into German in the following year under the title, ‘Versuche und
Beobachtungen über verschiedene Theile der Naturlehre,’ and contained
among other things the writer’s experiments on plants. His way of
managing them was eminently unsuitable, nor did he arrive at any
definite and important result, though he expressed the idea which
had led him to make them clearly enough, where he says, ‘If the air
exhaled by the plant is of better character (richer in oxygen) than
atmospheric air, it follows that the phlogiston of the air is retained
in the plant and used there for its nourishment, while the part which
escapes, being deprived of its phlogiston, necessarily attains a higher
degree of purity.’ After he had ceased his experiments with plants in
1778, he observed that there was a deposit of matter in the water in
some vessels which he had used for them, and that it gave off a very
‘pure air’; a number of further observations taught him that this
air was given off only under the influence of sun-light; Priestley
himself did not suspect that the deposit in question, afterwards known
as Priestley’s matter and found to consist of Algae, was a vegetable
substance.
In the same year (1779) appeared the first book by INGEN-HOUSS[132], in
which the subject was treated at length; it was called, ‘Experiments
on Vegetables, discovering their great power of purifying the common
air in the sunshine and of injuring it in the shade and at night,’
and was at once translated into German, Dutch and French. The title
itself shows that the author had observed more and more correctly
than Priestley. But he did not come to an understanding of the
inner connection of the facts, till Lavoisier completed his new
antiphlogistic theory. He says himself in his essay, ‘On the nutrition
of plants and the fruitfulness of the earth,’ which appeared in 1796,
and was translated into German with an introduction by A. v. Humboldt
in 1798, that when he published his discoveries in 1779, the new system
of chemistry was not yet fully declared, and that without its aid he
had been unable to deduce the true theory from the facts; but that
since the composition of water and air had been discovered, it had
become much easier to explain the phenomena of vegetation. But in order
to establish his priority he says on p. 56, that he had been fortunate
enough to find out the real cause why plants at certain times vitiate
the surrounding air, a cause which neither Priestley nor Scheele had
suspected. He had discovered, he says, in the summer of 1779, that
all vegetables incessantly give out carbonic acid gas, but that the
green leaves and shoots only exhale oxygen in sun-light or clear
daylight. It appears therefore that Ingen-Houss not only discovered the
assimilation of carbon and the true respiration of plants, but also
kept the conditions and the meaning of the two phenomena distinct from
one another. Accordingly he had a clear idea of the great distinction
between the nutrition of germinating plants and of older green ones,
the independence of the one, the dependence of the other, on light;
and that he considered the carbon dioxide of the atmosphere to be the
main if not the only source of the carbon in the plant, is shown by
his remark on a foolish assertion of Hassenfratz that the carbon is
taken from the earth by the roots; he replied that it was scarcely
conceivable that a large tree should in that case find its food for
hundreds of years in the same spot. There was a certain boldness in
these utterances of Ingen-Houss, and a considerable confidence in
his own convictions, for at that time the absolute amount of carbon
dioxide in the air had not been ascertained, and the small quantity of
it in proportion to the other constituents of air would certainly have
deterred some persons from seeing in it the supply of the huge masses
of carbon which plants accumulate in their structures.
Before Ingen-Houss in the work last mentioned explained the results
of his observations of 1779 in accordance with the new chemical
views, and laid the foundations of the doctrine of nutrition in
plants, JEAN SENEBIER[133], of Geneva, made protracted researches
into the influence of light on vegetation (1782-1788), and founded
on their results a theory of nutrition, which he published in 1800
in a tediously prolix work in five volumes entitled, ‘Physiologie
végétale.’ In this work some valuable matter was concealed in a host of
unimportant details and tiresome displays of rhetoric, which for the
most part are beside the question. But it must be acknowledged that
Senebier was better provided with chemical knowledge than Ingen-Houss,
and that he brought together all the scattered facts that the chemical
literature of the day offered, in order to obtain a more complete
representation of the processes of nutrition. It was of especial
importance at that time to insist on the principle that the processes
of nutrition within the plant must be judged by the general laws of
chemistry; organised beings, said Senebier, are the stage, on which
the affinities of the constituents of earth, water, and air mutually
influence each other; the decompositions however are generally the
result of the influence of light, which separates the oxygen of the
carbon dioxide in the green parts of plants. He insists (II. p. 304)
upon this among other facts, that the simple constituents of all plants
are the same, and the differences are only quantitative. He then brings
before us the simple and compound constituents of plants one after the
other, and among them light and heat figure as material substances,
in accordance with the view of the time. He treats at great length
the old question of the meaning of the salts in the plant, and it is
instructive to observe how he tries to decide whether the nitrates,
sulphates and ammonia, which are found in the sap of plants, are
introduced from without, or are formed in them from their constituent
elements; he concludes finally that the former is the more probable
opinion. That the greater part at least of the carbon of plants comes
from the atmosphere could scarcely be a matter of doubt with those
who knew the writings of Ingen-Houss; but Senebier devotes special
attention to this question; he endeavours to take all the co-operating
factors into the calculation, and especially to prove once more that
the oxygen given off from the plant in light comes from the carbon
dioxide which has been absorbed, that the green parts only and no
others are able to effect this decomposition, and that there is a
sufficiency of carbon dioxide in nature to supply the food of plants.
But although he convinced himself that green leaves decompose the
carbon dioxide which surrounds them in a gaseous form, he supposed that
it is chiefly through the roots that this substance finds its way with
the ascending sap into the leaves, and this view often gave occasion to
further error in later writers.
The tedious prolixity of Senebier’s book was one reason why it never
enjoyed the measure of appreciation and influence which it deserved;
but it was also thrown into the shade by the appearance of a work of
superior excellence, distinguished at once by the importance of its
contents, by condensation of style, and by perspicuity of thought.
This work was the ‘Recherches chimiques sur la végétation’ of THÉODORE
DE SAUSSURE[134] (1804), which contained new observations and new
results, and what was still more important, a new method. Saussure
adopted for the most part the quantitative mode of dealing with
questions of nutrition; and as the questions which he put were thus
rendered more definite, and his experiments were conducted in a most
masterly manner, he succeeded in obtaining definite answers. He knew
how to manage his experiments in such a manner that the results were
sure to speak plainly for themselves; they had not to be brought out
by laborious calculation from those small and, as they are called,
exact data, which less skilful experimenters use to hide their own
uncertainty. The directness and brevity with which precise quantitative
results are expressed, the close reasoning and transparent clearness
of thought, impart to the reader of de Saussure’s works a feeling
of confidence and security such as he receives from scarcely any
other writer on these subjects from the time of Hales to our own.
