MORPHOLOGY AND SYSTEMATIC BOTANY UNDER THE INFLUENCE OF THE HISTORY OF
DEVELOPMENT AND THE KNOWLEDGE OF THE CRYPTOGAMS.
1840-1860.
In the years immediately before and after 1840 a new life began to
stir in all parts of botanical research, in anatomy, physiology,
and morphology. Morphology was now specially connected with renewed
investigations into the sexuality of plants and into embryology, and
attention was no longer confined to the Phanerogams but was extended to
the higher and later on to the lower Cryptogams. These researches into
the history of development first became possible, when von Mohl had
restored the study of anatomy, and Nägeli had founded and elaborated
the theory of cell-formation about the year 1845. The success of both
these enquirers was due to the previous development of the art of
microscopy; it was the microscope which revealed the facts on which
the foundations of the new research were laid, while its promoters at
the same time started from other philosophical principles than those
which had hitherto prevailed among botanists. Investigation by means
of the microscope enforces on the observer the very highest strain
of attention and its concentration on a definite object, while at
the same time a definite question to be decided by the observation
has always to be kept before the mind; there are sources of error on
all sides to be avoided, and possible deceptions to be taken into
consideration; the securing of the facts demands all the powers
which specially display the individual character of the observer.
Thus serious attention to microscopy was one of the causes which
introduced the best observers to the practice of inductive enquiry,
and gave them an insight into its nature; and in a few years’ time
when the actual results of these investigations began to appear, and
when a wholly new world disclosed itself to botanists, especially in
the Cryptogams, then questions arose on which the dogmatic philosophy
had not essayed its ancient strength; the facts and the questions
were new and untouched, and presented themselves to unprejudiced
observation in a purer form than those, which during the first three
centuries had been so mixed up with the old philosophy and with the
principles of scholasticism. Von Mohl, who only occasionally occupied
himself with morphological subjects, was a firm adherent of the
inductive method, and was bent on the establishment of individual facts
rather than of general principles; but the founders also of the new
morphology, Schleiden and Nägeli, started from philosophical points
of view, which, different as they were in the two men, had yet two
things in common, a demand for severely inductive investigation as
the foundation of all science, and the rejection of all teleological
modes of explaining phenomena, in which latter point their opposition
to the idealistic nature-philosophy school was most distinctly
manifested. They had indeed one very important point of contact with
this school, the belief in the constancy of organic forms: but this
belief, not being connected with the Platonic doctrine of ideas,
was with them only a recognition of every-day observations, and was
therefore of less fundamental importance, being felt merely as an
inconvenient element in the science. Treating the question in this
way, and influenced by the results of the new researches, they either
inclined to entertain the idea of descent before the appearance of
Darwin’s great work, or gave a ready assent to the principle of the
new doctrine, though they expressed some doubts respecting matters
of detail. Hofmeister’s researches in morphology and embryology
(‘Vergleichende Untersuchungen,’ 1851) threw an entirely new light on
the relations of affinity between the great groups in the vegetable
kingdom, and were leading more and more to the view, that there must be
some special peculiarity in the question of the constancy of organic
forms. But the idea of evolution in the vegetable kingdom was brought
more distinctly home to men’s minds by palaeontological researches;
Sternberg (1820-1838), Brongniart (1828-1837), Goeppert (1837-1845),
and Corda (1845) made the flora of former ages the subject of careful
study, and compared fossil plants with living allied forms. Unger
especially, while advancing the knowledge of the structure of cells and
of vegetable anatomy and physiology, and generally taking a prominent
part in the development of the new botany, applied the results of its
investigations to the examination of primeval vegetation, and showed
the morphological and systematic relations between past and existing
floras. After twenty years of preliminary study he declared distinctly
in 1852, that the immutability of species is an illusion, that the new
species which have made their appearance in geological periods are
organically connected, the younger having arisen from the elder[51]. It
was shown in the former chapter, how about the same time the leading
representative of idealistic views, Alexander Braun, was driven to the
hypothesis, though in a more indefinite form, of an evolution of the
vegetable kingdom: and in the year that Darwin’s book on the origin
of species appeared, Nägeli (‘Beiträge,’ ii. p. 34) wrote:—‘External
reasons, supplied by the comparison of the floras of successive
geological periods, and internal reasons given in physiological and
morphological laws of development and in the variability of the
species, leave scarcely a doubt that species have proceeded one from
another.’
Though these words might not contain a theory of descent capable
at once of scientific application, yet they show that the latest
researches and candid appreciation of facts were compelling the most
eminent representatives of the botany of the day to give up the
constancy of forms. At the same time in the genetic morphology which
had developed itself mainly under Nägeli’s guidance since 1844, and
still more in embryology, which in Hofmeister’s hands was leading to
results of the greatest systematic importance, there lay a fruitful
element destined to correct and enrich Darwin’s doctrine of descent
in one essential point. That doctrine in its original form sought
to show that selection, the result of the struggle for existence,
combined with perpetual variation was the sole cause of progressive
improvement in organic forms[52]; but Nägeli, relying on the results
of German morphology, was able as early as 1865 to point out that this
explanation was not satisfactory, because it leaves unnoticed certain
morphological relations, especially between the large divisions of the
vegetable kingdom, which scarcely seem explainable by mere selection
in breeding. While Nägeli allowed that Darwin’s principle of selection
was well adapted to explain fully the adaptation of organisms to their
environment and the suitableness and physiological peculiarities
of their structure, he pointed out that in the nature of plants
themselves there are intimations of laws of variation, which lead to a
perfecting of organic forms and to their progressive differentiation,
independently of the struggle for existence and of natural selection;
the importance of this result of morphological research has since been
recognised by Darwin. Thus Nägeli supplied what was wanting in the
theory of descent and gave it the form, in which it is adequate to
explain the problem already recognised by the systematists of the old
persuasion, namely, how it is possible for the morphological affinity
of species in the system to be in so high a degree independent of their
physiological adaptation to their environment.
The modern teaching on vegetable cells, modern anatomy, and morphology,
and the improved form of the theory of selection are the product of
inductive enquiry since 1840, a product, the full importance of which
will be described in the following portions of our history. At present
we have to deal only with morphological and systematic results, and
therefore with a part only of the abundant labours of the botanists
who will be noticed in this chapter; the remainder will be reserved
for succeeding books, which contain the history of the anatomy and
physiology of plants.
It is one of the characteristic features of this period of botany, that
morphology enters into the closest connection with the doctrine of the
cell, with anatomy and embryology, and that researches, especially
into the process of fecundation and the formation of the embryo, form
to some extent the central point of morphological and systematic
investigations. A strict separation of these various enquiries, which
are all ultimately applicable to the purposes of systematic botany, can
therefore scarcely be maintained, and least of all in dealing with the
lower Cryptogams.
* * * * *
The condition of botanical literature about the year 1840 was highly
unsatisfactory; it is true that eminent service was rendered in
the several domains of systematic botany, morphology, anatomy, and
physiology, and a number of von Mohl’s best works were produced in
this period; Meyen also, Dutrochet, Ludolph Treviranus and others were
cultivating vegetable anatomy and physiology, and it has been already
stated that good and noticeable work was done in the previous years in
morphology and systematic botany. But there was no one to put together,
to criticise and apply the knowledge which had been accumulated in
all parts of the science; no one really knew what a wealth there was
at that time of important facts; least of all was it possible to form
a judgment on the matter from the text-books of the period, which
were deficient in ideas and facts, and crammed with a superfluous
terminology; their mode of treating their subject was trivial and
tasteless, and whatever was specially worth knowing and important to
the student they did not contain. Those who undertook really scientific
enquiries separated themselves from those who dealt with botany after
the old schematism of the Linnaean school; but botanical instruction,
the propagation of knowledge, was almost everywhere in the hands of
this school, though it was the one least fitted for the task; and thus
a mass of lifeless phrases was the instruction offered to the majority
of students under the name of botany, with the inevitable effect of
repelling the more gifted natures from the study. This was the evil
result of the old and foolish notion, that the sole or chief business
of every botanist is to trifle away time in plant-collecting in wood
and meadow and in rummaging in herbaria,—proceedings which could
do no good to systematic botany even as understood by the Linnaean
school. Even the better sort lost the sense for higher knowledge
while occupying themselves in this way with the vegetable world; the
powers of the mind could not fail after a time to deteriorate, and
every text-book of the period on every page supplies proof of this
deterioration.
But such a condition of things is dangerous for every science; of what
profit is it, that single men of superior merit advance this or that
part of the science when a connected view of the whole is wanting,
and the beginner has no opportunity of studying the best things in
their mutual relations. However, the right man was found at the right
moment to rouse easy indolence from its torpor, and to show his
contemporaries, not in Germany only but in all countries where botany
was studied, that no progress was possible in this way. This man was
MATTHIAS JACOB SCHLEIDEN, born at Hamburg in 1804, and for many years
Professor in Jena. Endowed with somewhat too great love of combat, and
armed with a pen regardless of the wounds it inflicted, ready to strike
at any moment, and very prone to exaggeration, Schleiden was just the
man needed in the state in which botany then was. His first appearance
on the scene was greeted with joy by the most eminent among those who
afterwards contributed to the real advance of the science, though their
paths it is true diverged considerably at a later period, when the time
of reconstruction was come. If we were to estimate Schleiden’s merit
only by the facts which he discovered, we should scarcely place him
above the level of ordinarily good botanists; we should have to reckon
up a list of good monographs, numerous refutations of ancient errors
and the like; the most important of the theories which he proposed, and
over which vigorous war was waged among botanists during many years,
have long since been set aside. His true historical importance has
been already intimated; his great merit as a botanist is due not to
what he did as an original investigator, but to the impulse he gave
to investigation, to the aim and object which he set up for himself
and others, and opposed in its greatness to the petty character of the
text-books. He smoothed the way for those who could and would do really
great service; he created, so to speak, for the first time an audience
for scientific botany capable of distinguishing scientific work from
frivolous dilettanteism. Whoever wished from this time forward to take
part in the discussion of botanical subjects must address all his
powers to the task, for he would be judged by another standard than had
hitherto prevailed.
Schleiden, who had commenced his botanical labours with some important
researches in anatomy and the history of development, the most valuable
of which in matter and form was an enquiry into the development of
the ovule before fertilisation (1837), composed also a comprehensive
text-book of general botany, which appeared first in 1842-3, and
in much improved editions in 1845 and 1846, and in two subsequent
years. The difference between this and all previous text-books is the
difference between day and night; in the one, an indolent carelessness
and an absence of ideas; in the other, a fulness of life and thought,
calculated to influence young minds all the more, because it was in
many respects incomplete and still in a state of fermentation. On
every page of this remarkable work, by the side of facts really worth
knowing, the student found interesting reflections, a lively and
generally coarse polemic, and praise and blame of others. It was not
a book to be studied quietly and comfortably, but one that excited
the reader everywhere to take a side for or against, and to seek for
further instruction.
The work is generally quoted as ‘Grundzüge der wissenschaftlichen
Botanik,’ but its chief title is ‘Die Botanik als inductive
Wissenschaft,’ which indicates the point on which Schleiden laid most
stress. His great object was to place the study, which had been so
disfigured in the text-books as scarcely to wear the semblance of
a natural science, on the same footing with physics and chemistry,
in which the spirit of genuine inductive enquiry into nature had
already asserted itself in opposition to the nature-philosophy of
the immediately preceding years. It may seem strange to us now to
see a text-book of botany introduced by a formal essay, 131 pages
long, on the inductive method of investigation as opposed to dogmatic
philosophy, and to find the principles of induction set forth again
and again in connection with a great variety of subjects in the
book itself. Many objections may be raised to the contents of this
introduction; it may be said that many philosophical dicta are
misunderstood in it; that Schleiden himself has frequently offended
against the rules there laid down, for instance, when he substitutes
a formative impulse (nisus formativus) for the vital force which he
rejects, which is only introducing vital force again under another
name; that it is superfluous to present the history of development
as a ‘maxim’ in Kant’s use of the word, instead of showing that the
history of development enters naturally and of itself into inductive
investigation, and so on. All this will not lessen the historical
importance of this philosophic introduction; the traditional way in
which descriptive botany was at that time presented to the student was
so thoroughly dogmatic and scholastic, trivial and uncritical, that it
was necessary to impress upon him in many words, that this is not the
method of true investigation of nature.
