his time, have had no important share in deciding the
fundamental questions of modern science.
In the preceding period it was necessary to take into consideration
the increasing improvement of the microscope, in order to understand
the development of opinion on vegetable structure; but it is scarcely
needful to do so after 1840. Since that time good and serviceable
instruments with strong magnifying powers and clear definition have
been within the reach of every phytotomist; and though improvements
are still being constantly made, yet the microscopes that were in the
hands of skilful observers between 1840 and 1860 were fully adequate
to deciding the new questions proposed to them. The chief improvement
effected in the microscope during this period was the fitting it with
apparatus for the polarisation of light, and for the more convenient
measurement of objects; we shall see presently what influence the
former improvement had on the perfecting of Nägeli’s molecular
theory. As microscopes improved and the questions to be solved grew
more difficult, it became necessary to bestow increased care on the
preparation of objects; it was no longer sufficient to cut or dissect
neatly, and so learn the form of the solid portions of vegetable
structure; measures of precaution and auxiliary measures of the most
various kinds were needed to obtain a clear view of the soft contents
of cells, and to observe the protoplasm as far as possible in a living
state and protected from prejudicial influences; all sorts of chemical
reagents were applied to make the objects more transparent, or to show
their physical and chemical characters. The method invented by Franz
Schulze before 1851 deserves to be specially mentioned; it consisted
in isolating the cells in a few minutes’ time by boiling them in a
mixture of nitric acid and potassium-chlorate, and thus shortening
Moldenhawer’s process of maceration or superseding it altogether. In a
word, the technicalities of the microscope were perfected in a variety
of ways by Schleiden, von Mohl, Nägeli, Unger, Schacht, Hofmeister,
Pringsheim, De Bary, Sanio, and others, and raised to an art which must
be learnt and practised like any other art. Young microscopists were
able after 1850 to learn this art in the laboratories of their elders,
and to profit by their technical experience and scientific counsels;
schools of phytotomy were formed at least in the German universities;
elsewhere, it is true, the old condition of things remained in which
everyone had to trust to himself from the beginning.
The general dissemination of good microscopes was accompanied by a
higher standard of requirement in the execution of drawings from
the instrument, especially after von Mohl had shown the way; and
the invention of lithography and the revival of wood-engraving
ministered to the needs of science, supplying the place of the old
costly copper-plate printing. Hence we find an increasing number of
beautiful drawings in scientific monographs; the text-books also
could now be supplied with an abundance of figures, and this greatly
promoted the general understanding of things which could otherwise
be seen only under the glass of each observer. From the close of the
16th century wood-cuts had fallen more and more into disuse, and had
been replaced by copper-plates; after 1840 wood-engraving was restored
to its old rights and was found to be a more convenient method of
pictorial illustration, especially for text-books; thus Schleiden’s
‘Grundzüge’ of 1842, von Mohl’s ‘Vegetabilische Zelle’ of 1851,
Unger’s and Schacht’s text-books were enriched with many and sometimes
very beautiful wood-cuts. Lithographs were generally preferred for
periodicals and monographs; the ‘Botanische Zeitung,’ founded by Mohl
and Schlechtendal in 1843, and till after 1860 the chief organ for
shorter phytotomic communications, was illustrated by a large number
of beautiful prints from the establishment of the Berlin lithographer,
Schmidt.
1. DEVELOPMENT OF THE THEORY OF CELL-FORMATION FROM 1838 TO 1851.
Since we are here dealing with questions of fundamental importance
not only to one branch of botanical study but to the whole science
of botany, and even to the rest of the natural sciences, it seems
imperative that we should follow step by step the founding and
perfecting of the theory of the cell, as far as is possible in the
limited space at our command; we shall deal with the sexual theory
further on in a similar manner.
As usually happens in the inductive sciences, the period of strict
inductive investigation into cell-formation was preceded by a still
longer time, during which botanists ventured to put forward general
theories in reliance on highly imperfect observations. We have already
seen how Caspar Friedrich Wolff in 1759 made cells originate as
vacuoles in a homogeneous jelly, and how this view was adopted in
all essential points by Mirbel at a late period in the 18th century;
how Kurt Sprengel, and with him a number of phytotomists, among them
Treviranus as late as 1830, supposed cells to be formed from granules
and vesicles in the cell-contents, an idea which Link it is true
opposed in 1807, but afterwards accepted to a great extent. Though
Moldenhawer as early as 1812 (‘Beiträge,’ p. 70) distinctly rejected
these theories, and published observations which if followed up would
have led to the right path, yet the botanists above-named and others
with them, long continued to adhere to the earlier views. Kieser,
for example (‘Mémoire sur l’organisation des plantes,’ 1812) further
developed Treviranus’ theory, that the fine granules in the latex of
plants are cell-germs which are afterwards hatched in the intercellular
spaces. Schultz-Schultzenstein in his work ‘Die Natur der lebenden
Pflanze,’ 1823-28, i, p. 607 rejected this view and adopted that
of Wolff and Mirbel. Scarcely better than the notion of cell-germs
represented by Sprengel, Treviranus, and Kieser was the theory
propounded by Karsten soon after 1840; that of the French botanists
Raspail and Turpin[85] (1820-1830), though conveyed in a different
terminology, corresponded in its main points with the views of Sprengel.
It had been the good fortune of Mirbel at the beginning of the century,
and again thirty years later, to promote the advance of phytotomy by
means of important observations, though he may have interpreted some of
them incorrectly; the same thing happened again thirty years later, and
it was a German enquirer, von Mohl, who corrected his observations and
views on both occasions.
In his famous treatise on Marchantia polymorpha, which appeared in 1835
in the Memoirs of the French Institute, the first part having been
laid before the Paris Academy in 1831-32, Mirbel distinguished three
modes of cell-formation; in the germination of the spores of Marchantia
new cells are formed from the germ-tube and new cells again from these
by a similar process, much in the same way therefore as that which
actually occurs in the germination of Yeast-fungi; he found a second
kind of cell-formation in the production of the gemmae of Marchantia,
where he distinctly observed the successive appearance of the dividing
walls, but formed an erroneous idea of the proceeding on the whole; in
the further development of the gemmae and in other cases of growth he
considered that new cells are formed between those that are already
present in the manner supposed in his earlier theory.
Von Mohl’s dissertation on the multiplication of vegetable cells
by division, published in 1835 and reprinted in ‘Flora’ of 1837,
shows how strange these processes even then appeared; in this work
he expresses some doubts about Mirbel’s statements, but he accepts
them on the whole, and only makes incidental mention of his own more
numerous and better observations on the development of spores (‘Flora,’
1833), though he had there seen several cases of cell-division and
free cell-formation with tolerable distinctness. Adolph Brongniart
(‘Annales des sciences naturelles,’ 1827) also had observed, though
imperfectly, the formation of pollen-grains in their mother-cells in
Cobaea scandens, and Mirbel, in the appendix to the work mentioned
above, had given a correct description and good figures of the
formation of pollen-cells; and yet von Mohl neglected to compare
these important observations of cases of cell-division with his own;
even in 1845, when he published the latter in a revised form in his
‘Vermischte Schriften,’ he overlooked the close relation between the
formation of those pollen-grains and spores, and the cell-division in
Cladophora. Still this treatise of von Mohl’s is of great importance
in the history of the theory of cell-formation, because it described a
case of cell-division for the first time step by step and brought all
the salient points into relief. Dumortier had observed the division
of cells as early as 1832[86], and Morren had seen it in Closterium in
1836, but had not given the needful details. Finally, von Mohl applied
the experience which he had gained from Cladophora to other filamentous
Algae, and pointed out the similarity between these processes and
the division of Diatoms, which he consequently claimed as plants in
opposition to Ehrenberg, who considered them to be animals (‘Flora,’
1836, p. 492).
Meyen next, relying on von Mohl’s observations on Cladophora, declared
in the second volume of his ‘Neues System’ that cell-division was a
very common occurrence in Algae, Filamentous Fungi and the Characeae,
but he neglected any closer investigation of the processes by which
the division is introduced and completed. His comparison of these
cases of cell-formation with the formation of spores, pollen-grains,
and endosperm-cells is moreover noticeable as the first attempt
to distinguish what is now known as free cell-formation from
cell-division; it was obviously the want of this distinction which long
prevented clearer views on the whole of this field of observation.
The due separation of these two modes of cell-formation was a short
step after the observations that had been already made; and if that
step had been taken, Schleiden’s theory would have been impossible,
and the development of the cell-theory would not have been prejudiced
by the mistake, introduced by Schleiden after 1838, of applying the
mode of free cell-formation, which he believed he had observed in the
embryo-sac of Phanerogams, to the multiplication of vegetative cells in
growing organs, and regarding it as the only mode of cell-formation.
This would have been the more impossible, since von Mohl in the same
year gave an excellent description of the development of stomata by
division of a young epidermis-cell and the later separation of the
dividing wall into two laminae. But von Mohl in the years immediately
following was over-cautious in refraining from all speculative
consideration of cases that lay clearly before him, and his views
were still undecided in 1845, when Unger and Nägeli had already made
good observations on the formation of tissue-cells in growing organs
(‘Vermischte Schriften,’ 1845, p. 336).
Schleiden’s theory of cell-formation arose out of a curious mixing
together of obscure observations and preconceived opinions, and reminds
us indeed strongly of the old notions of Sprengel and Treviranus; it is
true that he distinctly rejected their views, but he too made new cells
arise from very minute granules, and his theory like theirs did not
rest on any thorough course of observation.
Robert Brown, (see his Miscellaneous Writings, edited by T. T. Bennett,
I.) had discovered the nucleus in the cells of the epidermis of
Orchidaceous plants in 1831, and had shown that it was very generally
present in the tissue-cells of Phanerogams, but had obtained no results
from his discovery. The cell-nucleus lay undisturbed, till Schleiden
suddenly made it the soul of his theory and the starting-point of all
cell-formation. He considered that the nucleus was formed from the
mucilaginous content of the cell, which he assumed on insufficient
grounds to be of the nature of gum; this he called the cytoblastem, and
the nucleus itself the cytoblast. As he states that his cytoblastem
becomes yellow and granular in solutions of iodine, we may recognise in
it our own protoplasm.