The ‘Recherches chimiques’ have this in common with Hales’ ‘Statical
Essays,’ that the statements of facts which they contain have been
made use of again and again by later writers for theoretical purposes,
while the theoretical connexion between them was constantly overlooked,
as we shall have reason to learn in the following section. It is not
every one who can follow a work like this, which is no connected
didactic exposition of the theory of nutrition, but a series of
experimental results which group themselves round the great questions
of the subject, while the theoretical connection is indicated in short
introductions and recapitulations, and it is left to the reader to form
his own convictions by careful study of all the details. It was not de
Saussure’s intention to teach the science, but to lay its foundations;
not to communicate facts, but to establish them; the style therefore,
as might be expected, is dry and unattractive; the writer seems to
confine himself too anxiously within the limits of what is given in
experience, and there is no doubt that many errors in later times might
have been avoided if the inductive proof of de Saussure’s doctrines had
been accompanied with a deductive exposition of them of a more didactic
character.
The processes of vegetation examined by de Saussure were, for the most
part, the same as those which Ingen-Houss and Senebier had studied
at length and correctly described in their general outlines. But de
Saussure went beyond this, and by means of quantitative determinations
struck a balance between the amount of matter taken up and given off
by the plant, thereby showing what it retains. In this way he made two
great discoveries: that the elements of water are fixed in the plant
at the same time as the carbon, and that there is no normal nutrition
of the plant without the introduction of nitrates and mineral matter.
But we cannot form a due idea of de Saussure’s services to physiology
without going further into the detail of his work.
We will first consider his investigations respecting the assimilation
of carbon in plants. Here we have the important result, that larger
quantities of carbon dioxide in the atmosphere surrounding the plants
are only favourable to vegetation if the latter are in a condition to
decompose them, that is, if they are in sufficiently strong light; that
every increase in the amount of carbon dioxide in the air in shade or
in darkness is unfavourable to vegetation, and that if that increase is
greater than eight times in the hundred it is absolutely injurious. On
the other hand he found, that the decomposition of carbon dioxide by
the green parts in light is an occupation that is necessary to them,
that plants die when they are deprived of it. The first clear insight
into the chemical processes which accompany the decomposition of carbon
dioxide in the interior of the plant was obtained by perceiving, that
plants by appropriating a definite quantity of carbon make a much
more than proportionate addition to their dry substance, and that this
is due to the simultaneous fixation of the component parts of water.
The full significance of this fact could only be apprehended at a
later time, when the theory of the combinations of carbon, organic
chemistry, had been further developed. As regards the importance of the
decomposition of carbon dioxide by the green organs under the influence
of light to the whole nourishment of the plant, de Saussure arrived
by more definite proofs than Ingen-Houss had given at the result,
that only a small portion of the substance of plants is derived from
the constituents of the soil in solution in water, but that the great
mass of the vegetable body is built up from the carbon dioxide of the
atmosphere and the constituents of water; he convinced himself of this
partly by considering the small quantities of matter which the water is
able to dissolve from a soil capable of sustaining vegetation, partly
by experiments in vegetation and considerations of a more general
character.
Not less important were de Saussure’s investigations into
oxygen-respiration by plants, which taken simply as a fact, had been
previously discovered by Ingen-Houss. But de Saussure showed that
growth is impossible without this process of respiration, even in
germinating plants, though these are rich in assimilated matter. He
further showed that green leaves and opening flowers, and generally
the parts of plants which are distinguished by greater activity of
vital processes, require more oxygen for respiration than those in a
less active and resting state. He determined the loss of weight which
the organic substance of germinating plants suffers from respiration,
and found it to be greater than was proportionate to the weight of
carbon exhaled; but the chemical science of his day did not supply
him with a certain explanation of this fact. Lastly, de Saussure at a
later time (1822) discovered the chief relations between the internal
heat of flowers and their consumption of oxygen, and thus we see that
he supplied the most important elements in the modern theory of the
respiration of plants, though he did not fully explain their mutual
connection.
It evidently was the received opinion before the time of Ingen-Houss,
and in spite of Hales’ views, that plants derive the larger part of
their food from the constituents of earth and water. But when it became
known that the carbon, which is the chief constituent of vegetable
substance, comes from the atmosphere, and it was considered that much
the larger part of that substance is combustible, it naturally became
a question whether the incombustible ingredients which form the ash
take any part in the nutrition of plants. This question was by many
physiologists answered in the negative; but de Saussure maintained
the contrary view. He insisted that certain ingredients, which are
found in the ash of all plants, must not be regarded as accidental
admixtures, and that the small quantities in which they occur are no
proof that they are not indispensable; and he showed from a large
number of analyses of vegetable ash, which for a long time were
unsurpassed in excellence, that there are certain relations between
the presence of certain substances in the ash and the condition of
development of the organs of the plant; for instance, he found that
young parts of plants capable of development were rich in alkalies and
phosphoric acid, while older and inactive portions were richest in
lime and silicic acid. Still more important were the experiments in
vegetation, by which he showed that plants, whose roots grow not in
earth but in distilled water, only take up as much ash-constituents as
corresponds with the particles of dust which fall into the water; and
further, that the increase in the organic combustible substance of a
plant so grown is very insignificant, and consequently that there is
no normal vegetation where the plant does not take up ash-constituents
in sufficient quantity,—a result of the highest importance to the main
question. Unfortunately de Saussure neglected to state these results
with due emphasis and to point out their fundamental importance, and
consequently doubts were entertained even till after 1830 respecting
the necessity of the constituents of the ash to vegetation.
It was known in de Saussure’s time that nitrogen entered into the
substance of living plants; the question was, whence it was obtained.
As it was known that four-fifths of the atmosphere consists of
nitrogen, it was natural to suppose that it is this which the plant
makes use of for forming its nitrogenous substance. De Saussure
endeavoured to settle the question by the volumetric method, which,
as was afterwards discovered, was not in this case to be trusted.