Passing on to the more special problems of botanical enquiry, Schleiden
next dwells on the history of development as the foundation of all
insight into morphology, though he overshot the mark when he rejected
as unfruitful the simple comparative method, which had produced
considerable results in the hands of De Candolle, and was virtually the
fruitful element in the doctrine of phyllotaxis of Schimper and Braun.
Still he took an active part himself in the study of development in
plants, and gave special prominence to embryology; he also discussed
the doctrine of metamorphosis from the point of view of the history
of development, and pointed to Caspar Friedrich Wolff’s treatment of
that subject as much clearer than that which had been introduced by
Goethe. Finally, Schleiden’s mode of dealing with the natural system
must be reckoned among the good services which he rendered to method;
not because his classification of the vegetable kingdom presents any
specially interesting features or brought to light any new affinities,
but because we see an attempt made for the first time to give detailed
characters drawn from morphology and the history of development to the
primary divisions, and because by this means the positive and distinct
nature of the Cryptogams was from the first clearly brought out. The
old way of treating morphology, as though there were only Phanerogams
in the world, and then having recourse to unmeaning negatives in
dealing with the Cryptogams, was thus set aside, much to the profit
of the immediate future, which directed its attention specially to the
Cryptogams.
Schleiden however did not succeed in securing firm ground for the
morphology of the Cryptogams as founded on the history of their
development. His investigations into the morphology of the Phanerogams
were more successful. His theory of the flower and fruit is an
admirable performance for the time, even though we abandon his view of
the stalk nature of placentas and some other notions, as we obviously
must. As Robert Brown founded the history of the development of the
ovule, so Schleiden founded that of the flower, and his example
influenced other botanists. Soon investigations into the genesis of
the flower was one of the chief occupations of morphologists, and the
results of enquiry into development proved to be of great value for the
systematic arrangement of the Phanerogams, especially when more exact
attention was paid to the sequence of development in the organs of an
inflorescence, to abortion, doubling and branching of the stamens, and
to the like matters. Duchartre, Wigand, Gelesnoff and many others,
were soon working in the same direction with great success. Payer
deserves special mention for his enormous perseverance in examining the
development of the flower in all the more important families in his
‘Organogénie de la fleur,’ 1857, and thus producing a standard work,
distinguished alike for the certainty of the observations, the simple
unbiassed interpretation of the things observed, and the beauty and
abundance of the figures—a work which became more important every year
for the morphology of the flower.
Schleiden’s text-book was the first of its kind that supplied the
student with really good figures based on careful observations. With
all its many and obvious defects it had one merit which cannot be
rated too highly; its appearance at once put botany on the footing
of a natural science in the modern sense of the word, and placed it
upon a higher platform, extending its horizon by raising its point of
view. Botany appeared all at once as a science rich in subject-matter;
Schleiden had not only himself made many investigations and broached
new theories, but he everywhere drew attention to what was already
before the world and was important; for it is not sufficient as regards
the literature of a science that there should be good investigators; it
is as necessary that the scientific public, and especially the rising
generation of professed students, should be well and sufficiently
instructed in the art of distinguishing important from unimportant
contributions. It must be distinctly affirmed in this place, that if
Schleiden’s theory of cell-formation, his strange notion about the
embryology of Phanerogams and the like were very quickly shown to be
untenable, this does not in the least affect the great historical
importance which his writings possess in the sense here indicated.
That others besides Schleiden in the period following 1840 felt
strongly, that botany must thenceforward give up its complacent resting
in the old ideas, was shown among other things by the addition at this
time of new periodicals to the old journal ‘Flora.’ The ‘Botanische
Zeitung’ was founded by von Mohl and Schlechtendal in 1843, and the
‘Zeitschrift für wissenschaftliche Botanik’ by Schleiden and Nägeli.
The latter, however, only lived three years, from 1844 to 1846, and was
filled almost entirely with Nägeli’s contributions. Both publications
expressly set themselves the task of representing the new aims in the
science. The immediate consequence was that ‘Flora’ braced up its
energies, and endeavoured to do more justice to the modern spirit;
excellent notices of botanical works now appeared in it under the
exclusive management of Fürnrohr.
Schleiden’s productivity in the higher sense of the word expended
itself in his labours on the elements of scientific botany. His
later somewhat discursive writings exerted no great influence on the
further development of the science. The ideal which he had set up for
scientific botany and had sketched in its larger outlines, could only
be realised by the most persevering labour not of one man only, but of
whole generations of observers and thinkers, nor did he apply himself
with painful unremitting industry to the attainment of this exalted aim.
Soon after Schleiden’s ‘Grundzüge’ first stirred the scientific world,
a man of a very different character of mind began to address himself
to the great task. This was CARL NÄGELI, whose researches from this
time onwards laid the foundations of knowledge in every department of
botany. He showed what points were the most immediately attainable, and
aided in perfecting the inductive method of enquiry and in advancing
the study of the history of development. He did not make discoveries
here and there by desultory efforts, but worked with earnest endurance
at every question which he took up till he had arrived at a positive
result; and this was almost always an enlargement of previous
knowledge, and a new foundation on which others might build, and a
copious literature be developed.
Nägeli like others felt the necessity of first determining his position
with respect to the philosophical principles of the investigation of
nature, but he did not proceed to give a general exposition of the
inductive method as opposed to the dogmatism of the idealistic school.
He went straight to the application of the laws of induction to the
most general problems of organic nature, and specially of vegetation.
It is easy to say that the task of natural science is simply to deduce
conceptions and laws from the facts of experience by aid of exact
observation. Many considerations present themselves as soon as the
attempt is made to satisfy this demand; for it is not enough merely to
accumulate individual facts, the point to which the inductive enquiry
is to lead must be kept constantly and clearly before the mind. Nägeli
insisted that it is only in this way that facts and observations
have any scientific value; that the one important thing is to make
every single conception obtained by induction find its place in the
scheme of all the rest of our knowledge. With greater consistency of
reasoning than Schleiden, and in entire accordance with the nominalist
view of genuine investigation of nature in its sternest opposition to
the idealistic school, Nägeli’s first principle is not only to deduce
conceptions from the observation of phenomena, to classify them and
establish their subordination, but to treat these conceptions as mere
subjective products of the understanding and employ them as instruments
of thought and communication, and to be always ready to modify them
as soon as inductive enquiry renders such modification necessary.
Till this happens, the conception once laid down and connected with
a word is to be strictly adhered to, and every arbitrary change or
confusion with another conception is strictly forbidden. Since in
nature everything is in movement, and every phenomenon is transitory,
presenting itself to us in organic life especially as the history of
development, all due regard must be paid to this condition of constant
motility in forming scientific conceptions. The history of development
is not merely to be treated generally as one of various means of
investigation, but as identical with investigation into organic nature.
These views are expressed in Nägeli’s detailed observations on method
in the first and second volume of the journal which he brought out in
conjunction with Schleiden in 1844 and 1855, where the chief hindrance
to his carrying them out fully and consistently is also to be found;
for, like all his contemporaries, Nägeli believed at that time in the
constancy of species, and consistently with this view he looked upon
the natural system as a framework of conceptions, though these do not
take the form of Platonic ideas with him as with the systematists of
the idealistic school. It is equally consistent with his philosophical
position, which refused to regard a change in our conceptions as a
change in things themselves, that ‘the idea of metamorphosis’ in the
sense of Goethe and Alexander Braun disappears in Nägeli from the field
of scientific observation. It has been shown in the previous chapter
that what Goethe called the normal or ascending metamorphosis has
no scientific meaning unless species are supposed to be variable. It
appeared moreover that if the Cryptogams are made the chief subjects
of investigation, as Nägeli made them, the so-called metamorphosis of
the leaves is a phenomenon of secondary importance, and only attains
to its full importance in the Phanerogams. If Schleiden, illogically
from his point of view, conceived of metamorphosis as the principle
of development, Nägeli on the contrary scarcely employed the word. He
regarded the history of development as the law of growth of the organs,
and, in accordance with the theory of the constancy of species, the
law of growth of every species and every organ was invariable in the
same sense in which we apply the term to natural laws in physics and
chemistry. In a word, Nägeli’s considerations on the ‘present task of
natural history’ in the work above cited, are not only logically and
entirely consistent on the principles of the inductive method, but they
are also consistent where others have been misled by the theory of the
constancy of species into illogical conclusions.
Nägeli set himself in earnest to meet the demands of inductive
enquiry, such as he had himself described them. It will be shown
more in detail in the history of phytotomy, how he satisfied these
demands in his refutation of Schleiden’s doctrine of the cell, and
in the establishment of his own, and at a later time in the framing
of his theory of molecular structure and of the growth of organised
bodies, and how he made these investigations true models of genuine
inductive enquiry. Here we are concerned only with what he effected
in this way for morphology and systematic botany. In this field of
research he introduced two innovations of the profoundest importance,
which affected both the aim and method of enquiry for some years. He
connected his own morphological investigations, as far as possible,
with the lower Cryptogams, extending them afterwards to the higher
Cryptogams and to the Phanerogams; that is, he proceeded from simple
and plain facts to the more difficult, thus not only introducing the
Cryptogams into the field of systematic investigation, but making
them its starting-point. In this way morphology not only secured a
foundation in exact historical development, but it assumed a different
aspect, inasmuch as the morphological ideas hitherto drawn from
the Phanerogams were now examined by the light of the history of
development in the Cryptogams. This was one innovation; the second,
closely connected with it, was the way in which Nägeli made the new
doctrine of the cell the starting-point of morphology. Both the first
commencement of organs and their further growth were carried back to
the formation of the separate cells; and the remarkable result was to
show, that in the Cryptogams especially, whose growth is intimately
connected with cell-division, precise conformity to law obtains in the
succession and direction of the dividing walls, and that the origin and
further growth of every organ is effected by cells of an absolutely
fixed derivation. The most remarkable thing was, that every stem and
branch, every leaf or other organ has a single cell at its apex, and
that all succeeding cells are formed by division of this one cell
according to fixed laws, so that the origin of all cell-tissue can be
traced back to an apical cell; and as early as the years 1845 and 1846
Nägeli described in the ‘Zeitschrift für wissenschaftliche Botanik’
the three main forms, according to which the segmentation of an apical
cell proceeds, namely, in one, two, and three rows (Delesseria,
Echinomitrium, Phascum, Jungermannia, Moss-leaves). In this way the
separate points in the history of growth in the Cryptogams were brought
out with unusual clearness and decision; but on the other hand, Nägeli
showed in 1844 in the case of a genus of Algae (Caulerpa) that the
growth of a plant may show the usual morphological differentiation
into axis, leaf, and root, when the propagative cell undergoes no
cell-divisions in the process of development and further growth,
and similar conditions were for the first time demonstrated in 1847
in Valonia, Udotea, and Acetabularia. Beside other results it was
established by these facts, that morphological differentiation during
growth must not be regarded as an effect of cell-divisions, and from
such cases as these the conception of the cell experienced a notable
expansion.