We make acquaintance with Schleiden’s theory of cell-formation in its
original form, if we turn to his treatise, ‘Beiträge zur Phytogenesis,’
(in the Archiv für Anatomie, Physiologie, etc. von Johannes Müller,
1838). The work begins with some remarks on the general and fundamental
laws of human reason, etc., discusses the literature of cell-formation
in a few lines without mentioning von Mohl’s numerous observations,
goes on to mention the general occurrence of the nucleus which here
receives its new name, then occupies itself with gum, sugar, and
starch, and at last comes to the main subject. There are two spots,
says Schleiden, in the plant, where the formation of new organisation
may be most easily and most certainly observed, the embryo-sac and
the end of the pollen-tube, in the latter of which, according to his
theory of fertilisation, the first cells of the embryo are supposed to
be formed, but where in fact no cells are formed. At both spots small
granules soon arise in the gum-mucilage, which, before homogeneous,
now becomes turbid, and then single larger and more sharply defined
granules, the nucleoli, appear. Soon after, the cytoblasts are seen as
granular coagulations from the granular mass; they grow considerably in
this free condition, but as soon as they have reached their full size,
a delicate transparent vesicle is formed upon them; this is the young
cell, which at first presents the appearance of a very flat segment of
a sphere, whose plane side is formed by the cytoblast, the convex by
the young cell (the cell-membrane), which rests upon the cytoblast as
a watch-glass on a watch. Gradually the vesicle becomes larger and of
firmer consistence, and now the whole of the wall, except where the
cytoblast forms part of it, consists of a jelly. By-and-bye the cell
grows beyond the edge of the cytoblast and rapidly becomes so large
that the latter appears only as a small body inclosed in one of the
side walls. The shape of the cell becomes more regular with advancing
growth and under the pressure of adjoining cells, and often passes
into that of a rhombododecahedron, which Kieser for reasons drawn from
the nature-philosophy assumed to be the fundamental form. It is only
after the resorption of the cytoblast that the formation of secondary
deposits on the inner surface of the cell-wall commences, though some
exceptional cases are adduced. Schleiden thinks (p. 148) that he may
assume that the process here described is the general law of formation
of vegetative cell-tissue in Phanerogams. He adds particularly that the
cytoblast can never lie free inside the cell, but is always enclosed in
a duplication of the cell-wall, and he thinks that it is an absolute
law that every cell, except perhaps in cambium, begins as a minute
vesicle, and grows to the size which it reaches in its matured state.
The resemblance of this view to that of Sprengel and Treviranus is
increased by what we find further on, where we read that from the
cell-germs in the spores of Marchantia usually only from two to four
serve to form cells, the rest becoming overlaid with chlorophyll,
and being consequently withdrawn from the vital process. He who is
acquainted with the modern view of the processes of free cell-formation
founded on the numerous and careful investigations of later times will
scarcely discover in the above account of Schleiden’s theory a single
correct observation.
Soon after, von Mohl published in ‘Linnaea,’ 1839, p. 272, his
observations on the division of the mother-cells of the spores of
Anthoceros; these were carefully made and were correct in all the main
points; and in opposition to Mirbel’s former statements they establish
the fact, that the division is effected by the mucilaginous contents
of the cell, and consequently that it is not a passive division of
the contents of the mother-cell produced by the growth inwards of
projections of the cell-wall.
Unger[87] was the first to declare distinctly against Schleiden’s
doctrine, and his observations on the punctum vegetationis appeared
in the ‘Linnaea’ of 1841, p. 389; from the size and position of the
cells he concluded that the tissue-cells in this case are formed by
division, and not in the manner alleged by Schleiden. Soon after Nägeli
also (‘Linnaea,’ 1842, p. 252) observed the processes of cell-formation
in the extremities of roots, but he did not conceive them to be cases
of division; he saw two nuclei form in each mother-cell, and a new
cell form round each nucleus, and explained the origin of the dividing
wall as due to the meeting together of the two new cells; he thought
that a similar process takes place in stomata and in the mother-cells
of pollen; this conception was not absolutely incompatible with
Schleiden’s theory, but there was this difference, that in Nägeli’s
case essential processes were correctly observed, but were to some
extent incorrectly interpreted. In the same year appeared the first
edition of Schleiden’s ‘Grundzüge der wissenschaftlichen Botanik,’
in which his theory of cell-formation was repeated in a more precise
form. That he was thoroughly in earnest to maintain it is shown by the
fact that he gave still another exposition of it in his ‘Beiträge zur
Botanik’ in 1844, where he insists that his method of cell-formation
is the general one, though it has been distinctly ascertained in the
Phanerogams only. But how completely an observer may be led captive
by a preconceived opinion may be learnt from Schleiden’s suggestion,
that the formation of zygospores in Spirogyra is in accordance with his
views, though it is impossible to conceive of a case of cell-formation
more easy to observe, or less reconcilable with Schleiden’s theory.
It was mentioned in the first book, that Hedwig and Vaucher were
acquainted with the remarkable process of the formation of zygospores
in the alga-genus Spirogyra; but this as late as Schleiden’s time
was not regarded as an example of cell-formation, and his view was
really a step in advance, since it brought a process, so highly
peculiar according to existing ideas, under the general conception of
cell-formation.
The systematic elaboration of the theory of cells, founded on careful
observation and mature reflection, began with the year 1844. Almost
at the same time in this year appeared Nägeli’s detailed enquiries
into the occurrence of the cell-nucleus and into cell-division, von
Mohl’s observations on the primordial utricle and its behaviour in
the process of cell-division in young tissue, and lastly those of
Unger on merismatic cell-formation (cell-division) as a general
mode of proceeding in the growth of organs. As these observers were
chiefly concerned to test the correctness and general applicability
of Schleiden’s theory, they necessarily paid special attention to
the general occurrence of the cell-nucleus and to its position on
the side of the cell-wall, for these were the points most accessible
to observation and criticism. The discussion of these observations
disclosed a defect in the current phraseology, in which the word
cell was commonly understood to mean only the cell-membrane, but
sometimes included everything belonging to and contained in the cell;
hitherto moreover the protoplasm of the cell had not been sufficiently
distinguished from the rest of the cell-contents.
Nägeli and von Mohl arrived simultaneously at a clearer understanding
of these points; von Mohl recognised the primordial utricle (1844) as a
component part of the cell-contents and not belonging to the cell-wall,
and explained the part which it plays in cell-division; in 1846 he
arrived at a clear conception of the protoplasm as a peculiar substance
distinct from the other contents of the cell and gave it the name it
still bears. Meanwhile Nägeli had also distinguished the protoplasm
from everything else in the cell, and noticed its pre-eminent
importance in cell-formation and its nitrogenous character.
We must not omit to mention here, that investigations into the
processes of cell-formation compelled observers to search for the
spots where cell-formation actually takes place, and thus the fact was
ascertained, that cells in statu nascendi are not to be found in all
parts, not even in all growing parts of the plant, but that we must
look for them in the so-called puncta vegetationis in the stem and
root, in the youngest lateral organs, and between the bark and the wood
in woody plants. About this time a new idea began to be attached to
the word cambium, which Mirbel had used in the sense of a nourishing
juice saturating the plant; it was now applied to the tissue-masses in
which the formation of new cells takes place, and specially to the very
thin layer of tissue lying between the wood and the rind, from which
new layers of wood and rind in woody plants are formed—a layer, which
according to Mirbel’s theory had been a mass of sappy matter, in which
new cells arise as vacuoles.
Unger in an enquiry into the growth of internodes (‘Botanische
Zeitung,’ 1844) again declared himself as an opponent of Schleiden’s
theory. He maintained first of all and erroneously that the
cell-nucleus is not of general occurrence in tissue where division is
taking place, but he argued rightly from the position of the cells,
from the difference of thickness in their walls, and from their
relative size, in favour of their multiplication by the formation of
dividing walls; he noticed the part played by the cell-contents in
the multiplication of cells in hairs, and asserted that merismatic
cell-formation (cell-division) is the general rule in the growth
of organs of vegetation, while he distinctly declared that it was
not possible to bring all that is actually seen at the spots where
formation of cellular tissue is taking place into agreement with
Schleiden’s theory. But Unger did not observe the processes that take
place in cell-division step by step; his observations sufficed to make
Schleiden’s theory very improbable without offering enough foundation
for a new one, and Schleiden did not fail to reply to Unger’s
objections in the second edition of his ‘Grundzüge’ in 1845.
Earlier in the same year, von Mohl published in the ‘Botanische
Zeitung’ the treatise on the primordial utricle which has been already
mentioned; by the term primordial utricle he meant partly the very thin
layer of protoplasm, which in large cells full of sap lines the inside
of the cell-wall, and partly an outer layer of the protoplasm in young
cells, which are still rich in that substance. It is true that the
distinguishing the primordial utricle was not a very important matter;
but von Mohl applied it with his usual thoroughness to obtaining a
better insight into cell-formation by calling attention (p. 289) to
the circumstance, that the cells of the cambium-layer between the rind
and the wood fit into one another and leave no intercellular spaces;
from this he concluded that there are only two possible modifications
of cell-multiplication, either division of cells by formation of a
dividing wall or formation of cells within cells; in each of these
young cells is a primordial utricle, the origin of which must at
least be contemporary with that of the cell (cell-membrane). ‘Could
it then be distinctly shown, that two primordial utricles exist side
by side in cells, which are in the act of multiplying, before a
partition-wall is formed between them, it would be evident that in the
cambium layer and at the points of the stem and root the formation of
the primordial utricle precedes that of the cell.’ Von Mohl believed
that he had seen this process, but was not perfectly satisfied as to
the correctness of his observation; but he continues: ‘Since every
young cell contains a primordial utricle, this must either be absorbed
before a multiplication of the cell commences in order to make way
for two new ones formed in its stead, or the old primordial utricle
must separate into two.’ He considered the first supposition to be the
probable one, rejecting Unger’s statement that the nuclei are formed
after the division. It is surprising that after these considerations
von Mohl thought that his own observations necessarily confirmed
Schleiden’s theory of cell-formation, although he noticed beside that
the nucleus never forms a part of the cell-wall, an essential feature
in that theory; but in fact von Mohl took the membrane which according
to Schleiden separates from the nucleus for the primordial utricle.