Nevertheless he arrived at the right conclusion, that plants do not
assimilate the nitrogen of the atmosphere; this gas must therefore be
taken up by the roots in some form of chemical combination. He made
no experiments on growing plants to decide what that form was, but
contented himself with the conjecture that vegetable and animal matter
in the soil and ammoniacal exhalations from it supply the nitrogen in
plants. This question, first ventilated certainly by de Saussure, and
afterwards the subject of protracted discussion, was finally settled
fifty years later by the experiments of Boussingault.
In connection with his researches into the importance of the
constituents of the ash, de Saussure proposed the question whether
roots take up the solutions of salts and other substances exactly in
the form in which they offer themselves. He found first of all that
very various and even poisonous matters are absorbed by them, and
that there is therefore no such power of choice, as Jung had once
supposed; on the other hand, it appeared that the solutions do not
enter unchanged into the roots, for in his experiments in every case
the proportion of water to the salt absorbed was greater than the
proportion between them in the solution, and that some salts enter the
plant in larger, some in smaller quantities, under circumstances in
other respects the same. But at this time, and for a long time after,
it was not possible to understand and rightly explain these facts; the
theory of diffusions was not yet known, and fifty or sixty years were
to elapse before light was thrown on the questions thus raised by de
Saussure.
Such were the most important contents of de Saussure’s publication
in 1804. His later contributions to the knowledge of some important
questions in vegetable physiology will be mentioned further on. A
comparison of the contents of the ‘Recherches chimiques’ with what
was known of the chemistry of the food of plants before 1780 excites
the liveliest astonishment at the enormous advance made in these
twenty-four years. The latter years of the 18th century had proved
still more fruitful, if possible, as regards the theory of nutrition
than the latter years of the 17th; both periods have this in common,
that they developed an extraordinary abundance of new points of view
in every branch of botanical science. They resemble each other also in
the circumstance that they were both followed by a longer period of
inactivity; the time from Hales to Ingen-Houss was highly unproductive,
and so also were the thirty years that followed the appearance of de
Saussure’s great work, though it must be admitted that some good work
was done during that period in France, while in Germany the new theory
was grossly misunderstood by the chief representatives of botany, as we
shall see in the following section. It should be mentioned however that
one of these misconceptions, which was not removed till after 1860, was
caused by de Saussure himself. He had observed that the red leaves of a
variety of the garden Orache disengage oxygen from carbon dioxide, as
much as the green leaves of the common kind. In this case he was hasty,
and concluded from this single observation that the green colour is not
an essential character of the parts which decompose carbonic acid; if
he had only removed the epidermis of the red leaves he would have found
that the inner tissue is coloured as dark green as the ordinary green
leaves. He who was usually so extremely careful as an observer was for
once negligent, and later writers, as is apt to happen, fixed exactly
on this one weak point, and repeatedly called in question one of the
most weighty facts of vegetable physiology, namely, that only cells
which contain chlorophyll eliminate oxygen.
5. VITAL FORCE. RESPIRATION AND HEAT OF PLANTS. ENDOSMOSE. 1804-1840.
During the twenty years that followed the appearance of de Saussure’s
chemical researches the theory of the nutrition of plants can scarcely
be said to have been advanced in any one direction, while much that
had already been accomplished was not even understood. Various
circumstances worked together to introduce misconceptions in this
province of botany; above all others the inclination, more strongly
pronounced than ever at this period, to attribute to organisms a
special vital principle or force, which was supposed to possess a
variety of wonderful powers, so that it could even produce elementary
substances, heat, and other things out of nothing. Whenever any
process in such organisms was difficult to explain by physical or
chemical laws, the vital force was simply called in to bring about the
phenomena in question in some inexplicable manner. It was not that the
question was now raised, which at a later time engaged the attention
of profounder thinkers, whether there was a special agent operating in
organic bodies beside the general forces which govern inorganic nature;
for a careful examination of this question would certainly have led
to the most earnest efforts to explain all the phenomena of life by
physical or chemical laws. On the contrary, it was found convenient to
assume this vital force as proved, and to assign it as the cause of a
variety of phenomena, thus escaping the necessity of explaining the
way in which the effects were produced; in a word, the assumption of
a vital force was not a hypothesis to stimulate investigation, but a
phantom that made all intellectual efforts superfluous.
Another hindrance to the progress of physiology, especially where
questions of nutrition turned on the movement of the sap, was the
backward condition of the study of the inner structure of plants,
as described in the second book. For instance, the question of
the descending sap was complicated in the strangest way by Du
Petit-Thouars’s theory of bud-roots that descend between the bark
and the wood; Reichel’s unfounded idea of the rising of the sap in
the tubes of the wood was generally accepted, and a still worse
error was maintained by some, that the intercellular spaces of the
parenchyma are true sap-conveying organs. In 1812 Moldenhawer had
to insist, but without producing any general conviction, that the
vessels of the wood contain air, and Treviranus in 1821 that the
stomata serve for the entrance and exit of air. We need not notice
here what nature-philosophers like Kieser said about nutrition and
the movement of the sap; but even those who were far from adopting
the extravagancies of this school were incapable of either making
use of or carrying on the labours of Ingen-Houss, Senebier, and de
Saussure. We may adduce in proof of this statement the remarks of
Link on the function of leaves in his ‘Grundlehren der Anatomie und
Physiologie,’ 1807. He says at p. 202 that their function is according
to Hales transpiration, according to Bonnet absorption, according
to Bjerkander the exudation and secretion of a variety of fluids,
according to Hedwig the storing up of juices, and inasmuch as leaves
increase the green surfaces of plants, bear stomata and hairs, and
hold a quantity of juices in their abundant parenchyma, we may ascribe
all these functions, but none of them exclusively, to leaves; the only
thing peculiar to them is that they convey elaborated juices to the
young parts. Their great work, the decomposition of carbon dioxide, he
does not mention. But this neglect of the doctrines of Ingen-Houss,
Senebier, and de Saussure was common, especially in Germany; it is seen
in the efforts made to prove once more the existence of a descending
sap in the rind, just as it had been proved in the two previous
centuries, by the result of removing a ring of bark from the stem,
and by similar experiments; whereas the simple consideration that it
is only in the green leaves that carbonaceous vegetable substance is
formed, would have made the existence of what was known as a descending
sap appear to be a matter of course, and must have led to a much
clearer conception of the matter. But this consideration was either
quite overlooked or only mentioned incidentally by those who occupied
themselves with experiments on the movement of the descending sap. This
is the case in Heinrich Cotta’s ‘Naturbeobachtungen über die Bewegung
und Function des Saftes in den Gewächsen,’ 1806, in many respects an
instructive work, and in Knight’s otherwise serviceable experiments
on the growth in thickness of trees. It was not till after 1830 that
De Candolle and Dutrochet perceived that the fact that the green
leaves are assimilating organs must be decisive of the question of the
movement of the sap in the stem.