Moreover, Nägeli was not satisfied with seeking instructive examples
for general morphological axioms in the lower Cryptogams; he devoted
special study to the Algae for systematic and descriptive purposes;
and his ‘Neuen Algensysteme,’ which appeared in 1847, and ‘Gattungen
einzelliger Algen,’ of 1849, were the first successful attempts to
substitute serious investigation for the mere zeal of the collector in
this part of the vegetable kingdom, which had not indeed been hitherto
neglected, but had not been systematically worked since the time of
Vaucher. In the same spirit Alexander Braun also in his ‘Verjüngung’
contributed a rich material of new observations on the mode of life
of the Algae and the morphological conditions connected with it, and
his labours were followed in the succeeding years by the important
researches of Thuret, Pringsheim, De Bary, and others, to which we
shall recur in a later portion of this history.
But before the examination of the Algae, and soon after of the Fungi
also, led to such great results, the systematic botany of the higher
plants underwent important changes through the methodical study of
the embryology of the Muscineae and Vascular Cryptogams. These groups
had been frequently and carefully examined by good observers since
the last century, and the systematists, without penetrating deeply
into the peculiarities of their organisation, had brought the species
and genera, the families and even the higher divisions into tolerable
order. Comprehensive and methodically arranged catalogues of these
plants had been formed, and attempts had been made to explain their
morphology by that of the Phanerogams; Schmidel[53] published valuable
observations on the Liverworts in the year 1750, Hedwig especially
on the Mosses in 1782; these works were followed by Mirbel’s thorough
examination of Marchantia in 1835, by Bischoff’s of Marchantieae and
Riccieae, by Schimper’s study of the Mosses in 1850, and by Lantzius
Beninga’s[54] contributions to the knowledge of the structure of
the moss-capsule in 1847. The organisation, and to some extent the
germination, of the Vascular Cryptogams had become better known since
1828 through Bischoff’s[55] researches; Unger had as early as 1837
described the spermatozoids in the antheridia of various Mosses, Nägeli
had discovered them on an organ of the Ferns which had up to that time
been taken for the cotyledonary leaf of these plants, and on the same
part of the plant Suminski in 1848 observed the female sexual organs
and the entrance of the spermatozoids into them. The history of the
germination of the Rhizocarps, from which Schleiden thought that he
had proved his erroneous theory of fertilisation with more than usual
certainty, had been examined some years before by Nägeli, and also by
Mettenius, in great detail; here too Nägeli detected the spermatozoids.
Thus important fragments of the life and organisation of these plants
had been described up to the year 1848, but until they were more fully
understood and connected together they had but little scientific value,
the one fact perhaps excepted, that fertilisation in the Cryptogams
as in animals was effected by spermatozoids. A perfect insight into
the embryological conditions in question could only be obtained when
the embryology of the Phanerogams especially had been cleared up, for
according to Schleiden’s theory, which made the pollen-tube enter the
embryo-sac in the ovule and develop into the embryo, the ovule was no
longer to be regarded as a female sexual organ, but only as a place of
incubation for the embryo, which was thus really produced asexually.
This important question was set at rest by WILHELM HOFMEISTER’S work,
‘Die Entstehung des Embryos der Phanerogamen,’ which appeared in 1849.
In this work, and in a series of subsequent treatises, he showed that
the egg-cell is formed in the embryo-sac before fertilisation, and that
it is this which is excited to further development by the appearance
of the pollen-tube, and produces the embryo. Hofmeister had observed
the organisation of the ovule, the nature of the embryo-sac and of
the pollen-grain, and the formation of the embryo from the fertilised
egg-cell step by step and cell by cell, and his account of these
processes was aided by the light which Nägeli’s theory of the cell,
and his reference of all processes of development to the processes of
cell-formation, had thrown upon the history of development. He went
on to apply the same method to the study of the embryology of the
Muscineae and the Vascular Cryptogams, and followed the development
of the sexual organs cell by cell in a large number of species; he
observed the origination of the egg-cell which was to be subsequently
fertilised, and the formation of spermatozoids, and above all he
showed the divisions which take place in the fertilised egg-cell,
and the relation of its segments to the further growth of the sexual
product in course of formation. The whole course of development in the
Muscineae and Vascular Cryptogams displayed a return twice repeated to
the single cell as the starting-point in each case of a new phase of
development; the true relation between the asexually produced spore and
its germ-product on the one side, and the sexually generated embryo on
the other, and their significance in the history of development, were
brought out clearly by Hofmeister’s investigation, while the exactness
of his method rendered lengthy discussions on the subject unnecessary.
With these embryological processes, especially those of the Rhizocarps
and Selaginellae, in which the presence of two kinds of spores was
now for the first time correctly interpreted, Hofmeister compared the
embryology of the Conifers, and by their aid that of the Angiosperms
also.
The results of the investigations published in the ‘Vergleichende
Untersuchungen’ in 1849 and 1851 were magnificent beyond all that has
been achieved before or since in the domain of descriptive botany;
the merit of the many valuable particulars, shedding new light on
the most diverse problems of the cell-theory and of morphology, was
lost in the splendour of the total result, which the perspicuity of
each separate description revealed to the reader before he came to
the conclusion of the work, and there a few words in plain and simple
style gave a summary of the whole. Briefly to describe this result
in all its importance for botanical science is a difficult task; the
idea of what is meant by the development of a plant was suddenly and
completely changed; the intimate connection between such different
organisms as the Liverworts, the Mosses, the Ferns, the Equisetaceae,
the Rhizocarps, the Selaginellae, the Conifers, the Monocotyledons,
and Dicotyledons could now be surveyed in all its relations with
a distinctness never before attained. Alternation of generations,
lately shown to exist though in quite different forms in the animal
kingdom, was proved to be the highest law of development, and to
reign according to a simple scheme throughout the whole long series
of these extremely different plants. It appeared most clearly in the
Ferns and Mosses, though at the same time with a certain difference in
each; in the Ferns and allied Cryptogams a small inconspicuous body
grows out of the asexually produced spore, and immediately produces
the sexual organs; from the fertilisation of these organs proceeds
the root-bearing and leafy stem of the Fern, which in its turn again
produces only asexual spores. In the Muscineae, on the other hand, a
much differentiated and usually long-lived plant is developed from the
spore, and this plant proceeds again after some time to form sexual
organs, the product of which is the so-called Moss-plant. The first
generation that arose from the spore, the sexual, is in the Muscineae
the vegetative plant, while in the Ferns and their allies the whole
fulness of vital activity and of morphological differentiation is
unfolded in the second generation which is sexually produced. Here
all was at once clear and obvious: but Hofmeister’s researches also
showed that the same scheme of development holds good in the Rhizocarps
and Selaginellae where two kinds of spores are formed; and it appeared
plainly from their case that the recognition of the true relation
between the production of spores and sexual organs is the guide to the
morphological interpretation. When the processes in the large female
spore of the most perfect of the Cryptogams was known, the formation
of the seeds in the Conifers was at once understood; the embryo-sac in
these answered to this large spore, while the endosperm represented
the prothallium, and the pollen-grain the microspore; the last trace
of alternation of generations, so obvious in the Ferns and Mosses, was
seen in the formation of the seed in the Phanerogams. The changes,
which the alternation of generations passes through from the Muscineae
upwards to the Phanerogams, were, if possible, still more surprising
than the alternation of generations itself.
The reader of Hofmeister’s ‘Vergleichende Untersuchungen’ was presented
with a picture of genetic affinity between Cryptogams and Phanerogams,
which could not be reconciled with the then reigning belief in the
constancy of species. He was invited to recognise a connection
of development which made the most different things appear to be
closely united together, the simplest Moss with Palms, Conifers, and
angiospermous trees, and which was incompatible with the theory of
original types. The assumption that every natural group represents an
idea was here quite out of place; the notion entertained up to that
time of what was really meant by the natural system had to be entirely
altered; it could as little pass for a body of Platonic ideas as for a
mere framework of conceptions. But the effect of the work was great in
respect to the system also; the Cryptogams were now the most important
objects in the study of morphology; the Muscineae were the standard by
which the lower Cryptogams must be tried, the Ferns were the measure
for the Phanerogams. Embryology was the thread which guided the
observer through the labyrinth of comparative and genetic morphology;
metamorphosis now received its true meaning, when every organ could
be referred back to its parent-form, the staminal and carpellary
leaves of the Phanerogams, for example, to the spore-bearing leaves
of the Vascular Cryptogams. That which Häckel, after the appearance
of Darwin’s book, called the phylogenetic method, Hofmeister had
long before actually carried out, and with magnificent success. When
Darwin’s theory was given to the world eight years after Hofmeister’s
investigations, the relations of affinity between the great divisions
of the vegetable kingdom were so well established and so patent, that
the theory of descent had only to accept what genetic morphology had
actually brought to view.
So gorgeous a picture as Hofmeister had designed of the genetic
connection of the vegetable kingdom, except the Thallophytes, could
not possibly be completely perfect and correct in all its separate
features; there were still many gaps to fill up and particular
observations to correct. Hofmeister himself continued his labours; the
remarkable genera Isoetes and Botrychium were in the following years
more carefully studied by himself, the fertilisation and embryology of
the Equisetaceae by himself and Milde, and those of Ophioglossum by
Mettenius, and all were fitted into their place in the system. To the
present day it is always a profitable task to submit the different
forms of the Muscineae, the Vascular Cryptogams, and the Gymnosperms
to exact investigation in order to ascertain all the details in the
process of development in these plants, the formation of the embryo,
the succession of cells at the apex, the first appearance and further
growth of the lateral organs; and the more careful the observation, the
more clearly even to its farthest results does the correctness of the
alternation of generations asserted by Hofmeister everywhere appear. It
does not fall within the limit of this history to pursue the subject
further, and to show how the doctrine of alternation of generations
and the knowledge of the morphology of the Cryptogams were further
advanced by later and distinguished researches, such as those of Cramer
on the Equisetaceae, of Pringsheim on Salvinia (1862), of Nägeli and
Leitgeb on the formation of roots in the Cryptogams, of Hanstein on the
germination of the Rhizocarps, and of others.
THALLOPHYTES.
The method of investigation which starts from the first steps towards
the formation of the embryo before and after fertilisation, and
follows the advancing segmentation and growth through all the stages
of development up to the final completion of the embryo-plant, has
led since 1850 in the case of the Muscineae, Vascular Cryptogams, and
Phanerogams to great certainty in the morphological explanation of
the organs, while the determination of affinities has ceased to be
arbitrary and insecure; the way was now known which would lead to the
desired end, whenever it was sought to establish the affinities of a
genus of Cryptogams or of the larger groups of Phanerogams; the day
of ingenious guessing and trying was over; the only plan was patient
investigation, and this always yielded a result of lasting value.
The case was quite different with the Thallophytes still in 1850;
what was certainly known about them only showed how uncertain the
rest was; the Algae, Fungi, and Lichens presented a chaotic mass of
obscure forms in contrast with the well-ordered knowledge of the
Muscineae and Vascular plants. In the Mosses and Ferns the series of
developments within the limits of the species was so set forth in its
several stages, that all the important points in the advancing growth
were clearly ascertained, while the alternation of generations at once
sharply distinguished and connected together the chief sections in the
development; on the other hand the development of the Algae and Fungi
seemed to break up into a disorderly and motley throng of forms that
appeared and disappeared, and it seemed scarcely possible to discover
their regular genetic connection. Here the important point was to
determine which of the known forms belonged to one and the same cycle
of development, for these plants go back at the most various stages of
development to the segregation of single cells, which are the beginning
of a new development either repeating or carrying on the old one. The
beginnings of the most different species of Algae lay mixed up together
in the same drop of water, those of quite different Fungi grew together
and even upon one another on the same substratum; in the Lichens,
Fungus and Alga were united together. Such was the case with the small
and microscopic species; the large Seaweeds, the Mushrooms, and the
large Lichens were easier to distinguish specifically, but less if
possible was known of their development than of that of the microscopic
Thallophytes.