But these mistakes are soon followed by the right conjecture, that
the substance of the primordial utricle may be identical with the
mucilaginous mass, which commonly encloses the nucleus, and so with
that which von Mohl two years later named protoplasm. In this later
treatise (‘Botanische Zeitung,’ 1846), in which he proves that the
well-known movements in the interior of cells are made not by the
watery cell-sap but by the protoplasm, he states (p. 75) that it is
the protoplasm which produces the nucleus, that the organisation of
the nucleus ushers in the formation of the new cell, and that contrary
to Schleiden’s theory the protoplasm completely envelopes the nucleus,
which always occupies the centre of very young cells, as is the case
especially in the endosperm-cells observed by Schleiden. He then shows
how the protoplasm of young cells at first solid afterwards forms
sap-cavities and stretches between them in walls, bands or threads, the
substance of which exhibits the streaming movement. Von Mohl strangely
neglected on this occasion to compare carefully his former observations
on the origin of spores and the division of Alga-cells with his new
results, and to seek for the essential resemblances between them; on
the contrary he said emphatically that the cell-division in Cladophora
is probably a quite different process from the multiplication of
tissue-cells in higher plants.
The discoveries of Unger and von Mohl up to the year 1846 were quite
sufficient to refute Schleiden’s theory, but not to give a clear
and general view of the processes in the formation of cells; the
different kinds of cell-formation were neither carefully distinguished
from one another, nor could they be referred to a common principle.
Both observers had endeavoured to conjecture the course of events
from certain data, supplying by inference what they had not directly
observed.
Nägeli about the same time took up a different position as an opponent
of Schleiden’s theory. In an exhaustive treatise on the cell-nucleus,
cell-formation, and cell-growth in plants, the first part of which
appeared in 1844 in the periodical founded by himself and Schleiden,
he collected together all that had hitherto been observed by himself
and others from various points of view. All sections of the vegetable
kingdom were once more systematically examined with reference to
the occurrence of the cell-nucleus and the different kinds of
cell-formation; all cases of the latter were carefully compared
together in their resemblances and differences, in order to deduce
from the observed phenomena that which was essential and universal.
The first result was, that Schleiden found himself obliged, in the
second edition of his ‘Grundzüge’ in 1845, to accept the cell-division
established by Nägeli in Algae and the mother-cells of pollen as a
second kind of cell-formation; thus began the movement in retreat
which was destined to end in the following year with the overthrow of
Schleiden’s theory. This was effected by the continuation of Nägeli’s
treatise in the third volume of the periodical for 1846. In the first
part of his work Nägeli had set out by assuming the correctness of
Schleiden’s assertions, though he was even then compelled to modify
them considerably. In the second part, however, in consequence of
further observations Schleiden’s theory was declared in plain terms
to be utterly incorrect, and was refuted point by point. But Nägeli
was not obliged to confine himself to this negative result; his
comprehensive investigations supplied material at the same time for
constructing a new theory of cell-formation, which not only took in all
the various cases, but declared the principle which lay at the root
of all. If we compare this second part of Nägeli’s treatise with von
Mohl’s publications from 1833 to 1846, we shall see that von Mohl had
observed with accuracy a number of important facts, but that Nägeli
added largely to them, and, which is the main point, elaborated them
into a comprehensive theory embracing all kinds of cell-formation.
How important the correct distinction of the protoplasm from the rest
of the cell-contents was for the perfecting of the theory of cells is
seen from Nägeli’s declaration, that he retracts his former view which
rested on the authority of Schleiden, because it sprang from a time
when he was ignorant of the significance of the mucilage-layer (the
protoplasm), though it is true that he indicates at the same time other
points and new considerations which definitively set aside Schleiden’s
theory. After investigating the different modes of free cell-formation
and finding the processes there quite different from Schleiden’s
account of them, he proceeded to search for free cell-formation where
Schleiden had affirmed that it invariably occurs, namely in growing
vegetative organs in the higher plants. But this investigation led
him to the conclusion that all vegetative cell-formation is true
cell-division, and that even the reproductive cell-formation in some
Algae and Fungi is effected by division; the reproductive cells of
most plants are the result of free cell-formation, but it should be
observed that the term free cell-formation is here used not exactly in
the modern sense, inasmuch as Nägeli included in it the formation of
four-fold grains (tetrads) in spores and pollen. If the distinction
between cell-division and free cell-formation had often been suggested
by former observers, Nägeli was the first who distinctly defined
it, though not exactly as it is now defined. ‘In cell-division the
contents of the mother-cell separate into two or more portions; a
perfect membrane forms round each of these portions, which at the
moment of its appearance rests partly on the wall of the mother-cell
and partly on the adjacent walls of the sister-cells. In free
cell-formation a smaller or larger part of the contents of a cell, or
even the whole of them becomes isolated. On its surface is formed a
perfect membrane, which is everywhere free on its outer face. There are
two processes in the formation of a cell; the first is the isolation
or individualising of a part of the contents of the mother-cell,
the second the formation of a membrane round the individualised
portion.’ He then proceeds to show that the cell-wall is formed by
the separation of non-nitrogenous molecules from the nitrogenous
mucilage (protoplasm). These sentences contain all that is general
and essential in vegetative cell-formation. Further on he notices the
peculiarities in the various processes in cell-formation; he says that
the individualising of the cell-contents assumes four forms; first,
single small portions of the contents separate themselves inside the
rest, as occurs in the formation of free germ-cells in Algae, Fungi,
and Lichens, and of endosperm-cells in Phanerogams; secondly, the whole
contents of one cell, or of two by conjugation of associated cells,
collect into a free spherical or ellipsoidal mass, as in the formation
of germ-cells in the Conjugatae; thirdly, the whole contents of a cell
separate into two or more portions, which is now called cell-division;
from this Nägeli distinguishes as his fourth form, the process known as
abscision (Abschnürung), which occurs in the formation of germ-cells in
many Algae and Fungi.
Schleiden had declared it to be a general law in plants, that cells are
only formed inside mother-cells. Meyen however, Endlicher, and Unger,
had recently assumed the formation of new cells between the older
ones; Nägeli maintained that all normal cell-formation, vegetative and
reproductive, takes place only within mother-cells.
In opposition to the long-cherished notion that there must be one
general and fundamental form of cell, Nägeli pointed to the fact that
cells have very different forms at the moment of their production.
Those which arise by free cell-formation are, he says, at first always
spherical or ellipsoidal; those produced by cell-division have a shape
necessarily conditioned by the form of the mother-cell and the manner
of division. He showed further that changes in the shape of cells with
advancing growth depend materially on whether they enlarge equally
in all parts of their circumference or not. These considerations,
obvious as they are, were now for the first time pointed out and fully
appreciated.
The reader who is already familiar with our subject will recognise
in the passages adduced from Nägeli without further explanation the
essential principles of the modern theory of cells, especially if he
compares them with the views propounded at the same time and previously
by Schleiden, Unger, and von Mohl. But, as might be expected, the
further investigations, which were pursued with zeal during the
succeeding twenty years and produced a considerable literature, did
much to enlarge and perfect Nägeli’s theory in many of its details
and to correct it in some minor points; the theory itself facilitated
this process by supplying a scheme to which the investigation of
special questions could readily be referred. Whether the nucleus is
a solid body or a vesicle, whether in the division of a mother-cell
into compartments the wall of partition always grows from without
inwards or is formed simultaneously over its whole surface, whether it
is originally composed of two laminae or of one which is afterwards
differentiated,—these and many other questions were decided in course
of time.
Schleiden’s theory was now definitively set aside, a deeper insight was
obtained into the nature of the cell, and the ideas connected with the
word became broader and more profound. The knowledge of the formation
of cells showed that the cell-walls, which had been hitherto regarded
as the important part, are only secondary products, that the true
living body of the cell is represented by its contents and especially
by the protoplasm. Alexander Braun, relying on numerous researches
into the lower Algae, expressed himself in 1850 (‘Verjüngung,’ p.
244) to the effect that it is an inconvenience that the word cell is
used at one time to designate the cell with its wall, at another time
the cell without its wall, or again the wall without the cell. Since
the contents are the essential part of the cell and form a separate
and individual whole which has its own membrane-like boundary, the
primordial utricle, before the secretion of the membrane of cellulose,
we must either confine the term cell to the enveloping membrane or
to the chamber which it forms and find another name for the body
of the contents, or else call this the true and proper cell. This,
which presents itself at once as the correct mode of conception to
anyone who observes the formation of swarmspores in Algae and Fungi
and many other cases of cell-formation, was from this time forward
a vital point in the doctrine of the cell. Braun contributed also
to the clearing up of the ideas of botanists on this subject by
bringing together under one systematic view and classifying all the
varieties of cell-formation which were known to him up to the year
1850, and especially by a more searching investigation into modes of
conjugation. Henfrey’s contributions (‘Flora’ of 1846 and 1847) rested
entirely on the observations of German botanists, and brought to light
nothing that was independently and essentially new. On the other hand
Hofmeister’s new observations on the development of pollen (1848), and
his many remarks on cell-formation in his epoch-making researches into
embryology in 1851, contributed repeatedly to the deciding of doubtful
points, especially in the behaviour of the nucleus in cell-formation
and the production of the dividing walls. Von Mohl, who in spite
of his own excellent observations maintained up to 1846 a somewhat
undecided attitude of mind in respect to Schleiden’s theory, which was
at that time still in vogue, published in 1851, in his treatise ‘Die
vegetabilische Zelle,’ an excellent summary of the results which had
been so far achieved. In describing cell-division he notices specially
that the new nuclei occupy the centres of the future daughter-cells
before the division of the contents commences; but he still clung to
his old view, that in every instance of cell-division the parting-wall
must form progressively from without inwards, as in Cladophora,
contrary to Nägeli’s and Hofmeister’s correct statements, that cases
also occur of simultaneous formation at every point of the surface of
the partition-wall. As usual, however, von Mohl rested his opposition
on a good observation, and showed that in the case of the formation of
pollen in dicotyledonous plants it is possible to burst the membrane
of a mother-cell in the act of dividing, and set free the protoplasm
when it is already deeply divided into the four parts, and so to see
the half-formed partition-walls; but this only proved that such was the
process in the cases observed, the formation of the partition-walls
being simultaneous in others. It may be mentioned in this place, that
the idea of special mother-cells in the formation of pollen introduced
by Nägeli in 1842 was in entire accordance with the condition of the
science at the time, since he meant by the term the laminae of membrane
formed during the successive divisions of the mother-cell. To call
these still special mother-cells, as some modern phytotomists do,
is quite unjustifiable, because since 1846, when Nägeli propounded
his theory, the word cell, as we have seen, no longer designated the
mere membrane but the whole body of the cell, while the expression
special mother-cell rests on the older phraseology, in which cell and
cell-membrane are identical.