No progress was made with the general doctrine of nutrition between
1820 and 1840 except in one point, the absorption of oxygen by all
parts of plants; here something was done to consolidate the theory
and to enrich it with new facts; it was indeed a subject more adapted
to the views of the day, because it at once suggested a variety of
analogies with the respiration of animals. Grischow showed in 1819
that Fungi never decompose carbon dioxide, but absorb oxygen and
give off carbon dioxide. Marcet carried the subject further in 1834,
after de Saussure had published in 1822 an excellent investigation
into the absorption of oxygen by flowers; in this work we have the
basis laid for the theory of vegetable heat, to which we shall
return. But Dutrochet was the first who made an elaborate comparison
of the respiration of plants and animals (1837), and showed that
not only growth, as de Saussure had already perceived, but also the
sensitiveness of plants depends on the presence of oxygen, that is on
their respiration. The recognition of the fact, that the inhalation of
oxygen plays the same part in plants that it does in animals, prepared
the way for the view that heat in plants is simply a result of their
respiration, as it is in animals. It is not necessary to describe at
length the experiments which were made on heat in plants before 1822;
they were one and all vitiated by a want of clearness in the statement
of the question, which made success impossible; it was assumed that
this heat by raising the temperature of the plant would make itself
felt by surrounding objects, and it was sought for exactly where it is
least to be found, in the wood, in fruits and tubers, and generally in
resting, inactive parts. Moreover the previous experiments, collected
in Goeppert’s book ‘Ueber die Wärmeentwicklung der Pflanzen,’ 1830,
were so unskilfully managed that they could not possibly lead to any
result. Nor could the question whether plants really develope internal
heat, as animals do, be determined by a few cases of active development
of heat in flowers, because an idea was prevalent at the time in
connection with the theory of a vital force, that flowers as the organs
of reproduction alone possessed the power of generating heat.
Lavoisier had clearly perceived in 1777 that the combustion of
substances containing carbon by inhaled oxygen was the source of
animal heat, and had proved it by experiments. Senebier, who first
observed the rise of temperature in the inflorescence of Arum by
the thermometer, had at least suggested in his work on physiology
of 1800 (iii. p. 315) that a vigorous absorption of oxygen might be
the cause of the phenomenon. Bory de St. Vincent reported in 1804
that Hubert, the owner of a plantation in Madagascar, had observed
among other things that the air in which the flowering spike of one
of the Aroideae had developed its heat could support neither animal
respiration nor combustion. These indications were however disregarded,
until de Saussure in 1822 proved directly the connection between
the absorption of oxygen and the rise of temperature in flowers. It
was however a long time before heat in plants was conceived of as
a general fact necessarily connected with their respiration. This
conception would have swept away the whole mass of facts accumulated
by Goeppert in his book of 1830, from which he tried to prove (p. 228)
that plants at no period of their life possess the power of generating
heat—a view which he retracted however in 1832, when he had observed
a rise of temperature in germinating plants, bulbs, tubers, and in
green plants, when collected into heaps. How difficult it was for
physiologists under the dominion of the ‘vital force’ to hold firmly
to the simple principle of natural heat, and not to be led away by
isolated observations, is shown by the expressions of De Candolle in
1835, and still more by those of Treviranus in 1838. It is therefore
refreshing to see Meyen in his ‘Neues System’ (1838), vol. ii, warmly
asserting this principle, and making the development of heat in plants
a necessary consequence of their respiration and of other chemical
processes. Meyen himself produced no new observations; but Vrolik
and De Vriese showed by laborious experiments in 1836 and 1839 the
dependence of the generation of heat in the flowers of Aroideae on
the absorption of oxygen. A higher importance as regards the general
principle attaches to the attempt of Dutrochet in 1840 to prove that
even growing shoots generate small quantities of heat, as shown by a
thermo-electric apparatus. Some of the details in these observations
are open to objection; but it cannot be denied that they are based on
a clear recognition of the general principle, though they ignore the
consideration that the generation of heat in plants is not necessarily
accompanied with a rise in temperature, since cooling causes may be
acting at the same time with greater effect. However the doctrine
of the natural heat of plants was in the main established by the
observations of de Saussure, Vrolik, De Vriese, and Dutrochet, and
by Meyen’s and Dutrochet’s assertion of the principle laid down by
Lavoisier, though thirty years elapsed before it became an accepted
truth in vegetable physiology.
The crude idea of a vital force was deprived of one of its chief
supports when it was recognised that the natural heat of organisms
was a product of chemical processes induced by respiration, for this
had been regarded since the time of Aristotle as more peculiarly an
effect of the principle of life. And now another discovery was made,
equally calculated to promote the reference to mechanical principles of
those general and important phenomena of life which had hitherto been
indolently ascribed to the operation of the vital force. It appears
to be a matter of indifference whether Professor Fischer of Breslau
is or is not to be considered as the true discoverer of endosmose in
1822, for it is certain that it was DUTROCHET[135] who first studied
the subject with exactness, and above all perceived its extraordinary
value for the explanation of certain phenomena in living organisms.
He repeatedly called attention to this value in the years between
1826 and 1837, and endeavoured to refer a variety of phenomena in
vegetation to this agency. He had first observed the operation of
endosmose in its mechanical effects in living bodies; the escape of
the zoospores of an aquatic Fungus and the ejection of the sperm from
the spermathecae of snails first led him to the hypothesis, that the
more concentrated solutions inclosed in organic membranes exercise
an attraction on surrounding water, which, forcing its way into the
inclosed space, is there able to exert considerable powers of pressure.