Nevertheless the knowledge of individual forms in these organisms had
been considerably extended before 1850. Collectors and amateurs, intent
only on determining what is immediately presented to the eye and making
little enquiry into origin and affinities, were indefatigable in adding
to their collections, and made catalogues and proposed various systems
founded on external marks taken at pleasure. The names of species were
counted by thousands, their characters filled thick volumes and the
figures large folios; the abundance of forms in the Thallophytes proved
to be so great that many botanists devoted their whole attention to
them, many collected and described only the Algae, others only the
Fungi and Lichens. It is true that a deeper insight into the connection
of these forms of life with one another and with other plants was not
to be obtained in this way; still an empirical basis was formed for
a knowledge of the Cryptogams, such as had been established for the
Phanerogams by the herbals of the 17th century. All forms open to
observation were named and arranged in one way or another; and there
was no difficulty in understanding what form was meant, when names, or
tables and figures, were cited from the various books. Of such works,
those of Agardh[56], Harvey, and Kützing on the Algae, those of Nees
von Esenbeck[57], Elias Fries, Léveillé, and Berkeley on the Fungi, and
especially Corda’s elaborate work on the latter plants are the most
valuable.
The views entertained on the subject of the development and propagation
of the lower Cryptogams down to the year 1850 were very uncertain
and fluctuating. In some Algae, Fungi, and Lichens certain organs of
multiplication and propagation were known, in others they were quite
unknown; some forms appeared in places and under circumstances which
seemed to necessitate the assumption of spontaneous generation; in 1827
Meyen declared that the small Algae, known as ‘Priestley’s matter,’
which are formed in stagnant water and even in closed vessels, are
produced by free generation, and Kützing endeavoured to show this by
experiment in 1833; some Fungi were regarded as diseased growths from
other organisms, many were supposed to spring up spontaneously, though
they might be capable at the same time of propagating themselves by
spores; this view was shared by even the best botanists with regard to
the most simple Fungi up to 1850. But the systematic investigation of
the Algae and Fungi was as little hindered by the notion of spontaneous
generation after 1850 as that of Phanerogams had been in the 17th
century by the same notion; it was however at first affected by the
view put forth by Hornschuch in 1821 and by Kützing in 1833, that the
simplest of all Alga-cells (Protococcus and Palmella), once produced
spontaneously, could develop according to circumstances into a variety
of Algae, and even of Lichens and Mosses; as some observers even
now consider Penicillium and Micrococcus to be the starting-points
of very different Fungi. There was a difficulty also in drawing the
boundary-line between the lower animals and plants; the difficulty was
solved by classing all objects capable of independent movement with
animals; thus whole families of Algae (the Volvocineae, Bacillariaceae,
and others) were claimed by the zoologists, and when the swarmspores
of a genuine Alga were seen for the first time in the act of escaping,
the phenomenon was described as the changing of the plant into an
animal. Trentepohl in 1807, and Unger in 1830, explained in this way
the escape of the zoospores of Vaucheria. The remarkable thing is, not
that such views were entertained, but that the majority of botanists
combined with them a belief in the constancy of species. But this
dogma rendered good service to the science in this instance, for the
botanists, who at a later time applied themselves to the systematic
examination of the Algae and Fungi, confided in the constancy of the
processes of development in each species, which they expected would
assert itself in these forms as in the Mosses and higher plants.
With much that was obscure and doubtful, the result of occasional
observation accompanied by uncritical interpretation, the literature
of the subject had contained for some time a certain number of single
well-established facts of real importance, which were well adapted
to serve as starting-points for earnest and exact investigation.
Among the Algae the genera Spirogyra and Vaucheria especially had
supplied remarkable phenomena; Joseph Gärtner observed the formation
of zygospores in Spirogyra in 1788, Hedwig saw in the mode of their
production at least a suggestion of sexuality (1798), and Vaucher[58],
in his ‘Histoire de Conferves d’eau douce,’ which appeared in 1803
and was far in advance of its time, called conjugation distinctly a
sexual process; the optical means at his disposal did not enable him
to observe the fertilisation in Vaucheria (Ectosperma), which was
named after him, though he described the sexual organs accurately;
the movement also of the zoospores in this genus escaped him, and
Trentepohl first observed their escape and swarming in 1807[59].
Vaucher had also observed the formation of new nets in the old cells
of Hydrodictyon, and Areschoug repeated the observation in 1842, when
he saw the swarming of young cells in the old ones. Bischoff, as early
as 1828, saw the spermatozoids of Chara, though without understanding
them. Observations on conjugating Algae were multiplied; Ehrenberg in
1834 saw corresponding phenomena in Closterium, and Morren described
them more exactly in 1836. The formation of swarmspores in fresh-water
and salt-water Algae was frequently observed between 1820 and 1830,
and in his ‘Neues System,’ iii, which appeared in 1839, Meyen gave a
summary of all that was known up to that time of the propagation of
the Algae. But a new aspect was given to the knowledge of the Algae by
those researches of Nägeli between the years 1844 and 1849, which have
been already mentioned, and which are the first since Vaucher’s time
that can be regarded as systematic. Nägeli studied especially the laws
of cell-division in sexual multiplication and growth, but he considered
the Florideae to be the only Algae that were sexually differentiated,
and distinguished the rest as being without sexuality. Braun in his
‘Verjüngung’ (1850) made numerous contributions to the biology of the
fresh-water Algae, affording many and most interesting glimpses into
a connection still little understood between these forms; and in 1852
he gave an account of the history of growth in the Characeae, a work
conceived in Nägeli’s spirit and a model of scientific research, in
which the mode of derivation of every cell from the apical cell of
the stem was shown, the sexual organs were minutely examined, and
the relation established between the direction of the ‘streaming’ of
the cell-contents and the morphology of the organs. Gustav Thuret
had already made the zoospores of the Algae the subject of detailed
examination.
Such was the condition of affairs with respect to the Algae about the
year 1850, when Hofmeister made the formation of the embryo in the
Phanerogams, the Vascular Cryptogams, and the Muscineae the central
point of investigation in morphology and systematic botany. He made
it clear that a perfect insight into the whole cycle of development
in the plant and into its affinities can only be obtained, if we
succeed in making its sexual propagation, the first commencement of
the embryo, the starting-point of the investigation. It was natural
to expect as happy results from the embryology of the Algae, as had
been obtained in the case of the higher plants; it was important
therefore, that the observer should no longer rest satisfied with a
knowledge of the sexual multiplication of the Algae; he must enquire
into their asexual propagation, and by its aid discover the complete
history of their development. Former observations suggested the
probability that here too sexual propagation is the prevailing rule;
but it was easy to foresee that it would be a task of great labour
to make out a connected history of development, a task of which the
collectors who liked to call themselves systematists had never formed
a conception; but Nägeli’s and Hofmeister’s researches had made
botanists familiar with the highest demands of this kind, and the
men who were to gain new conquests for genuine science were already
engaged in the work in 1850. A splendid result appeared in 1853, in
Thuret’s account of the fertilisation of the genus Fucus; this was a
simple process as a matter of embryology; but the sexual act was so
clear, and even open to experimental treatment, that it threw light
at once upon other cases more difficult to observe. Then followed
discoveries of sexual processes in rapid succession; Pringsheim solved
the old enigma in Vaucheria in 1855, and between 1856 and 1858 in the
Oedogonieae, Saprolegnieae and Coleochaetae; in 1855 Cohn observed
the sexual formation of spores in Sphaeroplea. Pringsheim however was
not content with carefully observing the sexual act; he gave detailed
descriptions of growth in the same families in its progress cell by
cell, of the formation of the sexual organs, and the development of
the sexual product. The asexual propagations which are intercalated
into the vegetation and embryology were shown in their true connection.
Processes were recognised which often recalled the alternation of
generations in the Muscineae; it was shown that very different
forms of sexuality and of general development occur in the Algae,
and these led to the formation of systematic groups, quite different
from those founded on the superficial observation of collectors. It
soon appeared in the Algae, and later in the Fungi and Lichens, that
special investigation must lay new foundations for the system. From
the confused mass of forms not before understood, Pringsheim brought
out a series of characteristic groups, which, thoroughly examined and
skilfully described in words and by figures, stood out as islands in
the chaotic sea of still unexamined forms, and threw light in many ways
on all around them. In like manner the morphology of the Conjugatae
was thoroughly examined by De Bary before 1860; fragments of the
history of development in the Algae were added by Thuret, and he and
Bornet cleared up the remarkable embryology of the Florideae in 1867,
while Pringsheim established the pairing of the swarmspores in the
Volvocineae in 1869. The Algae offer at present a greater variety in
the processes of development than any other class of plants; sexual and
asexual propagation and growth work one into the other in a way which
opens entirely new glimpses into the nature of the vegetable world.
The old conceptions of the nature of plants had been greatly modified
by Hofmeister’s discovery of the alternation of generations, and the
reduction to it of the formation of the seed in Phanerogams; in like
manner the first beginnings of plant-life, the simplest forms of
Algae, exhibit phenomena, which compel us to revise our fundamental
conceptions of morphology, if we are ever to be able to give a
systematic view of the whole vegetable kingdom.
The methodical examination of the Fungi after 1850 led to similar but
still more comprehensive results. From earliest times the Fungi had
been objects of wonder and superstition; what Hieronymus Bock said of
them has been told in the first chapter; this was repeated by Kaspar
Bauhin, and similar notions existed till late into our own century;
about the middle of the 17th century Otto Von Münchausen thought that
mushrooms were the habitations of Polypes, and Linnaeus assented to
that view. What the nature-philosophers, as Nees von Esenbeck for
instance, had to say on the nature of Fungi, need not be reproduced
here.
Still some useful observations had been accumulating for some time on
this subject; as early as 1729 Micheli[60] had collected the spores
of numerous Fungi, had sown them and obtained not only mycelia but
also sporophores (fructifications), and Gleditsch confirmed these
observations in 1753; Jacob Christian Schaeffer[61] about the year
1762 published very good figures of all the Fungi of Bavaria and the
Palatinate, and collected the spores of many species. Yet Rudolphi
and Link at the beginning of the present century ventured to deny the
germination of the spores of Fungi; Persoon in 1818 thought that some
Fungi grow from spores, others from spontaneous generation. A decided
improvement appears after 1820 in the views of botanists with respect
to Fungi, and to this Ehrenberg’s elaborate essay, ‘De Mycetogenesi,’
published in that year in the Leopoldina, contributed greatly. In that
work he collected together all that was then known on the nature and
propagation of the Fungi, and communicated observations of his own
on spores and their germination; he gave figures also of the course
of the hyphae in large sporophores and in other parts, but his most
important service was a description of the first observed case of
sexuality in a Mould, namely, the conjugation of the branches of
Syzygites. In the same year Nees von Esenbeck sowed Mucor stolonifer
on bread, and obtained ripe sporangia in three days (Flora, 1820, p.