The additions made to the doctrine of cell-formation during the greater
part of the twenty years after 1851 were unimportant in comparison with
the mighty development which it had experienced during the preceding
ten years. These years had indeed been marked by the greatest possible
activity and fruitfulness in results in all parts of botanical study.
By the labours of Unger, von Mohl, Nägeli, Braun, and Hofmeister,
not only were the foundations laid for a true theory of cells, but
the details were worked out, and the conceptions connected with them
finally cleared up. Textbooks could now disseminate the new teaching
through wider circles, and with these works may be classed von Mohl’s
treatise already mentioned on the vegetable cell, since it came much
into use in a later and special edition, and was made by many teachers
of botany the foundation and guide in their lectures. It was now become
the fashion to compose not general text-books of botany, but compendia
of anatomy and physiology, and thus morphology and systematic botany
were neglected, as anatomy and physiology had been in the period
immediately preceding Schleiden’s time. Whoever therefore wished to
consult a complete manual of general botany was for some time obliged
to be content with Schleiden’s ‘Grundzüge’; and this had a great
deal to do with keeping alive his erroneous doctrines on cells and
fertilisation among general readers, while the professed botanists
had long given in their adherence to more modern and more correct
views. It is a misfortune in our science to be singularly poor in good
text-books, which might have given a general account from time to time
of the existing condition of research; this is one of the reasons why
for some time past even official representatives of botanical science
often differ so much from one another in their fundamental views on
method, and on the question of how much has been actually established
and how much still remains doubtful in the main divisions of the
subject, that a mutual understanding seems often impossible. That a
better state of things in this respect prevails in zoology, physics,
and chemistry, is certainly not a little due to the many good compendia
and text-books, which endeavour to give some account of the progress of
those sciences from year to year.
However, during the period from 1850 to 1870 Schacht and Unger
attempted to make the results of modern phytotomic investigation
accessible to general readers by means of text-books. Such was the
nature of Schacht’s[88] work, ‘Die Pflanzenzelle,’ published in
1852, a book which claimed to expound all parts of phytotomy by the
aid of the author’s own observations, with occasional reference only
to the writings of others; the attempt was so far impossible, as the
essential points had already been fully cleared up by the labours of
other botanists. The work had however the advantage of attracting the
attention of the reader by numerous good original drawings, and the
style was enlivened by the constant appeal to original observation;
at the same time, through insufficient use of the available
literature, the author’s views not unfrequently fell short of the
existing standard of knowledge. Worse than this however was a certain
defect of education, which led the writer into self-contradiction
and to incorrect classification of his facts; things fundamentally
important were sometimes neglected for unimportant details, and a
certain unreflecting empiricism was apparent in the whole work, in
marked contrast with the logical exactness of von Mohl, Nägeli, and
Hofmeister. In the second edition of the work, published in 1856 under
the title, ‘Lehrbuch der Anatomie und Physiologie der Gewächse,’ we
find many improvements in the details, but still on the whole the same
formal defects. It is not unimportant in a historical point of view to
notice this character of Schacht’s writings, because during this period
most young botanists and other persons also derived their knowledge
of phytotomy and of the nature of cells chiefly from him; his books
did not truly represent the condition of the science; their defective
reasoning had an injurious effect on the minds of younger readers,
and they introduced into phytotomy and vegetable physiology a habit
of accumulating a mass of undigested facts, such as has for some time
marked the condition of morphology and systematic botany.
Unger’s text-book ‘Anatomie und Physiologie der Pflanzen’ (1855) was
superior in conception and execution. It introduced the beginner to
the doctrine of cells with careful attention to all that was known
on the subject, if sometimes with some hastiness of decision, while
it brought the really important points everywhere into prominence
and employed individual facts to explain the general propositions,
as should always be done in a work intended for learners. But in
addition to this Unger’s book contained much that was really new and
valuable, and among other things some very important remarks on the
physiological characteristics of protoplasm; and it pointed out for
the first time the similarity between vegetable protoplasm and the
sarcode in Rhizopods, which Max Schulze had before carefully described.
In this year Nägeli also published investigations into the primordial
utricle and the formation of swarmspores in his ‘Pflanzenphysiologische
Untersuchungen,’ Heft I, which gave a new insight into the physical
and physiological characteristics of protoplasm. It has been
mentioned above that De Bary’s investigations into the Myxomycetes
in 1859 had thrown new light on the subject of protoplasm, and had
called attention to vital phenomena connected with it, which, though
analogous to what had been before observed, were rendered very striking
from the circumstance that in this case the protoplasm was not in
microscopically small portions enclosed by firm cell-walls, but moved
about and showed changes of shape in large, sometimes in very large,
masses, that were entirely free and unconfined. Here was the best
opportunity for making a nearer acquaintance with protoplasm and for
learning to recognise it as the immediate support of all vegetable
and animal life; in succeeding years the zootomists and physiologists
Max Schulze, Brücke, Kühne, and others established the fact that the
substance which lies at the foundation of cell-formation in animals
agrees in its most important characteristics with the protoplasm of
vegetable cells. A more detailed account of modern researches on this
subject, which would moreover involve the examination of Hofmeister’s
work of 1867, ‘Die Lehre von der Pflanzenzelle,’ does not fall within
the limits of our history.
2. FURTHER DEVELOPMENT OF OPINION ON THE NATURE OF THE SOLID FRAMEWORK
OF CELL-MEMBRANE IN PLANTS AFTER 1845.
Between 1840 and 1850 the most eminent representatives of phytotomy
were chiefly engaged, as we have seen, in observing the formation of
vegetable cells, and in framing the true theory of the subject by
process of induction. It was not to be expected that, while these
labours were bringing year by year new things to light and keeping
opinion on the formation of cells in a constant state of fluctuation,
their results would lead to very important changes in the theory of
the solid framework of cell-membrane founded by von Mohl. On the
contrary, it was at this time that his views such as we have seen them
on the connection of cells one with another, on the configuration of
their partition-walls and on their growth in thickness, attained their
greatest influence. His theory seemed to stand firm and complete when
contrasted with the unsettled state of opinion respecting the origin
of cells, and the question, how far it could be made to agree with the
new observations on the history of cell-formation, was hardly raised.
In the midst of the strife of opinion on the latter subject appeared
von Mohl’s ‘Vermischte Schriften’ in 1845, in which his views on the
structure of mature vegetable tissue were produced in a series of
monographs as the apparently irrefragable result of his observations.
And in fact phytotomic research up to 1860 followed the train of
thought initiated by von Mohl, till at last the inadequacy of his views
was rendered apparent between 1858 and 1863 by Nägeli’s new theory of
growth by intussusception, and by the profounder insight obtained into
the nature of cell-formation.
A sufficient proof of the correctness of these remarks is to be
found in the further development of the views of botanists on the
intercellular substance and the cuticle, which might have adapted
themselves before 1850 to the new theory of cells, but instead of
doing so were moulded by the ideas current before 1845. It has been
shown in the preceding chapter how von Mohl gradually restricted the
theory of intercellular substance which he had proposed in 1836, and
had come in 1850 to regard this substance as only a cement which might
in many cases be perceived between the cell-walls. It should be added
here, that Schleiden in connection with his theory of cells considered
both the intercellular substance and the cuticle to be supplementary
secretions from the cells, and made the former fill the intercellular
spaces, just as laticiferous and resiniferous passages are filled
with secretions from the adjacent cells (1845). Unger too in 1855
(‘Anatomie und Physiologie der Pflanzen’) thought the existence of a
cement between the cells necessary to prevent their falling asunder.
Schacht, who in his ‘Pflanzenzelle’ of 1852 had followed Schleiden in
explaining the intercellular substance and the cuticle as secretions or
excreta from the cells of the plant, still kept on the whole to this
view in 1858, though he modified it in some important points. This
theory of Schleiden and Schacht was first opposed by Wigand in a series
of essays (1850-1861), in which in strict adherence to von Mohl’s
theory of apposition he sought to prove, that the layers which are
visible in wood-cells as intermediate laminae in the partition-walls,
and which till then had been regarded as a cement between contiguous
cells, an intercellular substance, were nothing else than the thin
primary membranous laminae formed in the process of cell-division,
and subjected to subsequent chemical change, while the secondary
layers of thickening in von Mohl’s sense lie on both sides of them.
The cuticle on the epidermis was explained in a corresponding manner.
Though Sanio in 1863 raised a variety of objections to Wigand’s view,
he still adhered to it in principle, and found a strong confirmation
of it in the fact, that he succeeded in producing the well-known
cellulose-reaction in the intercellular substance of wood-cells when
freed from foreign admixtures.
The researches of Wigand and Sanio were sufficient to overthrow von
Mohl’s account of the intercellular substance and the cuticle, but
they had not proved that the intermediate laminae are in fact the
primary partition-walls on which von Mold’s secondary thickening-layers
had been deposited, on both sides in the case of the intercellular
substance, on one side in that of the cuticle. The structure of the
partition-walls and the existence of the cuticle could be explained
in a totally different way from the point of view now opened by
Nägeli’s theory of intussusception; there was no need now to see
either a secretion or a primary cell-wall in the intermediate lamina
of thickened cells or in the cuticle, for it was possible that
this lamination might be due to subsequent chemical and physical
differentiation of membranes thickened by intussusception. As
phytotomists are not yet quite agreed as to the correctness of this
view, we must be content with observing here that in the matter of the
cuticle and the intercellular substance lies one of the points, the
determination of which will involve the question of von Mohl’s earlier
theory of apposition. It is not the purpose of this history to give the
more modern views that have asserted themselves since 1860, especially
where the question is still in debate.
It was a part of von Mohl’s idea of the cell-tissue and one to which
he had firmly adhered since 1828, that except in the cross walls of
genuine wood-vessels and some very isolated cases the partition-walls
in cellular tissue are never perforated; that both simple and bordered
pits always remain closed by the very thin primary lamina of cellulose.