To Dutrochet must always belong the merit of having brought into
notice this mechanical effect of endosmose and of employing it to
explain a number of vital phenomena. Many things in which a mechanical
explanation had not been hitherto thought of could now be traced to a
mechanical principle, the effects of which could be exhibited and more
accurately studied by means of artificial apparatus. Dutrochet rightly
attached a special value to the fact, that all states of tension in
vegetable tissue could be at once explained by endosmose and exosmose,
though, as so often happens in such matters, he may have extended
his new principle to cases where it was not applicable, as we shall
see below. His account of the nature of endosmose itself must now be
considered to be obsolete, nor did the mathematician Poisson or the
physicist Magnus about 1830 succeed in framing a satisfactory theory
on the subject. It was discovered in the course of the succeeding
twenty or thirty years, that the phenomena observed by Dutrochet, and
which he called endosmose and exosmose, were only complicated cases of
hydro-diffusion, which with the diffusion of gas forms an important
part of molecular physics. Dutrochet, like his immediate successors,
conducted his investigations into osmose with animal and vegetable
membranes, the latter being of a complex structure; with these he
always observed in addition to the endosmotic flow of water into the
more concentrated solution, an escape of the solution itself, and from
this he concluded that there must always be two currents in opposite
directions through the membrane which separates the two fluids, that,
as he expresses it, the endosmose is always accompanied with exosmose.
This error, which was even developed later into a theory of the
endosmotic equivalent, has had much to do till recently with making it
impossible or difficult to refer certain phenomena of vegetation to
the processes of hydro-diffusion. To mention only one case, Schleiden
rightly observed that if endosmose, as Dutrochet understood it, is
the sole cause why water is absorbed by the roots, there must also
be a corresponding exosmose at the roots; and this, which was called
root-discharge, Macaire Prinsep thought he had actually discovered,
and even Liebig firmly believed in its existence till a recent period,
although the researches of Wiegman and Polstorff (1842) and later
more careful investigations showed, that there was no noticeable
discharge by exosmose to answer to the great quantity of water with
substances in solution in it which is taken up by the roots. Again,
Dutrochet’s theory of endosmose did not fully explain the way in which
the several substances which feed the plant find their way into and
are disseminated in it. But notwithstanding these and other defects it
deserved the greatest consideration, because it gave the first impulse
to the further development of the theory of diffusion, and contained
a mechanical principle which might serve to explain very various
phenomena in vegetation as yet unexplained. Dutrochet hastened to apply
it to this purpose, where it was at all possible to do so, and chiefly
in his treatise on the ascending and descending sap (‘Mémoires,’ 1837,
i. p. 365), which was superior to anything which had been written
on the movement of the sap in plants in its clear conception of the
question and in perspicuity of treatment. It should be especially
mentioned that Dutrochet formed a true estimate of the functions of the
leaves as regards both the ascending and descending sap, and to some
extent pointed out the fault which lies at the bottom of the earlier
experiments with coloured fluids. After communicating a number of good
observations on the paths of the ascending and descending sap, and
noticing particularly that in the vine the vessels of the wood serve
for the movement of the sap only in spring, when vines bleed, but
that they are air-passages in summer, when transpiration causes the
most copious flow of water in the wood, he proceeds to consider the
forces which effect the movement of the ascending sap in the wood both
in spring and summer. He first of all judiciously distinguishes two
things which had been before always mixed up together, the weeping of
severed root-stocks and the rise of the sap in the wood in transpiring
plants. The first is caused, he thinks, by impulsion, the other by
attraction; we should now say, that in weeping root-stocks the water
is pressed upwards, in transpiring plants drawn up. He then refers the
phenomenon of impulsion to endosmose in the roots, and without going
much into detail as regards the anatomical conditions, he compares
a weeping root-stock to his own endosmometer, in the tube of which
the fluid that has been sucked in rises by endosmose and even flows
over; it is true that no very thorough understanding of the matter
was gained in this way, but at any rate the principle which was to
explain it was indicated. He then endeavours to explain the movement
of the water which ascends in the wood of transpiring plants by the
action of endosmose from cell to cell. In this he failed entirely,
as was afterwards perceived; but he succeeded in showing that all
the mechanical explanations that had been previously attempted were
incorrect, and the whole treatise, though unsatisfactory in its main
result, contains a great number of ingenious experiments and acute
remarks.
With the exception of Théodore de Saussure, who occupied himself
exclusively with chemical questions in physiology, Dutrochet was the
only vegetable physiologist in the period between 1820 and 1840 who
studied all its more important questions thoroughly and experimentally;
his treatise on the respiration of plants, which has been already
mentioned, is excellent in itself, and was of the greatest importance
at the time it appeared, because it brought the chemical processes in
respiration, the entrance and exit of the gases, for the first time
into correct connection with the air-passages in the plant, with the
stomata, the vessels, and the intercellular spaces, and submitted the
composition of the air contained in the cavities of plants to careful
examination. It was the best work on the respiration of plants in
the year 1837 and for a long time after; and if Dutrochet made the
mistake of regarding the oxygen which is disengaged from the plant
itself in the light as the chief agent in respiration, and the oxygen
directly absorbed from the atmosphere as only subsidiary to this, he
compensated for it by recognising the importance of the fact, that only
cells which contain chlorophyll decompose carbon dioxide, and still
more by correctly distinguishing between respiration by the absorption
of oxygen and the decomposition of carbonic dioxide in light; these
two processes were at that time and afterwards very inappropriately
distinguished as the diurnal and nocturnal respiration of plants, and
this misleading expression maintained itself in spite of Garreau’s
protest in 1851 till after 1860, when a modern German physiologist
succeeded in establishing the true distinction between respiration and
assimilation in plants. Another mischievous complication arose about
1830 connected with the expression, circulation of the sap; it was
thought that an argument for such a circulation even in the higher
plants was to be found in the ‘circulation of the sap’ (protoplasm) in
the cells of the Characeae, which had been detected by Corti and more
exactly described by Amici; Dutrochet (Mémoires, I. p. 431) exposed
this confusion of ideas, and has the merit of refuting at the same time
the absurd theory of the ‘circulation of the vital sap,’ for which
Schultz-Schultzenstein had received a prize from the Academy of Paris.