528); Dutrochet proved in 1834 (Mém. ii. p. 173) that the larger Fungi
are only the sporophores of a filiform branching plant, which spreads
usually under ground or in the interstices of organic substances, and
had been till that time regarded as a peculiar form of Fungus under
the name of Byssus. Soon after, Trog (Flora, 1837, p. 609) carried
these observations further; he distinguished the mycelium from the
sporophore, and pointed out that the former is often perennial and is
the first product of the germinating spores. He made an attempt to
examine the morphology of the larger sporophores, and showed that it
was possible to collect the spores of mushrooms on paper, and that
those of Peziza and Helvella are forcibly ejected in little clouds
of dust; he also produced new proofs of Gleditsch’s statement, that
the spores of Fungi are disseminated everywhere by the air. Schmitz
published in ‘Linnaea,’ between the years 1842 and 1845 excellent
observations on the growth and mode of life of several of the larger
Fungi. It was not unnecessary at that time to make it clearly
understood that the spores of Fungi reproduce their species exactly.
But the lower, the small and simple Fungi, those especially which are
parasitic on plants and animals, were the most attractive objects in
the whole field of mycology. Here were difficulties in abundance, here
were the darkest enigmas with which botany has ever had to deal, here
was new ground to be slowly won by extreme scientific circumspection
and foresight. In these forms, as in the Algae, the first thing to
be done was to make out the complete history of development in a few
species; but it was much more difficult in the Fungi than in the Algae
to discover what properly belonged to one cycle of development, and
to separate it from casual phases of development of other associated
Fungi. The merit of first breaking ground in this direction belongs
to the brothers Tulasne, who published before 1850 the first more
exact researches into the Smuts and Rusts; these were followed by a
long series of excellent works on different forms of Fungi, especially
the subterranean, whose mode of life and anatomy were described and
illustrated by splendid figures; but their account of the development
of Ergot of rye (1853), their further investigations into the
formation of the spores and the germination of Cystopus, Puccinia,
Tilletia, and Ustilago, and their discovery of the sexual organs in
Peronospora before 1861, were of greater theoretical importance.
The ‘Selecta Fungorum Carpologia,’ which appeared in three volumes
from 1861 to 1865 with fine figures, some of which represented the
process of development, contributed greatly to the reformation of
mycology. Meanwhile, Cessati had published investigations into the
Muscardine-fungus of the silkworm-caterpillar, and Cohn into a
remarkable Mould, the Pilobolus.
But mycology owes its present form to none more than to ANTON DE BARY,
whose writings, the fruit of twenty years’ labour, it would take too
much space to enumerate one by one. With a correct understanding of
the only means which can lead to sure results in this difficult branch
of study, De Bary made it his first endeavour to perfect the methods
of observation, and not only sought for the stages of development of
the lower Fungi in their natural places of growth, but cultivated them
himself with all possible precautions, and thus obtained complete and
uninterrupted series of developments. By these means he succeeded in
proving that parasitic Fungi make their way into the inside of healthy
plants and animals, and that this is the explanation of the remarkable
fact, that Fungi live in the apparently uninjured tissue of other
organisms, a fact which formerly had led to the supposition that such
Fungi owe their origin to spontaneous generation, or to the living
contents of the cells of their entertainers. Pringsheim had already
observed these occurrences in 1858 in the case of an unusually simple
water-fungus (Pythium). De Bary showed that the intrusive parasite
vegetates inside the plant or animal which is its host, and afterwards
sends out its organs of propagation into the open air, and that at
a given time the organism attacked by the fungus sickens or dies.
These investigations were not only of high scientific interest to
the biologist, but they produced a series of results of the greatest
importance to agriculture and forestry, and even to medicine.
With the Fungi, even more than with the Algae, the chief difficulty in
making out a complete series of developments in the history of each
species arose from the frequent intercalation of the asexual mode of
multiplication into the course of its development, and in the further
peculiarity, that the several stages of development in some cases could
only be completed on different substrata. One of the most important
tasks was to find the sexual organs, the existence of which was
rendered probable by various analogies, and after De Bary had observed
the sexual organs in the Peronosporeae in 1861, he succeeded in 1863
proving for the first time that the whole fruit-body of an Ascomycete
is itself the product of a sexual act, which takes place on the threads
of the mycelium.
The literature of mycology based on De Bary’s methods of observation
and its actual results has been enriched by others also in various
directions since 1860; in the case of the Fungi, as in that of the
Algae, it is not possible yet to see to what results investigation
will ultimately lead; but it is one of the fairest fruits of strictly
inductive method, that it has succeeded in smoothing this thorny
and indeed perilous route, where the enquirer is constantly in
danger of being misled, and in satisfying the severest demands of
science. Conclusions have been already reached that are important for
morphology and systematic botany, and among these the establishment
of the nature of the large sporophores, and of processes similar to
the alternation of generations in the higher Cryptogams should be
especially mentioned. But the most important result remains to be
told; it is, that the two classes of Algae and Fungi, hitherto kept
strictly separate, must obviously be now united, and an entirely
new classification adopted, in which Algae and Fungi recur as forms
differing only in habit in various divisions founded on their
morphology[62].
A few words must be given here to the Lichens. They are the division
of the Thallophytes, whose true nature was last recognised, and
that only in modern times; till after 1850 scarcely more was known
of their organisation than Wallroth had discovered in 1825[63],
namely, that green cells, known as gonidia are scattered through the
fungus-like hyphal tissue of the thallus. After Mohl’s investigations
in 1833, it was known that free spores were formed in the tubes of
the fructifications (apothecia), and that a dust collected from the
thallus and consisting of a mixture of gonidia and hyphae was in a
condition to propagate the species. The genetic relation between
the chlorophyll-containing gonidia and the fungus-like hyphae long
continued to be obscure, till at last, after 1868, it was shown that
the gonidia are true Algae, and the hyphal tissue a genuine Fungus, and
that therefore the Lichens are not a class co-ordinating with the Algae
and Fungi, but a division of Ascomycetes, which have this peculiarity,
that they spin their threads round the plants on which they feed, and
take them up into their tissue. De Bary suggested this explanation, but
it was Schwendener who adopted it without reserve and openly declared
it, as much to the surprise as the annoyance of Lichenologists. It may
be foreseen that their opposition will yield to the weight of facts,
which already leave no doubt in the minds of the unprejudiced.
Thus researches in the domain of the Thallophytes have led during the
last twenty years to a complete revolution in the views entertained
with respect to the nature of these organisms, and enriched botany with
a series of surprising achievements; and the movement there is still
far from having come to an end. But we must regard it as one of the
great results for the whole science that through the examination of the
lower and higher Cryptogams, morphology and systematic botany have been
rescued from many ancient prejudices, that the survey has become freer,
the methods of investigation surer, the questions more clearly seen and
put in more definite form.
SECOND BOOK
HISTORY OF VEGETABLE ANATOMY
(1671-1860)
INTRODUCTION.
That the substance of the more perfect plants consists of layers of
different constitution was a fact that could not escape the most
untutored observation in primitive times; ancient languages had still
words to designate the most obvious anatomical components of plants,
rind, wood and pith. It was also easy to perceive that the pith
consists of an apparently homogeneous succulent mass, the wood of a
fibrous substance, while the rind of woody plants is composed partly
of membranous layers, partly of fibrous and pith-like tissue. The
obtaining of threads for spinning from the rind of the flax-plant,
for instance, must have suggested some idea, if only a vague one, in
the earliest ages of the way, in which the fibrous could be separated
from the pulpy part of the bark by decay and mechanical treatment.
Neither Aristotle nor Theophrastus failed to compare these components
of vegetable substance with corresponding ones in animal bodies, and
it has been already shown in the first book how Cesalpino, following
his masters, took the pith for the truly living part of the plant and
the seat of the vegetable soul, and applied this idea in his morphology
and physiology. He remarked that the root generally has no pith, and
that the part of the root which answers to the wood of the stem is
often soft and fleshy; the composition of the leaves from a green and
succulent substance and strands of fibres at once suggested a certain
resemblance to the green rind of the stem; and it was evidently this
which led him to consider that not only the leaves, but also the
leaf-forms of the flower-envelopes had their origin in the rind of
the stem, while the soft, pulpy, succulent condition of the unripe
seeds and seed-vessels seemed to point to their identity with the pith.
That not only are juices contained in plants, but that they must move
in them, could not escape the simplest reflection; and further, the
bleeding of the vine, the flow of gum from resiniferous trees, the
gushing of a milky juice from the wounds of certain plants, exhibited
so striking a resemblance to the bleeding of a wound in the body of
an animal, that the idea of canals inside the plant, which, like the
veins in animals, contain those juices and set them in motion, appeared
quite natural, as we see plainly from Cesalpino’s reflections on these
structural conditions. If we add that it was known that the seeds are
enclosed in the fruits, and that the embryo, together with a pulpy mass
(cotyledons and endosperm), are in their turn enclosed in the seed,
we have pretty well the whole inventory of phytotomic knowledge up to
about the middle of the seventeenth century.
With careful preparation and skilful dissection of suitable parts of
plants, and attentive consideration of the changes produced by decay
and corruption, anatomical knowledge might have been considerably
enlarged at an earlier time; but seeing is an art that must be learnt
and cultivated; a definite aim and end must stimulate the observer into
willingness to see exactly, and to distinguish and connect together
correctly what he sees. But this art of seeing was not far advanced in
the middle of the 17th century. All that was achieved in this direction
did not go beyond the distinguishing the outer organs of leaf-forms and
stem-forms, and we have seen in the first book how unsuccessful was the
attempt to distinguish the minuter parts of the flower and fruit.
The invention of the microscope made small things seem large, and
revealed to sight what was too small to be seen without it; but the
use of magnifying glasses brought an advantage with it of a different
kind—it taught those who used them to see scientifically and exactly.
In arming the eye with these increased powers the attention was
concentrated on definite points in the object; what was seen was to
some extent indistinct, and always only a small part of the whole
object; perception by means of the optic nerve had to be accompanied
by conscious and intense reflection, in order to make the object,
which is observed in part only with the magnifying glass, clear to
the mental eye in all the relations of the parts to one another and
to the whole. Thus the eye armed with the microscope became itself a
scientific instrument, which no longer hurried lightly over the object,
but was subjected to severe discipline by the mind of the observer
and kept to methodical work. The philosopher Christian Wolff observed
very truly in 1721, that an object once seen with the microscope can
often be distinguished afterwards with the naked eye; and this, which
is the experience of every microscopist, is sufficient evidence of
the effect of the instrument in educating and training the eye. This
remarkable fact appears also in another way. We saw in the history
of morphology and systematic botany that botanists for a hundred
years scarcely attempted to make themselves masters in a scientific
sense of the external and obvious relations of form in plants, and to
consider them from more general points of view. Jung was the first who
applied systematic reflection to the morphological relations of plants
which lay open before his eyes, and it was not till late in our own
century that this part of botany was again handled in a scientific
and methodical manner. This extremely slow progress in obtaining a
mental mastery over external form in plants on the part of those
who are continually occupied with them appears to be due chiefly to
the fact, that the unassisted eye glances too impatiently over the
form of the object, and the attention of the observer is disturbed
by its hasty movements. In direct contrast to this customary want
of thoughtfulness in contemplating the external form of plants, we
find the first observers with the microscope, Robert Hooke, Malpighi,
Grew, and Leeuwenhoek in the latter half of the seventeenth century,
endeavouring by earnest reflection to apply the powers of the mind
to the objects seen with the assisted eye, to clear up the true
nature of microscopic objects, and to explain the secrets of their
constitution. If we compare the works of these men with the utterances
of the systematists of the same period on the relations of form in
plants, we cannot fail to see how superior the matter of the former is
in intellectual value. This appears most strikingly when we put what
Malpighi and Grew tell us of the construction of the flower and fruit
side by side with the knowledge of Tournefort, Bachmann, and Linnaeus
on the same subject.