But between 1850 and 1860 several cases were discovered which were
at once exceptions to von Mohl’s rule, and of great importance to
physiology. Theodor Hartig, in his ‘Naturgeschichte der forstlichen
Kulturpflanzen Deutschlands’ (1851), described peculiar rows of
cells in the bast-system, in which the transverse and sometimes the
longitudinal walls appear to be pierced like a sieve by numerous
minute holes, and to these cells he gave the name of sieve-tubes.
Von Mohl (1855), while in other points confirming and extending
Hartig’s discovery, declared against the perforation of the walls,
believing that the appearances were due to lattice-like thickenings
of the cell-walls; he proposed therefore to call Hartig’s sieve-tubes
latticed cells. Then Nägeli showed in 1861 that in some cases at least
there can be no doubt that the walls are actually perforated, and
that the sieve-plates serve for the passage of mucilaginous matter
in bast-tissue, and the author of this history, it may be remarked
in passing in 1863, and Hanstein in 1864, suggested means by which
it may be ascertained with certainty that Hartig’s sieve-plates
are perforated. Meanwhile a number of laticiferous organs had been
recognised as forms of vessels in von Mohl’s sense, and it was found
that such canals are produced by dissolution of the septa of adjacent
cells. But the knowledge of the laticiferous organs continued till
towards 1865 to be very unsettled and defective, and the examination
of resin-passages, and the discovery that they are formed by simple
parting of cells from one another, belong to modern phytotomy;
Hanstein, Dippel, N. J. C. Müller, Frank, and others have since
1860 enlarged our knowledge of these forms of tissue. Schacht in
1860 established one of the most important exceptions to von Mohl’s
view above-mentioned, by demonstrating the formation and true form
of bordered pits in the wood of Conifers and in dotted vessels in
Angiosperms from the history of their development, and by showing
moreover that in all cases where bordered pits are formed on both sides
of a partition-wall and the adjacent cells afterwards convey air, there
the original very thin partition-wall in the bordered pit disappears,
and that consequently in such cases the bordered pits represent so many
open holes, through which adjacent cells and vessels communicate. At
the same time another hitherto inexplicable phenomenon received its
explanation. Malpighi, and after him the phytotomists at the beginning
of the present century had remarked, that the large vessels in the wood
are not unfrequently filled with parenchymatous cell-tissue, for the
origin of which no one could account. The phenomenon, however, could
now be explained quite simply after Schacht’s discovery; the formation
of thylosis in vessels only takes place when these border on closed
parenchyma-cells in the wood; when this is the case, the very thin
membrane which separates the bordered pits from the contiguous cells is
not absorbed, but it bulges inwards into the cavity of the vessel under
the pressure of the sap of the neighbouring parenchyma-cell, there
swells up like a bladder, and may by the formation of partition-walls
give rise to parenchymatous tissue; this, if proceeding from a number
of pits, fills up the cavity of the vessel.
3. HISTORY OF DEVELOPMENT AND CLASSIFICATION OF TISSUES.
It has been already stated, that the first step to a real understanding
of the structure as a whole of the higher plants was made by
Moldenhawer, who beginning with the study of the Monocotyledons,
first formed an idea of the vascular bundles as a distinct whole, a
system composed of various forms of tissue, and applied this idea
to explain the construction of the stems of Dicotyledons, upsetting
thereby Malpighi’s earlier theory of the growth in thickness of stems.
It was also observed, that von Mohl, advancing further in the same
direction, gave a more exact description of the epidermis and of the
tissues connected with it, and classified them, that is, introduced
a terminology founded on real investigation, but did not succeed in
bringing the subject to an entirely satisfactory conclusion; this could
in fact be reached only by the study of the history of development,
the only decisive method of investigation, whether the object be
to determine the true nature of cells and their subordinate forms,
or the solid fabric of vegetable structure, or as in the present
case to distinguish and classify forms of tissue; it is this method
which supplies the morphological points of view necessary for the
understanding of the inner structure of the plant by investigating
tissues in those states of development, in which they are not yet
adapted to subsequent physiological functions. The combination of
morphological and physiological points of view in the determination of
facts has maintained itself longer in this part of botanical study than
in any other; but here too ideas and opinions were gradually sifted and
cleared up under the influence of the modern study of the history of
development, though it was not till after 1850 that the determination
of the chief points in the theory of cell-formation left the leading
phytotomists at liberty to devote themselves to histological questions.
How little advance had been made towards the true understanding of
the varieties of forms of tissue in the higher plants before 1850 is
shown, for instance, by Schleiden’s account of tissues on page 232 of
his ‘Grundzüge’ of 1845, where parenchyma, intercellular substance,
vessels, vascular bundles, bast-tissue, bast-cells in Apocyneae and
Asclepiadeae, laticiferous vessels, felted tissue, epidermal tissue,
are discussed in this succession in co-ordinated sections of the text.
It is obvious that no well-ordered view of the whole cellular structure
of a plant of the higher order could be obtained in this way. Further
on in the same work, where Schleiden attempts a classification of
vascular bundles, which he distinguishes into closed and open, and
assigns the latter to Dicotyledons, we find the cambium-layer named as
the outer boundary of these open vascular bundles; the bast which lies
outside the cambium was therefore not considered to be a part of the
open vascular bundles, and this necessarily excluded any profitable
comparison of the circumstances in Monocotyledons and Dicotyledons.
The case is still worse in many respects in Schacht’s work already
mentioned, ‘Die Pflanzenzelle’ of 1852, where under the heading ‘Kinds
of vegetable cells’ the histology is discussed in the following
co-ordinated sections; the swarm-filaments of Cryptogams, the spores of
the same, pollen-grains, cells and tissue of Fungi and Lichens, cells
and tissue of Algae, parenchyma and its cells, vessels of the plant,
wood and its cells, bast-cells, stomata, appendicular organs of the
epidermis, cork; then follows a paragraph on the thickening-ring, and
then to the no small astonishment of the reader comes an account of the
vascular bundles, after the vessels, the wood, and the bast-cells have
been already dismissed. That such a mode of presenting the subject is
due to the little insight possessed by the writer into the structure of
the plant as a whole is apparent from simply reading the book, and a
similar confusion of ideas is found in his text-book of 1856.
We find a much better classification of tissues in 1855 in Unger’s
‘Anatomie und Physiologie der Pflanzen’; an account of cells is
followed by a description of cell-complexes, as one of the chief
divisions of the book, and herein of cell-families, cell-tissues, and
cell-fusions. Another chief section is occupied with cell-groups,
and here epidermal formations, air-spaces, sap-receptacles, glands
and vascular bundles are noticed; here certainly the fact has been
overlooked that vascular bundles may be co-ordinated with epidermal
formations, but not air-spaces, sap-receptacles and glands. His last
chief division gives an account of tissue-systems and of the way in
which the vascular bundles are united together in different plants, and
secondary growth in thickness and the activity of the cambium-layer are
described quite in the right connection.
In this branch of the science, as in every case where it is a question
of establishing fundamental conceptions, of surveying facts from
extensive points of view, and of seeking the requisite principles
by means of the history of development, we find that it is Nägeli
who opens the way and lays the foundation. In his ‘Beiträge zur
wissenschaftlichen Botanik’ of 1858, he proposed a classification
of tissues from purely morphological points of view. His first
division was into generating and permanent tissue; in each section
he distinguished two forms, prosenchymatous and parenchymatous
tissue. Parenchymatous generating tissue, the original component of
every young organ, he named primary meristem as distinguished from
prosenchymatous generating tissue, which is differentiated in the
form of strands and layers, and received from him the general name of
cambium; this was certainly not a happy distinction, because Nägeli’s
cambium by no means consists entirely of prosenchymatous tissue. By
the term secondary meristem Nägeli designated the tissue-strands and
tissue-layers which are formed between the permanent tissue of older
parts. The cambium he regards as the first product of the primary
meristem. The second chief form, permanent tissue, he divides into
two classes, not according to the form of the cells or physiological
relations, but according to its origin; all permanent tissue, which
is derived immediately from primary meristem, is protenchyma, all
that comes directly or indirectly from cambium is epenchyma. And
since the tissue-strands, till then known as vascular bundles, do not
contain vessels only but always fibrous elements also, as Bernhardi
had shown in 1805, Nägeli thought that they should therefore be
called fibrovascular strands. If it cannot be denied that the obvious
distinction between epidermal and other tissue did not find suitable
expression in this classification, and though other points of view
may at the present day be proposed for the genetic arrangement of
tissues, yet Nägeli’s classification and terminology have the merit of
having for the first time exhibited the general histology of plants
on comprehensive and genetic principles. It contributed materially to
impart a better understanding of the collective structure of plants.
The vascular bundles or fibrovascular strands especially demanded
further investigation of the genetic and morphological kind; for a
correct insight into the origin and subsequent transformation of this
tissue-system is as important for phytotomy as a similar knowledge with
respect to the bony system in vertebrate animals is for zootomy. But
a knowledge of the vascular bundles and their course in the stem has
a special importance in phytotomy, because it is the only way to the
understanding of secondary growth in thickness in true woody plants.
It was noticed above, that von Mohl had proved in 1831 the separate
character of the bundles which begin in the stem and bend outwards into
the leaves where they end, so that the entire system of bundles in a
plant consists of single bundles isolated when formed and subsequently
brought into connection with one another. Nägeli had already examined
the corresponding circumstances in the vascular Cryptogams in 1846,
when Schacht took the retrograde step of making the vascular system in
the plant originate in repeated branching, instead of in subsequent
blending of isolated strands; Mohl declared unhesitatingly against this
mistake in 1858, but it was refuted at greater length and still more
clearly by Johannes Hanstein in 1857, and by Nägeli in 1858. Hanstein
in a treatise on the structure of the ring of wood in Dicotyledons
confirmed Nägeli’s previous statements, and proved in the case of
Dicotyledons and Conifers that the first woody circle in the stem is
formed from a number of vascular bundles, which are identical with
those of the leaves and originate in the primary meristem of the bud.