We shall recur in the next chapter to Dutrochet’s minute investigations
into the movements connected with irritability in plants, which he
also endeavoured to refer to endosmotic changes in the turgidity of
the tissues, but he did not do justice to the anatomical conditions of
the problem. And here we may take occasion to remark, that Dutrochet’s
works were often undervalued, especially in Germany, greatly to the
detriment of vegetable physiology. His younger German contemporaries,
von Mohl and Schleiden, and at a later time Hofmeister, were right
in pointing out what was erroneous and sometimes arbitrary in his
mechanical explanations of various movements in plants, and it cannot
be denied that he was sometimes led into obscure and doubtful views,
as for instance when without any apparent connection he regarded the
inhalation of oxygen as a mechanical condition of the rising of the
sap and also of heliotropic curvatures, and that his attempts at
explanation were not seldom forced and improbable; but all this does
not prevent it from being true, that an attentive reader will still
gain much instruction from his physiological writings and be excited
by them to examine for himself. Dutrochet was a decidedly able man and
an independent thinker, who it is true was often led astray by his
prejudices, but at the same time manfully protested against the old
traditional way of dealing with physiological ideas, and substituted
careful examination both of his own and others’ investigations for
the accumulation and comfortable retailing of isolated observations
which was then the fashion. After de Saussure’s ‘Recherches chimiques’
Dutrochet’s ‘Mémoires pour servir a l’histoire anatomique et
physiologique des végétaux et des animaux,’ 1837, are without doubt
the best production, which physiological literature has to show in the
long period from 1804 to 1840. If later botanists, instead of dwelling
on his faults, had developed with care and judgment all that was
really good in his general view of vegetable physiology, this branch
of botanical science would not have declined as it did in the interval
between 1840 and 1860. We shall discover the greatness of Dutrochet as
a vegetable physiologist by comparing his work above-mentioned with the
best text-books of the subject of the same time, those of De Candolle,
Treviranus, and Meyen; not one of them comes up to Dutrochet’s Mémoires
in acuteness or depth.
The three text-books just mentioned contained little or nothing new
either in facts or ideas on the subject of the nutrition of plants;
all three were rather compilations of what was already known, and
differed from each other only in their selection of material and in
the form which each sought to give to the general theory; but this is
a reason why we should take a nearer look at them, that we may learn
how the spirit and tendencies of the time were reflected in vegetable
physiology, and made themselves felt particularly in the theory of
nutrition.
De Candolle’s work appeared in French in 1832 in two volumes, the
first only being devoted to the subject of the nutrition of plants,
and in German in 1833 with many valuable annotations by the translator
Roeper, under the title, ‘Pflanzen-physiologie oder Darstellung der
Lebenskräfte und Lebensverrichtungen der Gewächsc.’ It suffers, in
common with the other two books we have mentioned on the same subject,
and with the earlier works of Du Hamel, Mustel, and other writers, from
a too discursive mode of treatment, which has the effect of burying
the points of fundamental importance under a huge mass of facts and
statements from other writers. It contains much that might have been
omitted as obsolete, and much empirical material of a purely chemical
nature, which could not at that time be applied to the purposes of
physiology. Nevertheless, it deserved the great consideration which
it enjoyed for a long time, especially in Germany, for its author had
undertaken to treat vegetable physiology as a separate and peculiar
branch of knowledge, not ignoring at the same time its connection with
and dependence on physics, chemistry, phytotomy, and biology proper,
and thus to give a full and complete delineation of vegetable life;
whereas the best works that had been written since Du Hamel’s time,
especially on the nutrition of plants, had proceeded from chemists and
physicists or from plant-growers like Knight and Cotta, who treated
the subject in a one-sided manner, each from his own point of view,
and made no attempt to give a connected account of all the phenomena
of vegetation. For this reason De Candolle’s ‘Physiologie végétale’ is
the most important performance that appeared after Du Hamel’s ‘Physique
des Arbres’; and if we wish to know what progress was made in vegetable
physiology generrally, and in the doctrine of nutrition particularly,
in the period from 1758 to 1832, we have only to compare the contents
of these two books. That this progress was a considerable one, appears
plainly from a short summary at the end of the first volume of the
general theory of nutrition, as De Candolle himself conceived it; this
summary will show us at the same time that he aimed rather at giving
a clear account of the whole of the internal economy of the plant,
than at searching into the moving forces, the causes and effects. From
this he was necessarily withheld by his assumption of a vital force.
He distinguished four kinds of forces; the force of attraction which
produces the physical, and that of elective affinity which causes
the chemical phenomena; then the vital force, the original source of
all physiological, and the soul-force, the cause of all psychical
phenomena. Only the first three of these forces operate in the plant,
and though it is necessary to find out what phenomena in vegetation are
due to physical or chemical causes, yet the main task of the vegetable
physiologist is to discern those which proceed from the vital force,
and the chief mark of such phenomena is that they cease with the death
of the plant (p. 6). Of course therefore all the peculiar phenomena of
nutrition, which are manifested only in the living plant, come within
the domain of the vital force. It must be allowed, however, that De
Candolle has made a very moderate use of the vital force, and confines
himself wherever he can to physical and chemical explanations; and when
he has recourse to the vital force, it is owing less to the influence
of his philosophical point of view than to the fact that his account is
based rather on tradition and information at second hand than on actual
research. It is true that De Candolle was perhaps better acquainted
than any contemporary botanist with the physics and chemistry of his
day, and it is part of his great merit that he should have acquired
so much knowledge on these subjects while engrossed in his splendid
labours as a systematist and morphologist; but he betrays, at least
in his later years, a want of practice in the study of physics and
a want also of the habit of mind which this imparts, and which is
more important to the physiologist than a knowledge merely of many
facts. But this defect is still more apparent in Treviranus and Meyen,
whose works on physiology were published soon after that of the great
systematist.