This enhancement of the mental capacity of the observer by the
microscope is however the result of long practice; the best microscope
in unpractised hands is apt soon to become a tiresome toy. It would be
a great mistake to suppose that progress in the study of the anatomy
of plants has simply depended on the perfecting of the microscope. It
is obvious that the perception of anatomical objects must grow more
distinct as the magnifying power of the instrument is increased, and
the field of sight is made brighter and clearer, but these things by
themselves would not add much to real knowledge. In examining the
structure of plants, as in every science, it is necessary to work with
the mind upon the object seen with the eye of sense, to separate the
important from the unimportant, to discover the logical connection
between the several perceptions, and to have a special aim in the
examination; but the aim of the phytotomist can only be to obtain so
clear an idea of the whole inner structure of the plant in all its
connections, that it can be reproduced by the imagination at any moment
in full detail with the perfect distinctness of sense-perception. It is
not easy to attain this end because the more the microscope magnifies,
the smaller is the part of the whole object which it shows; skilful
and well-considered preparation is required, careful combination of
different objects and long practice. The history of phytotomy shows
how difficult a task it is to combine the separate observations and
to fashion what has been seen bit by bit into a clear and connected
representation.
It appears then that progressive improvement of the microscope was
not in itself sufficient to ensure the advance of phytotomy. It would
not indeed be too much to say, that the progress which microscopic
anatomy made step by step with the aid of imperfect instruments
repeatedly gave the impulse to energetic efforts to improve them. Only
practical microscopists could tell where the real defects of existing
instruments lay; it was their anxiety to make them more manageable,
their constant complaints of the poor performance of the optical
part—complaints loudly expressed, especially at the end of the previous
and the beginning of the present century, which urged the opticians
to turn their attention to the microscope and to endeavour to make it
more perfect. Moreover, essential improvements in the instrument were
made by microscopists themselves. Thus Robert Hooke was the first who
in 1760 gave the compound microscope a form convenient for scientific
observation, and Leeuwenhoek developed the powers of the simple
microscope to their highest point. The modern microscope is greatly
indebted for its perfectness to Amici; nor ought the name of von Mohl
to be omitted here, who invented improved methods for microscopic
measurement, and in his work ‘Mikrographie’ (1846) on the construction
of the microscope gave many practical hints to the opticians.
We shall not then make the most important advances in the anatomy of
plants depend as a matter of course and quite passively on the history
of the microscope; they were determined here as in other parts of
botany by a logical necessity of their own; here as elsewhere we have
to fix our eye on the objects pursued by successive enquirers. If for
this purpose we cast a glance over the history of the subject, it
will appear that its founders in the latter half of the seventeenth
century, Malpighi and Grew, were chiefly bent on determining the
connection between the cellular and fibrous elements in the structure
of plants. Two fundamental forms of tissue were assumed from the first,
the succulent cellular tissue composed of chambers or tubes, and, in
contrast to this, the elongated usually fibrous or tubular elementary
organs, the distinction of which into open canals or vessels and
fibres with closed ends continued to be doubtful. The characteristic
feature of this period is, that the investigation of the more delicate
structure is everywhere closely interwoven with reflections on the
function of the elementary organs, and that thus anatomy and physiology
support each other, but not without mutual injury through the
imperfections of both. But the physiological interest far outweighed
the anatomical with the first phytotomists, who used anatomical
research for the purposes of physiology.
The imperfectness of the microscope during the whole of the eighteenth
century produced a certain disinclination to anatomical studies, which
were after all only regarded as auxiliary to physiology. The latter
had made very important progress without the help of anatomy in the
hands of Hales, and later on towards the end of the 18th century in
those of Ingen-Houss and Senebier, and thus the interest in phytotomy
was almost extinguished. Not only was very little addition made to the
contributions of Malpighi and Grew during the 18th century, but they
had to some extent ceased to be understood.
However towards the end of that time the microscope came again into
fashion; in the compound form it had become somewhat more convenient
and manageable; Hedwig showed how it revealed the organisation of the
smallest plants, and especially of the Mosses, and he examined also
the construction of cell-tissue and vascular bundles in the higher
plants. But with the beginning of the present century the interest in
phytotomy suddenly rose high again; Mirbel in France, Kurt Sprengel in
Germany made the microscopic structure of plants once more the subject
of serious investigation. The performances of both men were at first
extremely weak and contradicted one another; a lively dispute on the
nature of cells, fibres, and vessels grew up during the succeeding
years, and many German botanists soon took part in it; life was once
more infused into the whole subject, especially when the academy of
Göttingen in 1804 offered a prize for the best essay on the disputed
points, for which Link, Rudolphi and Treviranus contended, while
Bernhardi occupied himself with private researches into the nature
of vessels in plants. It was not much that was attained in this way;
men began once more from the beginning, and after 130 years Malpighi
and Grew were still the authorities to whom everybody appealed. Yet
the questions now discussed were in the main different from the old
ones; Malpighi, Grew and Leeuwenhoek had chiefly set themselves the
task of studying the different tissues in their mutual connection; the
moderns were chiefly concerned to get a clearer understanding of the
more delicate construction of the various tissues themselves, to know
what was the true account of cell-structure in parenchymatous tissue,
and the real nature of vessels and fibres. That very slow progress was
made at first in this direction was due partly to the imperfectness
of the microscope, and still more to very unskilful preparations, to
the influence of various prejudices, and to too slight exertion of the
mind. But a comprehensive work by the younger Moldenhawer in 1812 was
a considerable step in advance. It is marked by careful and suitable
preparation of the objects, and by critical examination of what was
observed by the writer himself and of what had been written by others;
in fact it is a fresh commencement of a strict scientific treatment
of phytotomy. Hugo von Mohl continued Moldenhawer’s work after 1828,
and Meyen was a contemporary and a zealous student of phytotomy; but
the period in the study of vegetable anatomy which reaches to 1840 may
be said to have been brought to a conclusion chiefly by von Mohl’s
contributions. Weak as the beginnings were at the commencement of this
period (1800-1840), and important as was the advance made by von Mohl
towards the end of it, yet we may include all that was done during that
time in one view, since the questions examined were essentially the
same; like Mirbel and Treviranus, Moldenhawer and Meyen, von Mohl was
chiefly occupied up to the year 1840 in deciding the questions, what
is the nature of the solid framework of cellulose in the plant in its
matured state, whether a single or double wall of membrane lies between
two cell-spaces, what is the true account of pits and pores, and of the
various forms of fibres and vessels; one great result of these efforts
must be mentioned, namely, the establishment of the fact that all the
elementary organs of plants may be referred to one fundamental form,
the closed cell; that the fibres are only elongated cells, but that
true vessels are formed by cells which are arranged in rows, and have
entered into free communication with one another.
Phytotomists before 1840, and von Mohl especially, had occasionally
paid attention among other things to circumstances connected with the
history of development, and single cases of the formation of various
cells had been described by von Mohl and Mirbel between 1830 and
1840, but greater interest was taken in the right understanding of
the structure of mature tissues; physiological questions also, though
no longer of the first importance in anatomical investigations, were
still of weight, so far as the enquiry was influenced by the relation
of anatomical structure to the functions of elementary organs. But
with Schleiden and Nägeli the question of historical development and
the purely morphological examination of interior structure assumed
an exclusive prominence in phytotomy. The first commencement of
vegetable cells especially and their growth were the subjects now
discussed. Schleiden had proposed a theory of cell-formation before
1840, which, resting on too few and inexact observations, referred
all processes of cell-formation in the vegetable kingdom to a single
form; it attracted great attention in the botanical world, but could
not easily be reconciled with what was already known; and in 1846 it
was completely refuted by Nägeli, who substituted for it the history
of the formation of the various kinds of vegetable cells in their
main features, based on profound and extensive investigations. It was
natural that these researches into the formation of cells should turn
the attention of observers, which had hitherto been almost exclusively
devoted to the solid framework of cell-tissue, to the juicy contents of
cells. Robert Brown had already discovered the cell-nucleus; Schleiden
recognised its more constant presence, but misunderstood its relation
to cell-formation; Nägeli and von Mohl next demonstrated the peculiar
nature of protoplasm, the most important component of vegetable cells,
and especially the weighty part which it plays in their origination.
Unger in 1855 called attention to the great resemblance which exists
between the protoplasm of the vegetable cell and the sarkode of the
more simple animals,—a discovery which was subsequently brought into
prominence by observations on the behaviour of the Myxomycetes,
and after 1860 finally led zootomists as well as phytotomists to
the conclusion, that protoplasm is the foundation of all organic
development, vegetable and animal. But there is yet another direction
in which the study of the history of development by the phytotomist led
to new points of view and to new results; we have already pointed in
the end of the first book to the way in which Nägeli after 1844 made
the sequences of cell-division in the growth of organs the basis for
his morphology, and how in this way the Cryptogams especially revealed
their inner structure; we also noticed the splendid results which
Hofmeister achieved by his study of the development of the embryo; here
we have further to show, how after 1850 the various forms of tissue,
especially the vascular bundles, were examined by observation of the
history of their development, and how in this way botanical science
has succeeded in explaining the inner histological connection between
leaves and axes, shoots and parent-shoots, primary and secondary roots,
and above all in gaining a correct insight into subsequent growth in
thickness and so learning to understand the true mode of formation of a
woody body and of the secondary rind.
It is then the task of the following chapter to give a more detailed
account of the history of phytotomy, the salient points in which have
now been indicated.
CHAPTER I.
PHYTOTOMY FOUNDED BY MALPIGHI AND GREW.
1671-1682.
The foundation of vegetable anatomy, indeed of all insight into the
structure of the substance of plants, is the knowledge of their
cellular structure. We find the first perception of this truth in
a comprehensive work of ROBERT HOOKE[64], which appeared in London
in 1667 under the title of ‘Micrographia or some physiological
descriptions of minute bodies made by magnifying glasses.’ The author
of this remarkable book was not a botanist, but an investigator of
nature of the kind more especially to be found in the seventeenth
century; he was mathematician, chemist, physicist, a great mechanician,
and later an architect, and moreover a philosopher of the new school
then rising. Beside many discoveries in various subjects he succeeded
in 1660 in so far improving the compound microscope, that with
considerable increase in magnifying power it had tolerably clear
definition. With this instrument Henshaw in 1661 is stated to have
discovered the vessels in walnut-wood, a fact not of importance for our
history. Hooke himself was anxious to show the world how much could be
seen with his improved instrument; as an adherent of the inductive
method he desired to aid in perfecting the perceptions of sense which
are the foundation of all human knowledge; with this feeling he
submitted all sorts of objects to his glass, that it might be known how
much the unassisted eye fails to perceive. He made what he saw texts
for discussions on a multiplicity of questions of the day. The book
therefore was not devoted to phytotomy; the structure of the substance
of plants is noticed in the same incidental manner, as the discovery
of parasitic fungi on leaves, or other similar matters. And what Hooke
saw of the structure of plants was not much, but it was new, and on
the whole fairly judged. It appears that he discovered the cellular
structure in plants by examining charcoal with his glass, and that he
then tried cork and other tissues. He says that a thin section of cork
on a black ground (by direct light therefore) looks like honey-comb; he
distinguishes between the hollow spaces (pores) and the dividing walls,
and to the former he gives the name which they yet bear; he calls
them cells. The arrangement of the cork-cells in rows misleads him
into taking them for divisions of elongated hollow spaces, separated
by diaphragms. These, he says, are the first microscopic pores, which
he or any one else had ever seen, and he regards the cell-spaces of
plants as examples of the porousness of matter, as do the text books
of physics up to modern times. Hooke employed his discovery especially
to explain the physical qualities of cork; he estimates the number
of pores in a cubic inch at about twelve hundred millions. He draws
another botanical conclusion; he gathers from the structure of the
cork that it must be an outgrowth from the bark of a tree, and appeals
to the statements of one Johnston in proof of this hypothesis. The
fact, that cork is the bark of a tree, was therefore not yet known
to all educated people in England. Hooke afterwards says that this
kind of texture is not confined to cork; for as he examined the pith
of elder and other trees with his microscope and the pulp of hollow
stems, such as those of fennel, teasel and reed, he found a similar
kind of structure with the difference only, that in the latter the
pores (cells) are arranged lengthwise, in cork in transverse rows. He
says that he has never seen any passages for communication between the
cells, but that they must exist, because the nourishing juice passes
from one to another; for he has seen how in fresh plants the cells are
filled with sap, as are the long pores in the wood; but these he found
empty of sap in the carbonised wood, and filled with air.