These primordial bundles pass downwards through a certain number of
internodes in the stem independent and separate, and either retain
their isolation to the point where they end below or unite with
adjacent bundles which originated lower down. Hanstein happily termed
the portions of the vascular bundles, which enter the stem from the
base of the leaf and traverse a certain portion of it in a downward
direction, leaf-traces, so that it may be stated briefly, that the
primary wood-cylinder in Dicotyledons and Conifers consists of the sum
of the leaf-traces. Nägeli’s observations were of a more comprehensive
character, and supplied, as we have seen, a terminology for tissues.
He distinguished three kinds of vascular bundles according to their
course; the common bundles, which represent Hanstein’s leaf-traces in
the stem, and whose upper ends bend outwards into the leaves; the
cauline bundles, which extend above to the punctum vegetationis of the
stem without bending outwards into leaves; and leaf-bundles, which
belong to the leaves only. He laid it down as a general rule as regards
the common bundles in Dicotyledons and Conifers that they begin to form
where their ascending and descending halves meet, at the spot therefore
where they bend outwards into the leaf, and continue to form as they
descend into the stem and ascend into the leaf by differentiation of
suitable tissue. It follows from the nature of these common bundles,
that a more thorough understanding of their course and origin
presupposes a more accurate knowledge of the order of formation of the
leaves at the end of the stem and of the changes in the phyllotaxis
during growth; these relations Nägeli took into detailed consideration,
and even derived from them new points of view for the examination of
the genetic arrangement of leaves, pointing out at the same time the
unsatisfactory nature of the principles of the doctrine propounded
by Schimper and Braun. Nägeli was also the first who compared the
anatomical structure of roots with that of stems, and drew attention to
the peculiar character of the fibrovascular body in these organs. As
his previous discovery of the apical cell and its segmentation promoted
further research, so now his treatise on fibrovascular strands called
forth many others from various quarters; among them that of Carl Sanio
on the composition of the wood (‘Botanische Zeitung,’ 1863) must be
mentioned as one of the first and most important, and as serving in
conjunction with the works of Hanstein and Nägeli to throw light upon
the processes of growth in thickness of stems. It has been already
said that neither von Mohl nor Schleiden, neither Schacht nor Unger
succeeded in finding the true explanation of growth in thickness. It
was impossible that they should do so, for they were insufficiently
acquainted with the origin, true course, and composition of the
vascular bundles before growth in thickness commences; the study of
the subject was greatly perplexed by the confounding together in
thought and language of totally different things which came under
consideration, the so-called thickening-ring, in which the first
vascular bundles were supposed to originate close under the summit
of the stem, being confounded with the cambium of true woody plants
which is formed at a much later period, and both of them again with
the very late-formed meristem-layer in arborescent Liliaceae, in
which new vascular bundles are continually being produced and cause a
peculiar enlargement of the stem[89]. Sanio’s treatise first removed
this confusion of ideas, which appears in von Mohl himself to some
extent even in 1858, by sharply distinguishing the thickening-ring
beneath the point of the stem, in which the vascular bundles begin to
be formed, from the true cambium, which is formed at a later time in
and between the vascular bundles, and produces the secondary layers of
wood and rind; Sanio also occupied himself with submitting the various
elements of the wood to a more careful examination, and with giving
them a better classification and terminology. The peculiar instance
of secondary growth in thickness in the arborescent Liliaceae, which
had long been known and had helped to mislead von Mohl and Schacht,
was fully explained for the first time by A. Millardet in 1865. The
later works of Nägeli, Radlkofer, Eichler and others on abnormal
wood-formations contributed materially to enlarge the knowledge of
normal growth also; but these coming after 1860, and Hanstein’s later
investigations into the differentiation of tissues at the end of the
stem in Phanerogams, do not fall within the limits of our history.
4. NÄGELI’S THEORY OF MOLECULAR STRUCTURE AND OF GROWTH BY
INTUSSUSCEPTION.
This theory, the importance of which to the further development of
phytotomy and vegetable physiology has been already pointed out,
will form the conclusion of our history of the anatomy of plants. It
was a remarkable coincidence that this molecular theory of organic
forms, which is not without results for zootomy also, was brought
to completion at about the same time, namely, the year 1860, that
Darwin first published his theory of descent. At the first glance
the two theories seem to have no connection with one another, and so
the coincidence in time appears to be quite accidental. But if we
go deeper into the matter, we find a resemblance between them which
is of great historical importance; they both of them exchange the
purely formal consideration of organic bodies, which had prevailed
up to that time, for a consideration of causes; as Darwin’s doctrine
endeavours to account for the specific forms of animals and plants from
the principles of inheritance and variability under the disturbing
or favouring influence of external circumstances, so the object of
Nägeli’s theory is to refer the growth and inner structure of organised
bodies to chemical and mechanical processes. The future will show,
whether the views which we owe to Nägeli will not contribute to the
laying a deeper foundation for the theory of descent, since it is
not improbable that a more thorough understanding of the molecular
structure of organisms may add light and certainty to the still obscure
conceptions of inheritance and variation.
The first beginnings were, as is usual in similar cases, small
and inappreciable, and no one could have foreseen from the first
observations of the facts in question what the ultimate development
would be. We have said above, that von Mohl observed as early as
1836 the striation of certain cell-walls, and that this led Meyen,
on the ground of further but to some extent inaccurate observations,
to conceive of vegetable cell-walls as composed of spirally twisted
threads. It was also noticed that von Mohl next distinguished true
striation from spiral thickenings (1837), the two having been confused
together by Meyen, and advanced so far as to form some idea of the
molecular structure of cell-walls, without arriving however at any
satisfactory conclusion. Agardh, who discovered some new instances
of cell-striation, was still less successful in his speculations.
Von Mohl resumed the subject in 1853 in the ‘Botanische Zeitung,’
and insisted on the fact that it was not possible to separate the
striae or apparent fibres by mechanical or chemical means, but he
left it still undecided whether the lines which cross each other
in the surface-view belong to the same or to different layers of
membrane. The communications of Crüger and Schacht, made shortly
after, did not help to advance the question; Wigand also took part
in the discussion in 1856, but wandered at once from the right path
by supposing the cross-striations to belong to different layers of
membrane. As long as botanists adhered to von Mohl’s theory, that
the concentric stratification of cell-walls was due to deposition of
new layers, it was scarcely possible for them to arrive at a correct
decision with respect to striation; it became possible, when Nägeli
proved in his great work ‘Die Stärkekörner’ (1858) that the concentric
stratification of starch-grains and of cell-membranes generally does
not mean, that similar layers lie simply one on another, but that
denser and less watery layers alternate with layers that are less
dense and contain more water; and that it is not possible to explain
this mode of stratification by deposition as understood by von Mohl,
but that it may be explained by intercalation of new molecules between
the old ones and by corresponding differentiation of the amount of
water. That surface-growth in cell-walls does take place by this kind
of intussusception had been incidentally suggested by Unger, and
the appearance, known as the striation of the cell-wall might now
be referred to the same principle as the concentric stratification,
namely to the intercalation of more and less watery matter in regular
alternation. But Nägeli pointed out a fact which had escaped other
observers, namely, that the difference of structure which usually
appears on the surface-view as double cross-striation, passes through
the whole thickness of a stratified cell-wall. Thus Nägeli arrived
at a differentiation in three directions in space of the substance
of every minute portion of cell-membrane, and made better use than
von Mohl himself had made of the comparison which he had suggested,
namely, that the structure of a cell-wall with cross-striation and
at the same time with concentric stratification resembles that of a
crystal cleaving in three directions. He first gave expression to this
conception of the structure of the cell-wall in 1862 in his ‘Botanische
Untersuchungen,’ I. p. 187, and further developed it in the second
volume of the same work at p. 147.
But the true starting-point of Nägeli’s theory of molecular structure
is to be found in his searching investigations in 1858, into the
constitution of starch-grains. From the way in which they resist the
effects of pressure, drying, distention, and withdrawal of a part of
their substance, he arrived at the conclusion that the whole substance
of a starch-grain is composed of molecules, whose shape must be not
spherical but polyhedral, that these are separated from one another
in their normal condition by envelopes of water, and that the amount
of water in the stratified substance depends on the size of these
molecules, the water being less when the molecules are larger; this
view could at once be applied to the structure of the cell-wall,
the growth of which may be explained as the increase in size of the
molecules already present, and the intercalation of new small molecules
between the old ones. These molecules of Nägeli are themselves very
compound bodies, for the smallest of them would consist of numerous
atoms of carbon, hydrogen and oxygen, and ordinarily a molecule would
be composed of thousands of those aggregates of atoms, which the
chemists call molecules.
In examining starch-grains Nägeli came to the conclusion that molecules
of different chemical character are grouped together at every visible
point; the material which colours blue with iodine, the granulose,
could be removed from the grains, and then there remained behind a
skeleton of the starch-grain very poor in substance, but showing
exactly the original stratification and giving no blue colour with
iodine; this Nägeli named starch-cellulose. It followed from this
behaviour, that two chemically different molecules lie everywhere side
by side in the grain of starch, much as if red and yellow bricks had
been so employed to build a house, that when all the yellow bricks
were afterwards removed, the red alone would still represent the
wall in its original form as a whole though in a looser condition.
He arrived at similar results in the case of the crystalloid proteid
bodies, which Theodor Hartig discovered, and Radlkofer had examined
crystallographically, Maschke chemically. Since it is possible in
the same manner to extract the so-called incrusting matters from
cell-membranes without essentially altering their form, and to obtain
ash-skeletons of them which imitate all the delicacies of their
structure, the comparison adopted above may also be applied in still
more complex manner to the molecular structure of these membranes;
and indeed many considerations lead to the belief, that the ideas
which Nägeli obtained from starch-grains may be applied with some
modifications to the structure of protoplasm also.