De Candolle first brings together all the facts in physiology which
have been discovered from the beginning, not omitting the chemical
researches of more modern times into the substance of plants, and then
gives a general delineation of the processes of nutrition in the plant:
‘The spongioles (an unfortunate invention of his own which has not
yet disappeared from French books, and plays a great part in Liebig’s
latest work)—the spongioles of the roots, being actively contractile
and aided by the capillarity and hygroscopic qualities of their tissue,
suck in the water that surrounds them together with the saline organic
or gaseous substances with which it is laden. By the operation of
an activity which is manifested principally in the contractility of
the cells and perhaps also of the vessels, and is maintained by the
hygroscopic character and capillarity of the tissue of the plant and
also by the interspaces produced by exspiration of the air and by other
causes, the water sucked in by the roots is conducted through the wood
and especially in the intercellular passages to the leaf-like parts,
being attracted in a vertical direction by the leaves and in a lateral
direction by the cellular envelope (cortical parenchyma) at every
period of the year, but chiefly in the spring; a considerable part of
it is exhaled all day long through the stomata into the outer air in
the form of pure water, leaving in the organs in which the evaporation
takes place all the saline, and especially all the mineral particles
which it contained. The crude sap which reaches the leaf-like parts of
the plant there encounters the sun-light, and by it the carbonic acid
gas held in solution by the sap, whether derived from the water sucked
in by the roots or from the atmospheric air, or being part of that
which the oxygen of the air produced with the surplus carbon of the
plant, is decomposed in the day-time; the carbon is fixed in the plant
and the oxygen discharged as gas into the air. The immediate result of
this operation appears to be the formation of a substance which in its
simplest and most ordinary state is a kind of gum consisting of one
atom of water and one of carbon, and which may be changed with very
little alteration into starch, sugar, and lignine, the composition
of which is almost the same. The nutrient sap thus produced descends
during the night from the leaves to the roots, by way of the rind and
the alburnum in Exogens, by way of the wood in Endogens. On its way it
falls in with glands or glandular cells, especially in the rind and
near the place where it was first formed; these fill themselves with
the sap and generate special substances in their interior, most of
which are of no use in the nutrition of the plant, but are destined
either to be discharged into the outer air or to be conducted to other
parts of the tissue. The sap deposits in its course the food-material,
which becoming more or less mixed up with the ascending crude sap in
the wood, or sucked in with the water which the parenchyma of the
rind draws to itself through the medullary rays, is absorbed by the
cells and chiefly by the roundish or only slightly elongated cells,
and is there further elaborated. This storing up of food-material,
which consists chiefly of gum, starch, sugar, perhaps also lignine,
and sometimes fatty oil, takes place copiously in organs appointed for
the purpose, from which this material is again removed to serve for
the nourishment of other organs. The water, which rises from the roots
to the leaf-like parts of the plant, reaches them in an almost pure
state, if it passes quickly through the woody parts, the molecules of
which are but slightly soluble. If, on the other hand, the water flows
through parts in which there is much roundish cell-tissue filled with
food-material, it moves more slowly and mixes with this material and
dissolves it; when it is drawn away from these places by the vital
activity of the growing parts, it reaches them not as pure water but
charged with nutrient substances. The juices of plants appear to
be conveyed chiefly through the intercellular passages. The vessels
probably share in certain cases in these functions, but serve generally
as air-canals. The cells appear to be the really active organs in
nutrition, since decomposition and assimilation of the juices take
place in them. Cyclosis (of Schultze’s vital sap[136]) is a phenomenon
which appears to be closely connected only with the preparation of
the milky juices, and to be caused by the actively contractile nature
of the cell-walls or of the tubes. Woody and other substances are
deposited in every cell in different quantities according to their
kinds and the accompanying circumstances, and clothe their walls; the
unequal thickness of the layer so deposited appears according to Hugo
von Mohl to have given rise to the supposition of perforated cells;
that is, the parts of the cell-wall that remain transparent appear
under the microscope as pores. Every cell may be regarded as a body
which prepares juices in its interior; but in vascular plants their
activity stands in such a connection with a complex of organs, that a
single cell does not represent the whole organism, as may be said of
the cells of certain cellular plants, which are all like one another.
There is no circulation in plants like the circulation in animals, but
there is an alternating ascent and descent of the crude sap and of the
formative sap which is often mixed with it. Both these phenomena depend
perhaps on the contractile power in cells that are still young, and if
so, this power would be the true vital energy in plants.‘
What is strange to us in De Candolle’s theory of nutrition is due
chiefly to the predominance of the vital force; yet at the same time
it gives the facts in their general connection, and its best feature
is, that the true function of the leaves, the decomposition of carbon
dioxide in light and the production of organisable substance, is made
the central point of the whole circle of the processes of nutrition.
Very different in this respect were the views of the two most eminent
German vegetable physiologists at the close of the period before
us, Treviranus and Meyen, though they are not in accord with one
another in their general conception of the subject. It may be said
that all the prejudices and errors, built up on the foundation of the
hypothesis of a vital force during the first thirty years of the 19th
century, culminated in Treviranus; while others were already setting
up the mechanical explanation of the phenomena of vegetation as the
one object to be attained, Treviranus produced once more the whole
machinery of the obsolete doctrine of the vital force, and with such
effect, that his ‘Physiologie der Gewächse’ was already obsolete when
it appeared in 1835. The second volume of Meyen’s ‘Neues System der
Pflanzenphysiologie’ was a striking contrast to the work of Treviranus;
Meyen endeavours as far as possible to trace back the phenomena of
vegetation to mechanical and chemical causes, though he does not
often succeed in bringing anything to light that is new or of lasting
service. He, like Treviranus, was deficient in sound training in
chemistry and physics; they did not stand in this respect, as Hales and
Malpighi once did, at the highest point of knowledge of their time. At
the same time there was a great difference in the way in which each
dealt with the writings of his predecessors; Treviranus, who had done
good service in former years in phytotomy, was not equal to the task
which he had now undertaken; his physiological expositions are marked
by feebleness of thought and by an inability to survey as from a higher
ground the connection between the facts; he distrusts all that had been
done during the previous thirty years, and almost everywhere appeals
to the publications of the 18th century; he lives indeed in the ideas
of the past, without gaining vigour from the forcible reasoning and
freshness of thought of a Malpighi, a Mariotte, or a Hales. Meyen’s
treatment of his subject is on the contrary fresh and vigorous; he does
not disregard the old, but he holds chiefly to the modern conquests
of science; while Treviranus with singular ill-luck constantly
overlooks what is valuable in itself and important in its results,
Meyen generally picks out the best things from the books before him;
Treviranus timidly avoids expressing any view decidedly and maintaining
it; Meyen, amid the multiplicity of the labours which we have already
described, finds no time to arrange his thoughts, is hasty in judgment
and often contradicts himself. But with all these defects, he is still
the champion of the new tendencies that were being developed, while
Treviranus lives entirely in the past, and shows no trace of the
actively creative spirit which was soon to burst forth so mightily in
every branch of natural science.