It is plain that it was not much that Hooke saw with his improved
microscope; thin cross-sections of the stem of balsam or gourd, two
plants that grew at that time in every garden, would have shown the
naked eye as much or even more of vegetable structure. At the same time
there is proof here of what was said above on the influence of the
microscope on the use of the eye; the pleasure in the performance of
the new instrument must first direct attention to things which can be
seen without it, but were never seen.
About the time of the appearance of Hooke’s ‘Micrographia’ Malpighi
and Grew had already made the structure of the plant the subject of
detailed and systematic investigations, the results of which they
laid before the Royal Society in London almost at the same time in
1671. The question to which of the two the priority belongs has been
repeatedly discussed, though the facts to be considered are undoubted.
The first part of Malpighi’s large work, the ‘Anatomes plantarum idea,’
which appeared at a later time, is dated Bologna, November 1, 1671;
and Grew, who from 1677 was Secretary to the Royal Society, informs
us in the preface to his anatomical work of 1682, that Malpighi laid
his work before the Society on December 7, 1671, the same day on
which Grew presented his treatise, ‘The Anatomy of plantes begun,’ in
print, having already tendered it in manuscript on the eleventh of
May in the same year. But it must be observed that these are not the
dates of the larger works of the two men, but only of the preliminary
communications, in which they gave a brief summary of the researches
they had then made; the fuller and more complete treatises appeared
afterwards; the preliminary communications formed the first part of the
later works and to some extent the introduction to them. Malpighi’s
longer account was laid before the Society in 1674, while Grew produced
a series of essays on different parts of vegetable anatomy between 1672
and 1682; and these appeared together with his first communication in
a large folio volume under the title, ‘The anatomie of plantes,’ in
1682. Thus Grew had opportunity to use Malpighi’s ideas in his later
compositions; he actually did so, and the important point as regards
the question of priority is, that where he makes use of Malpighi he
distinctly quotes from him. No more is necessary to remove the serious
imputation which Schleiden has made against Grew in the ‘Grundzüge’
(1845), i. p. 207.
Whoever has not himself read the elaborate works of Malpighi and Grew,
but knows them only from the quotations in later phytotomists, may
easily imagine that these fathers of phytotomy had found their way
to a theory of the cell, such as we now possess. But it is not so;
their works have very little resemblance to modern descriptions of
vegetable anatomy; the difference lies chiefly in this, that modern
writers in their accounts of the structure of plants start with the
idea of the cell, and afterwards treat of the connection of cells
into masses of tissue. The founders of phytotomy on the contrary, as
might naturally be expected, consider first and foremost the coarser
anatomical circumstances; they describe the rind, bast, wood, and
pith chiefly of woody dicotyledons, and the histological distinctions
between root, stem, leaf, and fruit in their broader relations, and
examine the detail of the structure of buds, flowers, fruits, and seeds
for the most part only so far as it can be seen with the naked eye.
The more delicate structural conditions are afterwards discussed as a
supplement to this less minute anatomy and always in close connection
with it. The chief emphasis is laid on the consideration of the way
in which the fibrous tissue connects with the succulent parenchyma,
while such questions as the nature of the cell, the fibre, and the
vessel are only incidentally touched upon or discussed at greater
length in the course of the exposition. The mode of investigation and
exposition is therefore chiefly analytic, while in modern compendiums
of phytotomy it is essentially synthetic. It need scarcely be said
that with this mode of treatment the questions which are now regarded
as fundamentally important are either treated as of secondary moment,
or are disregarded; we must not therefore, in judging of the merit of
these men, approach their works with the demands upon them which our
more advanced science would lead us to make. It would be quite wrong
even to think of measuring the value of their books by the extent to
which their contents agree with the modern cell-theory. Both of them
had enough to do to find their way at all in the new world which the
microscope had revealed; many questions which have become trivial for
us had then to be solved for the first time, and the chief merit of
both lies in this very effort to understand first of all the coarser
relations of the anatomical structure of plants; in this respect the
study of their works may yet be recommended to beginners, because
modern phytotomical books are generally very imperfect on these
points. And yet we must not undervalue what Malpighi and Grew had to
say on the more delicate anatomy, and especially on the nature of the
solid framework of cell-membrane in the plant; imperfect and crude as
their views on such points may be, yet they continued for more than a
hundred years to be the foundation of all that was known about cellular
structure; and when phytotomy took a new flight at the beginning of the
present century, Malpighi’s and Grew’s scattered remarks on the union
of cells with one another, and on the structure of fibres and vessels,
were adopted by the later phytotomists and connected with their own
investigations.
If the views of Malpighi and Grew agreed in the main on the points here
mentioned, yet the style and manner of the two were very different.
Malpighi kept more closely to that which could be directly seen;
Grew delighted in tacking on a variety of theoretical discussions to
his observations, and especially endeavoured to follow the path of
speculation beyond the limits of what was visible with the microscope.
Malpighi’s account reads like a masterly sketch, Grew’s like an
elaborate production of great and almost pedantic carefulness; Malpighi
displays a greater formal cultivation, and deals with the questions
with light touches, allusively, and almost in the tone of conversation.
Grew on the other hand is at pains to reduce the new science to a
learned and well-studied system, and to bring it into connection with
chemistry, physics, and above all with the Cartesian philosophy.
Malpighi was one of the most famous physicians and zootomists of his
time, and treated phytotomy from the points of view already opened
in zootomy; Grew too occupied himself occasionally with zootomy, but
he was a vegetable anatomist by profession, and gave himself up,
especially after 1688, almost exclusively to the study of the structure
of plants with a devotion hardly to be paralleled till we come down to
Mirbel and von Mohl.
As in medicine in the 17th century human anatomy was intimately
connected with physiology, and the latter was not yet treated as
a distinct study, so the founders of phytotomy naturally combined
the physiological consideration of the functions of organs with the
examination of their structure. Considerations on the movement of sap
and on food appear in the front of every anatomical enquiry; relations
of structure, which the microscope could not reach, were assumed
hypothetically on physiological grounds, although little positive was
known at the time about the functions of the organs of plants; hence
recourse was had to analogies between vegetable and animal life, and
it is true that vegetable physiology received its first great impulse
by this means, but occasion was given at the same time to many
errors, which in their turn often misled the anatomist. At present,
when vegetable anatomy has separated itself more than is desirable
from physiology, that is, from the investigation of the functions of
organs, it is difficult, nay impossible, to give the reader a brief
account of the contents of these two books which form an epoch in the
science. I must confine myself to noticing a few chief points, which
are historically connected with the further development of phytotomy,
though some of these are just the questions to which Malpighi and Grew
only gave occasional attention, and which it is therefore a little
unjust to them to bring into prominent notice. I shall recur to the
physiological portion of their writings in the third book of this
history, confining myself here to that which concerns the structural
relations of plants.
The phytotomical work of MARCELLO MALPIGHI[65] appeared under the title
‘Anatome Plantarum,’ and to it was added a treatise on hens’ eggs
during the process of incubation (1675). The phytotomical portion of
the book separates into two main divisions, the first of which, the
‘Anatomes Plantarum idea,’ was, as was stated above, completed in 1671,
and contains a general abstract and survey of Malpighi’s views on the
structure and functions of vegetable organs in fourteen-and-a-half
folio pages; the second and much larger portion illustrates in detail
by numerous examples and with the help of many copper-plates the views
expressed in the first part; it will answer our purpose best to turn
principally to the connected expression of the author’s views in the
first part.
He begins his remarks with the anatomy of the stem, and as the rind
first attracts the eye, he takes it first. The outer part of it, he
says, the cuticle, consists of utricles or little sacs arranged in
horizontal rows; these die in time and decay, sometimes forming a dry
epidermis. On the removal of the epidermis, layer after layer of woody
fibre is disclosed, and these layers, usually forming reticulations and
lying one on another, follow the longitudinal direction of the stem.
These fibrous bundles are composed of numerous fibres, and each single
fibre of tubes which open into one another (‘quaelibet fibra insignis
fistulis invicem hiantibus constat’) and so on. The interspaces of the
network are filled with roundish tubes, which usually have a horizontal
direction towards the wood. If the rind is removed the wood appears,
chiefly composed of elongated fibres and tubes, and consisting of rings
or vesicles open towards one another and arranged in longitudinal rows.
The fibres also of the wood do not run parallel to one another, but
allow a network of angular anastomosing spaces to be formed between
them, the larger of which are filled with bundles of tubes, which run
from the rind through these interspaces to the pith, etc., etc. Between
the fibrous and fistulose bundles of the wood lie the spiral tubes
(‘spirales fistulae’), smaller in number but of larger size, so that in
cross sections of the stem they appear with open orifices. They lie in
different positions, but the majority in concentric circles. He says
that in the course of ten years’ examination (from 1661 therefore) he
found these spiral tubes in all plants, and it may be added here that
Grew in the introduction to his book expressly concedes the priority
in this discovery to Malpighi; but Malpighi’s ideas on the subject
of these tubes are extremely indistinct[66], and this gave occasion
to much misinterpretation and to gross errors on the part of later
writers. Malpighi thought he observed a peristaltic movement in these
vessels, a delusion to which many of the nature-philosophers were
particularly fond of surrendering themselves at the beginning of the
present century.
In addition to the bundles of fibres and the tracheae, Malpighi
observed a number of tubes in Ficus, Cupressus, and other plants, which
allowed the escape of a milky juice, and he concludes that similar
special tubes might be present also in the wood of stems from which
milk, turpentine, gum, and the like exude.
Such are the elementary organs of plants, as far as they were known to
Malpighi; in the subsequent part of his book we find them applied to a
histology of the stem, and here a mistake at once makes its appearance,
which, resting on his authority, was reproduced by the phytotomists of
the 18th and even of the early part of the 19th century,—the theory,
namely, that the young layers of wood in the stem originate in the
periodic transformation of the innermost layers of bark (secondary
bast-layers); Malpighi was led into this mistake, as it appears, partly
by the softness and light colour of the alburnum, partly by its fibrous
character. In this substance the spiral tubes are gradually formed, and
as the mass becomes more solid and compact, it subsequently forms the
true wood.
The pith lies in the centre of the stem, and, according to Malpighi,
consists of numerous rows of spheres (‘multiplici globulorum ordine’)
arranged longitudinally one after another, and composed of membranous
tubes, as may be clearly seen in walnut, elder, and other trees. In
this place also he mentions the milk-vessels in the pith of the elder.
Passing over many and various matters, it may be mentioned next that
Malpighi recognises the connection of the layers of tissue in young
shoots with those of the parent-stem, and very expressly notices the
same continuity of structure between the leaf and the axis of the
shoot. He then briefly touches on the anatomical relations of the
fruit and the seed, the existence of the embryo in the seed and its
structure, and then goes on to the roots. ‘The roots of trees are a
part of the stem, which divides into branches and ultimately ends
in capillary threads (‘capillamenta’); so that, in fact, trees are
simply fine tubes, which run separate from one another underground but
gradually collect into bundles; these bundles unite further on with
other and larger bundles, and all together ultimately join to form a
single cylinder, the stem, which then by separation of the tubes at the
opposite extremity stretches out its branches, and by continued gradual
separation of the larger into smaller finally expands into leaves, and
so reaches its furthest limits.’ The conclusion of the whole account is
chiefly concerned with the part played by the various kinds of tissue
in the nourishment of the plant.