We said that the appearances in the starch-grains led Nägeli to suppose
that their molecules are not spherical but polyhedral, and the question
naturally arose whether they are really crystalline. The point could
be settled by the use of polarised light, to which different observers
had already turned their attention. Erlach in 1847, Ehrenberg in 1849,
had employed polarised light for the determination of microscopic
objects, without however arriving at any conclusions on the subject
of molecular structure; Schacht indeed at a later time declared
such observations to be a pretty amusement, but without scientific
value. But soon we have once more one of von Mohl’s careful and solid
investigations (‘Botanische Zeitung,’ 1858), in which with the aid
of technical improvements in the apparatus he arrived at conclusions
respecting the nature and substance of cell-membranes, starch-grains,
&c., which proved that in the hands of a reflecting observer perfectly
familiar with the physics of polarised light the instrument is no
toy, but a means for penetrating deeply into nature’s secrets. Yet on
this occasion also appeared that peculiarity in von Mohl which twenty
years before had prevented him from founding a conclusive theory upon
his profound and extended observations on cell-formation; he was
content once more to observe thoroughly and correctly, to describe
what he observed carefully, and to connect it with proximate physical
principles in such a manner as to supply rather a classification of
phenomena, than a new and deeper insight into the essence of the
matter. He wanted the creative thought, the intense mental effort,
to arrive by analysis at the ultimate elements in the results of his
investigations and to frame for himself a clear representation of
the inner structure of the organised parts. Von Mohl in this case
also stopped short at induction and did not pass on to deductive and
constructive elaboration of the question before him; this was left to
Nägeli, as we shall see.
Meanwhile a more exhaustive work appeared in 1861 from the pen of
Valentin on the investigation of vegetable and animal tissue in
polarised light, in which the author, equipped with great knowledge of
the subject itself and its literature, examined in detail the phenomena
of polarisation, gave a good account of the instrument and the mode
of using it, and explained generally the theory and practice of
investigations of the kind. But he overlooked one fact noticed by von
Mohl, that vegetable cell-membranes, through which rays of polarised
light pass perpendicularly to their surface, show interference-colours,
and this was sure to lead him to an incorrect explanation of their
inner structure.
Nägeli from 1859 onwards made the phenomena of polarisation the
subject of protracted study, practical and theoretical; the results
were published in 1863 in his ‘Beiträge,’ Heft 3, but he had in the
previous year made known that portion of them which bore on the
molecular structure of cell-walls and starch-grains (‘Botanische
Mittheilungen,’ 1862). The phenomena of polarisation led him once
more and by a different path to the view that the organised parts
of the vegetable cell consist of isolated molecules surrounded by a
fluid, and his renewed investigations of these phenomena resulted in
more definite conceptions of the nature of these molecules, which
from the optical behaviour of the objects examined he concluded were
not only polyhedral but crystalline; in effect, the molecules of
the substance of the organised parts of plants behave, according to
Nägeli, as crystals with two optic axes, which therefore possess three
different axes of elasticity; in starch-grains and cell-membranes these
crystalline molecules are so arranged that one of these axes is always
perpendicular to the stratification, while the two others lie in its
plane. The effect of the organised parts of the cells on polarised
light is the sum of the effects of the single molecules, whereas the
fluid that lies between them is optically inactive, and only comes into
consideration because according to its quantity the molecules separate
more or less far from or approach one another.
THIRD BOOK
HISTORY OF VEGETABLE PHYSIOLOGY
(1583-1860)
INTRODUCTION.
All that was known in the 16th and at the beginning of the 17th
centuries of the phenomena of life in plants was scarcely more than
had been learnt in the earliest times of human civilisation from
agriculture, gardening, and other practical dealing with plants. It was
known, for instance, that the roots serve to fix plants in the soil and
to supply them with food; that certain kinds of manure, such as ashes
and, under certain conditions, salt, strengthen vegetation; that buds
develope into shoots; and that the blossom precedes the production
of seeds and fruits. These and a variety of minor physiological
phenomena were disclosed by the art of gardening. On the other hand,
the physiological importance of leaves in the nourishment of plants
was quite unknown, nor can we discover more than a very indistinct
perception of the connection between the stamens and the production of
fruitful seeds. That the food-material taken up from the soil must move
inside the plant in order to nourish the upper parts was an obvious
conclusion, which it was attempted to explain by comparing it with the
movement of the blood in animals. Writers on the subject up to the end
of the 17th century make very slight mention of the influence of light
and warmth on the sustentation and growth of plants, though doubtless
the operation of these agencies in the cultivation of plants, as in
other matters, must have been early recognised.
So scanty was the stock of knowledge which the founders of vegetable
physiology in the latter half of the 17th century found ready to their
hand. While the physiological significance of the different organs of
the human body and of most animals were known to every one, at least
in their more obvious features, the study of vegetable life had to
begin with laborious enquiries, whether the different parts of plants
are generally necessary to their maintenance and propagation, and
what functions must be ascribed to individual parts for the good of
the whole. It was no easy matter to make the first step in advance in
this subject; something can be learnt of the functions of the parts
of animals from direct observation, scarcely anything in the case of
plants; and it is only necessary to read Cesalpino and the herbals
of the 16th century to see how helpless the botanists were in every
case in presence of questions concerning the possible physiological
meaning of vegetable organs, when they ventured beyond the conceptions
of the root as the organ of nourishment, and of the fruit and seeds
as the supposed ultimate object of vegetable life. The physiological
arrangements in vegetable organs are not obvious to the eye; they
must be concluded from certain incidental circumstances, or logically
deduced from the result of experiments. But experiment presupposes the
proposing a definite question resting on a hypothesis; and questions
and hypotheses can only arise from previous knowledge. An early attempt
to connect the subject with existing knowledge was made in the use
of the comparison of vegetable with animal life, a comparison which
Aristotle had employed with small success. Cesalpino, provided with
more botanical and zoological knowledge, endeavoured to arrive at more
definite ideas of the movement of the nutrient juices in plants, and
when Harvey discovered the circulation of the blood in the beginning of
the 17th century, the idea at once arose that there might be a similar
circulation of the sap in plants. Thus a first hypothesis, a definite
question was framed, and attempts were made to decide it by more exact
observation of the ordinary phenomena of vegetation, and still better
by experiment; and though a discussion which lasted nearly a hundred
years led to the opinion that there is no circulation of sap in plants
corresponding to the circulation of blood in animals, the result was
obtained by the aid of this hypothesis derived from a comparison
between animals and plants. The important discovery that leaves play
a considerable part in the nourishment of plants, was to some extent
an incidental product of the investigation of the former question, and
it preceded that of the decomposition of carbon dioxide by the green
parts of plants by more than a hundred years. To give another example;
it was obviously a comparison of certain phenomena in vegetable life
with the propagation of animals which paved the way for the discovery
of sexuality in plants; long before Rudolf Jacob Camerarius made his
decisive experiments (1691-1694) on the necessary co-operation of the
pollen in the production of seeds capable of germination, the idea
had been entertained that there might be an arrangement in plants
corresponding to the sexual relation in animals, though that idea was
highly indistinct and distorted by various prepossessions. In like
manner the interest excited by the discovery of the irritability of
the Mimosae in the 17th century, and of similar phenomena of movement
in plants at a later time, was mainly due to the striking resemblance
suggested between animals and plants; and the first researches into
the subject were obviously intended to answer the question whether
the movements in plants are due to conditions of organisation similar
to those in animals. In all cases of this kind it was matter of
indifference whether the analogies presupposed were finally confirmed
after prolonged investigation, as in the question of sexuality, or
disproved as in that of the circulation of the sap. The result was
of less importance than the obtaining points of departure for the
investigation. It answered this purpose to adopt certain actual or only
apparent analogies between plants and animals, and to assume, to some
extent to invent, certain functions for the apparently inactive organs
of plants, and to interrogate them upon the point. Scientific activity
was set in motion, and it mattered not what the result might be. In
all questions connected with the phenomena of life, our own life is not
only the starting-point but also the standard of our conceptions; what
animate nature is as opposed to inanimate we discern first by comparing
our own being with that of other objects. From our own vital motions we
argue to those of the higher animals, which we comprehend immediately
and instinctively from their conduct; by aid of these the motions
of the lower animals also become intelligible to us, and further
conclusions from analogy lead us finally to plants, whose vitality
is only in this way made known to us. While plants were thus even in
ancient times regarded as living creatures and allied to animals,
further reflection naturally suggested the idea that the phenomena of
animal life would be reproduced in plants even in details. We learn
from the botanical fragments of Aristotle that this was in fact the
way in which the first questions in vegetable physiology arose; they
assumed a more definite form with Cesalpino, and later physiologists
repeatedly made use of similar conclusions from analogy. The historian
of this branch of botanical science must seek no other beginning of
it, for it had no other and could have no other from the nature of
the case. And if preconceived analogies between plants and animals
often proved deceptive and mischievous, yet continued investigation
gradually brought to light more important and more essential points of
agreement between the two kingdoms; it has become more and more evident
in our own days, that the material foundations of vegetable and animal
life are in the main identical,—that the processes connected with
nourishment, movement of juices, sexual and asexual propagation present
the most remarkable similarities in both kingdoms.
If the first founders of scientific vegetable physiology surrendered
themselves thoroughly to teleological views, this was owing to the
circumstances of the time, and it served indeed to promote the
first advances of the science. There was no need in the 17th and
18th centuries that a man should be an Aristotelian to presuppose
design and arrangements in conformity with design in all parts of
physiological investigation. This is everywhere and always the original
point of view which precedes all philosophy; but it is the part of
advanced science to abandon this position; and as early as the 17th
century philosophers recognised the fact that the teleological mode
of proceeding is unscientific. But the first vegetable physiologists
were not philosophers in the stricter sense of the word, and in their
investigations they accepted the teleological conception of organic
nature without question, because they regarded it as a self-evident
fact, that every organ must be purposely and exactly so made as to be
in a condition to perform the functions necessary for the permanence
of the whole organism. This conception was in accordance with views
then prevailing, and was even useful; it was no disadvantage in the
first beginnings of the science, that it should be supposed that
every, even the minutest, part of a plant was expressly contrived
and made for maintaining its life, for this was a strong motive for
carefully examining the organs of plants, which was the first thing
requisite. This is exemplified in Malpighi, Grew, and Hales, and
we shall see that even towards the end of the 17th century Konrad
Sprengel made splendid discoveries respecting the relations of the
structure of the flower to the insect world, while strictly carrying
out his teleological principles. The teleological view was injurious
to the progress of morphology from the first, though the history
of systematic botany shows how hard it was for botanists to free
themselves from such notions. The case was different with physiology;
so long as it was a question of discovering the functions of organs,
and learning the connection between the phenomena of life, teleology
proved highly useful if only as a principle of research. But it was
another matter when it became requisite to investigate causes, and to
grasp the phenomena of vegetation in their causal connection. To this
the teleological mode of view was inadequate, and it became necessary
indeed to discard it as a hindrance, in spite of the difficulty of
explaining adaptation in the arrangements of organisms from any other
than the teleological point of view. It is sufficient here to say that
this difficulty is satisfactorily removed by the theory of selection.