In the second part published in 1674, the different kinds of tissue in
the stem are discussed at greater length; here there is much that is
really good, but at the same time much that is imperfect to an extent
which cannot be attributed solely to the inferiority of his microscope.
Very excellent is the way in which he endeavours to make out the more
obvious anatomical relations of the rind, the wood, and the pith, and
in the texture of the rind and the wood connects the longitudinal
course of the vessels and woody fibre with the horizontal course of the
medullary rays and the ‘silver-grain.’ The magnifying powers which he
used must, to judge from his figures, have been very considerable; how
much of what is imperfect in them is due to the indistinctness of the
field of view, and how much to inaccurate observation, we cannot say.
For instance, he sees the bordered pits in the wood of Conifers without
perceiving the central pore, and represents them as coarse grains lying
on the outside of the wood-cells; it was unfortunate for Malpighi, as
for his successors, that the large vessels in the wood of dicotyledons,
to which they gave most of their attention, are often filled with
secondary tissue (thylosis), which Malpighi figures Tab. vi, fig. 21,
but the true nature of which was not understood till 150 years later.
Malpighi, like succeeding phytotomists till as late as 1830, lays great
stress on the structure of the spiral vessels or tracheae, and mentions
particularly that they are surrounded by a sheath of woody fibre;
but he did not fall into the strange notions which Grew and other
phytotomists entertained with regard to the nature of these vessels.
We may at present omit the numerous remarks on assimilation and the
movement of the sap; the descriptions and figures of the parts of
buds and of the course of the bundles of vessels in different parts
of plants, and especially the analyses of the flower and fruit and
the examination of the seed and embryo, conducted with a carefulness
remarkable for that time, deserve a fuller notice, but this would
detain us too long from our main subject.
If Malpighi’s work reads like a masterly sketch in which the author
is bent only on giving the outlines of the architecture of plants,
the much more comprehensive work of NEHEMIAH GREW[67], ‘The anatomy
of plantes’ (1682), has the appearance of a text-book of the subject
thoroughly worked out in all its details; the tasteful elegance of
Malpighi is here replaced by a copiousness of minute detail that is
often too diffuse; while in Malpighi we only occasionally encounter
the philosophical prejudices of his time, which usually lead him
into mistakes, Grew’s treatise is everywhere interwoven with the
philosophical and theological notions of the England of that day;
but we are compensated for this by the more systematic way in which
he pursues the train of thought, and especially by the constant
effort to give as clear a representation as possible of what he sees.
Though he too everywhere introduces physiological considerations
into his anatomical investigation, yet he keeps himself free from
many preconceptions which his successors imported in this way into
phytotomy. To mention one point by anticipation, he avoided the
erroneous notion so common at a later time, and first definitively
removed by von Mohl in 1828, that the cell-walls must have visible
openings to serve for the movement of the sap.
Grew’s work, as has been said, separates into two main divisions;
the first, ‘The anatomy of plants begun, with a general account of
vegetation founded thereupon,’ was printed in 1671, and contains a
brief and rapid account of the general anatomy and physiology of
plants in forty-nine folio pages. Then the anatomy of roots, stems,
leaves, flowers, fruits and seeds appeared as separate treatises in the
following years up to 1682. We may pass over the chemical researches
embodied in this work and the enquiries into the colours, taste and
smell of plants, as well as the previously issued treatise, ‘An idea of
a philosophical history of plants,’ which, as it was first laid before
the Royal Society in 1672, we may imagine to have been intended as a
counterpart to Malpighi’s ‘Anatomes plantarum idea,’ though it is very
different in character and admits much that is foreign to vegetable
anatomy and physiology.
With Grew as with Malpighi the main point of enquiry is not the
individual cell, but the histology; after distinguishing, like
Malpighi, between the parenchymatous tissue and the longitudinally
elongated fibrous forms, the true vessels and the sap-conducting
canals, he is chiefly bent on explaining the combination of these
tissues in the different organs of the plant; and in this point he is
superior to Malpighi both in carefulness of description and in the
beauty of his delineations. Grew’s numerous figures on copper plates,
more carefully executed than Malpighi’s, give in fact so clear an idea
especially of the structure of the root and stem that a beginner may
still use them with advantage; such figures as those on plates 36 and
40 and elsewhere show that he knew how to fashion his observations by
aid of much reflection into a clear representation of the thing seen;
there are, as might be expected, many errors in the details of the more
delicate structure of the various forms of vessels and cells.
Malpighi had not said, whether he considered the cells of the
parenchyma (the term parenchyma comes from Grew) to be perfectly closed
or porous, nor how they cohere; Grew leaves no doubt on this point; he
says distinctly on page 61 that the cells or vesicles of the parenchyma
are closed, that their walls are not traversed by any visible pores,
so that the parenchyma may be compared to the foam of beer. He quotes
Malpighi’s view respecting the vessels of the wood, and supplements
it by saying that the spiral band is not always single, but that two
or more bands entirely separate from one another may form the wall of
the vessel, and also that the spiral thread is not flat but roundish
like a wire, and its turns are more or less close together according to
the part of the plant. He also notices that the spiral tubes are never
branched, and that when they run straight, as in Arundo Donax, they can
be seen throughout considerable distances. The view of the structure of
spiral vessels, which began with Malpighi and was maintained through
the whole of the 18th century, Grew (p. 117) expresses still more
distinctly than Malpighi; but it is to be observed that neither of
them clearly distinguished true spiral vessels with separable spiral
threads from vessels of the kind which occurs in secondary wood, and
only shows a spiral structure on being torn. From the way, says Grew,
in which the threads are woven, it comes to pass that the vessels often
unroll into a flat surface, as we may imagine a narrow ribbon wound in
a spiral about a round staff so that edge meets edge; and if the staff
is drawn out, the ribbon so wound will remain behind in the form of a
tube, and this would answer to an air-vessel in the plant. We should
notice specially that Grew, better taught than the phytotomists of the
18th century, considers the vessels of the wood as air-passages, though
they sometimes convey water. But he goes on with his description of
the wall of the vessel; the flat surface disclosed by the unwinding of
a vessel is, he says, itself composed of many parallel threads, as in
an artificial ribbon, and the threads that are spirally wound answer
to the warp in an artificial tissue, being held together by transverse
threads, which correspond to the woof. To realise to ourselves this
very strange idea of the structure of a spiral vessel as it appeared to
Grew, we ought to know that he thinks that all cell-walls, even those
of the parenchyma, are composed of an extremely fine web; his previous
comparison of cell-tissue with foam was only intended to make the more
obvious circumstances clear to the reader; his real idea is, that the
substance of the walls of vessels and cells consists of an artificial
web of the finest threads. He hints at this on pages 76 and 77, and on
page 120 he returns once more to this conception and dwells upon it at
great length. The most exact comparison, he says, which we can make of
the whole body of a plant is with a piece of fine lace-tissue, such as
women make upon a cushion; for the pith, the medullary rays, and the
parenchyma of the rind are an extremely delicate and perfect tissue of
thread. The threads of the pith run horizontally like the threads in a
piece of woven stuff, and form the boundaries of the numerous vesicles
of the pith and the rind, as the threads in a web bound the interstices
in it. But the woody fibres and the air-vessels are perpendicular to
this tissue, and therefore at right angles to the horizontal threads
of the parenchyma, just as the needles in a piece of lace work that
lies on the cushion are perpendicular to the threads. To complete the
comparison we ought to suppose the needles to be hollow and the tissue
of thread-lace in a thousand layers one above another. Grew himself
states incidentally, that he lit upon this notion from looking at
shrivelled masses of tissue, when he naturally saw wrinkles and folds,
which he took for threads. Besides he seems to have used blunt knives,
which might easily tear the cell-walls into threads; so we might gather
from the figure in Plate 40, where what he supposes to have been a
tissue of thread from the walls of a cell is depicted quite plainly.
Lastly the observation of vessels with reticulated thickening, and
parenchyma-cells with crossed striation may have contributed to his
view.
It will hardly be superfluous to remark here, that Grew’s idea of this
very delicate structure of cell-walls has evidently given rise to the
common expression cell-tissue (contextus cellulosus) when speaking
of plants and animals, an expression which has become naturalised in
microscopy, and is still retained though we no longer think of Grew’s
comparison of cell-structure with artificial lace. But the word tissue
has often misled later writers, as words are apt to do, and made them
found their conception of vegetable structure on the resemblance to an
artificial tissue of membranes and threads.
Grew, like Malpighi, derives the young layers of wood in the stem from
the innermost layers of the rind. The true wood, he says on page 114,
is entirely composed of old lymph-vessels, that is of fibres, which lay
originally in the inner circumference of the rind. But by true woody
substance he understands the fibrous components of the wood, excluding
the air-vessels; his lymph-vessels are the bast-fibres and similar
forms; for, he goes on, the air-vessels with the medullary rays and
the true wood form what is commonly called the wood of a tree; he uses
the term air-vessels, not because these forms never contain sap, but
because they only contain a vegetable air during the proper period of
vegetation, when the vessels of the rind are filled with sap.
The above is certainly a very imperfect account of Grew’s services to
phytotomy; for the points here made prominent were treated by him as
accessories only to the coarser histological relations with which he
chiefly occupied himself.
These two works of Malpighi and Grew, so important not only for botany
but for the whole range of natural science, were not followed during
the course of the next hundred and twenty years by a single production,
which can claim in any respect to be of equal rank with them; that
long time was a period not of progress but of steady retrogression,
as we shall see in the next chapter. But before the beginning of the
18th century ANTON VON LEEUWENHOEK[68] made some contributions to
the knowledge of the details of vegetable anatomy, if not exactly
to the settling of very important points in it; he communicated his
observations on animal and vegetable anatomy in numerous letters to the
Royal Society of London, and these appeared for the first time in a
collected form in Delft in 1695 under the title of ‘Arcana naturae.’ It
is not easy to gain a clear idea of Leeuwenhoek’s phytotomic knowledge
from his scattered statements. He too discussed the less minute anatomy
of fruits, seeds and embryos, and among other things he made occasional
observations on germination, and many on the structure of different
woods. But all bears the stamp of only occasional study of plants; he
was led to his observations by questions of the nature-philosophy then
in vogue, and especially by such as were connected with the theory of
evolution, not unfrequently by mere curiosity and pleasure in things
obscure and inaccessible to ordinary people, but he did not gain from
them a general idea of the structure of plants. In the course of
these observations he did unquestionable service in perfecting simple
magnifying glasses; he made a large number with his own hands, and
these possessed magnifying powers evidently not at the command either
of Malpighi or Grew. By aid of such glasses he discovered the vessels
of secondary wood which are not spirally thickened but beset with
pits, the true character of which however he did not investigate. He
was the first moreover who perceived the crystals in vegetable tissue,
namely in the rhizome of Iris florentina and in species of Smilax,
and this could only be done with strong magnifying powers. In other
matters he repeats the histological views of Malpighi and Grew, and on
the whole his numerous communications seem painfully fragmentary and
unscientific in presence of Malpighi’s elegance and perspicuity, and
Grew’s systematic thoroughness. His figures too, which were not drawn
by himself, are with some exceptions inferior to those of his great
contemporaries.
CHAPTER II.
PHYTOTOMY IN THE EIGHTEENTH CENTURY.