This theory is become as important in this respect to physiology, as
the theory of descent is to systematic botany and morphology. If the
theory of descent finally liberated the morphological treatment of
organisms from the influence of scholasticism, it is the theory of
selection which has made it possible for physiology to set herself free
from teleological explanations. Only an entire misunderstanding of the
Darwinian doctrine can allow anyone to reproach it with falling back
into teleology; its greatest merit is to have made teleology appear
superfluous, where it seemed to naturalists in former times, in spite
of all philosophical objections, to be indispensable.
If the comparison of plants with animals as well as the teleological
conception of organisms promoted the first attempts at the
physiological investigation of plants, other influences of decisive
importance came into play when the time came for endeavouring to
conceive and explain the causes and conditions of the functions, which
had then been ascertained at least in their most obvious features.
Phytotomy was here the chief resource. In proportion as the inner
structure of plants was better known and the different kinds of tissue
better distinguished, it became possible to bring the functions of
organs, as made known by experiment, into connection with their
microscopic structure; phytotomy dissected the living machine into its
component parts, and could then leave it to physiology to discover from
the structure and contents of the tissues, how far they were adapted to
perform definite functions. Obviously this only became possible when
the phenomena of vegetation had been previously studied in the living
plant. For example, the microscopic examination of the processes
which take place in fertilisation could first be made to yield further
conclusions, after sexuality itself, the necessity of the pollen to
the production of fruitful seeds, had been proved by experiment; in
the same way the anatomical investigation of wood could only supply
material for explaining the mode in which water rises in it, when it
had first been ascertained by experiment that this happens only in the
wood, and so in other cases.
The relation between physiology and physics and chemistry suggests
similar considerations; it is necessary to make some preliminary
remarks in explanation of this relation, because we often meet with
the view, especially in modern times, that vegetable physiology is
virtually only applied physics and chemistry, as though the phenomena
of life could be simply deduced from physical and chemical doctrines.
This might perhaps be possible, if physics and chemistry had no further
questions to solve in their own domains; but in fact both are still
as far distant from this goal, as physiology is from hers. It is true
indeed, that modern vegetable physiology would be impossible without
modern physics and chemistry, as the earlier science had to rely on
the aid of the physics and chemistry of the day, when she was engaged
in forming a conception of ascertained vital phenomena as operations
of known causes. But it is equally true, that no advance which physics
and chemistry have made up to the present time would have produced
any system of vegetable physiology, even with the aid of phytotomy;
history shows that a series of vital phenomena in plants had been
recognised in the 17th and 18th century, at a time when physics and
chemistry had little to offer, and were in no condition to supply
explanations of any kind to the physiologist. The true foundation of
all physiology is the direct observation of vital phenomena; these must
be evoked or altered by experiment, and studied in their connection,
before they can be referred to physical and chemical causes. It is
therefore quite possible for vegetable physiology to have reached a
certain stage of development without any explanation of the phenomena
of vegetation from physics or chemistry, and even in spite of erroneous
theories on those subjects. What Malpighi, Hales, and to some extent
Du Hamel produced, was really vegetable physiology, and of a better
kind than some moderns are inclined to believe; and their knowledge was
derived from observations on living plants, and not from the chemical
and physical theories of their time. The discovery even of important
facts, for example, that green leaves only can form the food suitable
to effect the growth and formation of new organs, was made a hundred
years before that of the decomposition of carbon dioxide by the green
parts of plants, at a time indeed when chemistry knew nothing of carbon
dioxide and oxygen. A whole series of physiological discoveries might
be mentioned, which were distinctly opposed to chemical and physical
theories, and even served to correct them. We may give as examples,
the establishment of the facts that roots absorb water and the
materials of food without giving up anything in return, which seemed
quite unintelligible on the earlier physical theory of the endosmotic
equivalent; and that the so-called chemical rays of the physicists are
of subordinate importance in vegetable assimilation, while contrary
to the prevailing notions of physicists and chemists the yellow
portions of the spectrum and those adjacent to it actively promote the
decomposition of carbon dioxide. From what doctrines of the physicists
could it have been concluded, that the downward growth of roots and the
upward growth of stems was due to gravitation, as Knight proved in 1806
by experiments on living plants; or could optics have foreseen that
the growth of plants is retarded by light, and that growing parts are
curved under its influence. Our best knowledge of the life of plants
has been obtained by direct observation, not deduced from chemical and
physical theories. After these preliminary remarks we may proceed to
give a rapid survey of the progress of vegetable physiology.
1. That the first beginnings of vegetable physiology were made about
the time that chemistry and physics began to take their place among
the true natural sciences, is no proof that they called vegetable
physiology into existence. She, like general physiology, mineralogy,
astronomy, geography, owed her origin to the outburst of the spirit of
enquiry in the 16th and 17th centuries, which feeling the emptiness
of the scholastic philosophy set itself to gather valuable knowledge
by observation in every direction. It was in the second half of the
17th century that societies or academies for the study of the natural
sciences were founded in Italy, England, Germany, and France under the
influence of this feeling; the first works on vegetable physiology
play a very prominent part in their transactions; not to speak of less
important cases, it was the Royal Society of London which published
between 1660 and 1690 the memorable works of Malpighi and Grew; the
first communications of Camerarius, which form an epoch in the history
of the doctrine of sexuality, appeared in the journals of the German
Academia Naturae Curiosorum, and the French Academy undertook about the
same time to organise methodical researches in vegetable physiology
under Dodart’s direction, though the results it is true did not answer
to the goodness of the intention. This period of movement in all
branches of science, when the greatest discoveries followed one another
with marvellous rapidity, witnessed also the first important advances
in vegetable physiology; such were the first investigations into
the ascending and descending sap, especially those made in England,
Malpighi’s theory which assigned to leaves the functions of organs of
nutriment, Ray’s first communications on the influence of light on
the colours of plants, and above all the experiments of Camerarius,
which proved the fertilising power of the pollen. It was the period of
first discoveries; the attempts at explanation were certainly weak;
but phytotomy which was just commencing its own work lent aid from
the first to physiology, while physics and chemistry could do but
little for her. On the other hand, the predilection for mechanics
and mechanical explanation of organic processes in Newton’s age bore
fair fruit in Hales’ enquiries into the movement of sap in plants;
his ‘Statical Essays’ of 1727 connect closely with the works before
mentioned which had laid the foundations of the science, and with
this important performance the first period of its history reaches a
distinctly marked conclusion.
This time of vigorous advance was followed by many years, in which
no notable work was done and no great discovery effected; there was
active disputation on what had been already ascertained, but it did not
lead to any deeper conception of the questions or to new experimental
determinations.
2. About the year 1760 new life was infused into the consideration of
various branches of vegetable physiology. Du Hamel’s ‘Physique des
arbres’ (1758) gave a summary of former knowledge and added a number of
new observations, and from that time till the beginning of the present
century a series of important discoveries was made. The doctrine of
sexual propagation, which had scarcely been advanced since the time of
Camerarius, and was disfigured by the theory of evolution, found an
observer of the first rank in Koelreuter (1760-1770), who threw new
light upon the nature of sexuality by his experiments on the artificial
production of hybrids; he was the first who carefully studied the
arrangements for pollination, and pointed out the remarkable connection
between them and insect-life. These relations were afterwards (1793)
examined in greater detail by Konrad Sprengel, who arrived at such
astonishing and far-reaching results, that they were not even
understood by his contemporaries, nor was their significance fully
appreciated till quite modern times and in connection with the theory
of descent.
No less important was the advance made in the doctrine of the
nourishment of plants. Between 1780 and 1790 Ingen Houss proved, that
the green parts of plants absorb carbon dioxide under the influence
of light and eliminate the oxygen, and thus obtain the carbon which
plants accumulate in organic combinations, but that all parts of
plants also absorb at all times smaller quantities of oxygen, and
exhale carbon dioxide, and so perform a process of respiration exactly
corresponding to that of animals. He was soon followed by Theodore
de Saussure with more thorough investigation of these processes,
and with proofs that the ash-constituents of a plant are no chance
or unimportant addition to its food, as had been hitherto commonly
supposed (1804). The influence also of general physical forces on
vegetation was established in some important points, though not yet
submitted to searching examination. Thus Senebier showed in the period
between 1780 and 1790 the great effect which light exercises on the
growth and green colour of plants, and De Candolle at a later date
discovered its operation in the case of leaves and flowers that show
periodic movements. Still more important was Knight’s discovery in 1806
that the upright growth of stems and the downward direction of the main
roots are determined by gravitation.
3. This second period of important discoveries was also followed by
a relapse, and again doubts were raised as to the correctness of the
very facts which had been best established; attempts were made under
the influence of preconceived opinions to invalidate or ignore these
facts, and to substitute for them theories that wore the guise of
philosophy. The so-called nature-philosophy, which had long been a
great hindrance to morphology, proved in like manner injurious to
vegetable physiology; the doctrine of the vital force especially stood
in the way of every attempt to resolve the phenomena of life into their
elementary processes, to discern them as a chain of causes and effects.
The ash-constituents of plants, and even their carbon, were traced to
this vital force, and misty notions connected with the word polarity
were used to explain the direction of growth and much beside. In like
manner the influence of the nature-philosophy was brought to bear on
the established results of the sexual theory to the destruction of all
sound logic, and the sexuality of plants was once more openly impugned
in the face of Koelreuter’s investigations. This state of things
continued till some time after 1820, but then it began to improve once
more. L. C. Treviranus examined and refuted the errors of Schelwer and
Henschel in 1822; in England Herbert conducted new and very valuable
investigations into the question of hybridisation; and it was in this
period that Carl Friedrich Gärtner studied and experimented on normal
fertilisation and the production of hybrids during more than twenty
years; his conclusions, published in exhaustive works in 1844 and in
1849, finally settled the more important questions connected with the
sexual theory about the same time that Hofmeister established the
microscopic embryology of Phanerogams on a firm foundation.