If we examine what both these writers have said on the subject of
the nutrition of plants, we shall find that the difference in their
general views in physiology as described above appears at once in
their treatment of the work of suction in the roots, and of the
means by which the sap ascends; here in Treviranus the vital force
is everything; it makes the vessels of the wood conduct the juices
from the roots into the leaves, with other antiquated notions of the
kind; Meyen on the contrary adopts Dutrochet’s position, and even
rejects De Candolle’s spongioles. Treviranus knows not what to make of
respiration; Meyen explains it without hesitation as a function that
answers to respiration in animals, and finds in it the main cause of
the natural heat which Treviranus derives in the old mystical fashion
from the vital force. In one point however they agree, namely, in a
complete misconception of the connection between the decomposition of
carbon dioxide in the leaves and the general nutrition of the plant.
It is necessary to the understanding of the confusion of ideas which
had crept at this time into the doctrine of nutrition, and to a right
estimate of the services of Liebig and Boussingault on this point, that
we should look a little more closely into the chemical part of the
theory of nutrition in Treviranus and Meyen.
Treviranus in the introduction to his book repudiated the idea of a
vital force separable from matter, but he was in fact a prisoner
within that circle of ideas, and he made a much freer use of the vital
force than De Candolle; he went even farther than this, and in his want
of chemical experience he hit upon the grossly materialistic notion of
a vital matter [I. p. 6]. This vital matter is a half-fluid substance,
which may be obtained from all bodies that were once alive by boiling
and by decay; it is formed from other elements, but it is itself the
true elementary matter with which alone physiology has to do; it is
common to the animal and vegetable kingdom, and is purest when in the
form of mucilage, albumen, and gelatine; that animals and plants alike
consist of this vital matter explains the circumstance, that plants
serve as food for animals and animals as food for plants. He goes on to
show that a similar unctuous substance, called by chemists extract of
the soil, and considered by many of them to be an important ingredient
in the nutrition of plants, is their true and proper food. This extract
of the soil was therefore the vital matter which plants take up; it
was natural that Treviranus should no longer attribute any importance
to the decomposition of carbon dioxide in the leaves, especially
as he was unable to understand the chemical connection of all that
Ingen-Houss, Senebier, and de Saussure had written. He explained the
co-operation of light in the nutrition of plants to be a merely ‘formal
condition,’ and the salts in solution in the water of the soil were in
his opinion stimulants for the use of the extremities of the roots,
which were thus put into a condition of ‘vital turgescence’; and as the
functions of the leaves, such as Malpighi and Hales had conjectured,
and Ingen-Houss, Senebier, and de Saussure had proved it to be, had no
existence for Treviranus, he made the assimilation of the soil-sap take
place on its way, as it flowed upwards and downwards through the plant.
We see that nothing can be conceived more deplorable than this theory
of nutrition; it would have been bad at the end of the 17th century, it
is difficult to believe that it could have been published thirty years
after de Saussure’s work.
There is much in the details of Meyen’s views on the chemical
processes in the nutrition of plants that is better than what we find
in Treviranus; it is a great point that he concluded from earlier
experiments, that the salts which find their way with the water into
the roots are not merely ‘stimulants’ but food-material, and, as was
before said, he explained the respiration of oxygen by plants correctly
in accordance with de Saussure’s observation. But he too stumbled over
the assimilation of carbon; he, like so many before and after him, was
confused by the simple fact, that gaseous matter takes part both in the
nutrition and the respiration of the plant; and taking the processes in
both cases for processes of respiration, he considered the absorption
of oxygen to be the only important and intelligible function, and the
decomposition of carbon dioxide in light to be a matter of indifference
as regards the internal economy of the plant. Instead of ascertaining
by a simple calculation, whether the apparently small quantity of
carbon dioxide in the atmosphere was perhaps sufficient to supply
vegetation with carbon, he simply declared it to be insufficient,
and because plants will not flourish in barren soil merely by being
supplied with water containing carbon dioxide, he gave up the
importance of that gas altogether. He too found the humus-theory, which
had been constructed by the chemists, more convenient for his purpose,
and like Treviranus derived the whole of the carbon in plants from
‘extract’ of the soil, without any close attention to the facts of the
case; he refused to believe that the soil is rendered not poorer but
richer in humus by the plants that grow on it. It is obvious then that
the account given by Treviranus and Meyen of the chemical processes
that take place in the nutrition of plants, though correct in some of
the details, could afford no true general view of the processes of
nutrition, because it entirely misconceived the cardinal points in the
whole theory, namely the source of the carbon, and the co-operation
of light and of the atmosphere; and thus the best results of the
observations of Ingen-Houss, Senebier, and de Saussure were lost upon
the German vegetable physiologists.
6. SETTLEMENT OF THE QUESTION OF THE FOOD-MATERIAL OF PLANTS. 1840-1860.
We have noticed in the previous section the rise of views during
the period between 1830 and 1840 which were calculated to make the
hypothesis of a vital force appear superfluous, at least as an
explanation of certain important phenomena in vegetation; such were
the referring the natural heat of plants to chemical processes, and
the movement of the sap to osmose; in the domain of chemistry also,
in which Berzelius had in the year 1827 made the distinction between
organic and inorganic matter to consist in the fact, that the former
is produced under the influence of the vital force, the opinion was
openly expressed that such an intrusion of the vital principle could
not be allowed, since organic compounds had been repeatedly produced
from inorganic substances by artificial means, and therefore without
its aid. The general tendency of scientific thought was now in fact
unfavourable to the nature-philosophy of former days; it inclined to
free itself from the obscurity that attended the idea of a vital force,
and to assert the belief, that chemical and physical laws prevail alike
outside and inside all organisms; this idea became an axiom with the
more eminent representatives of natural science after 1840, and if not
always expressed in words, was made the basis of all their attempts to
explain physiological phenomena.
Thus a freer course began to open for the intellectual movement of the
time even before the year 1840, and strict inductive research, and
above all the establishment of facts and closer reasoning were now
demanded in the question of the nutrition of plants, as they were also
in the domain of morphology and phytotomy. But in dealing with the
theory of nutrition, the first thing required was not the discovery
of new facts so much as the forming a correct appreciation of the
discoveries of Ingen-Houss, Senebier, and de Saussure, and clearing
away the misconceptions that had gathered round them. The chief modern
representatives of vegetable physiology, De Candolle, Treviranus, and
Meyen, had increased the difficulty of the task by neglecting to keep
the several questions of their science, the chemical especially and
the mechanical, sufficiently distinct from one another. The question,
what are the materials which as a rule compose the food of plants,
though one of the first and most immediate importance, had been very
imperfectly investigated, while attention had been diverted to a
confused mass of comparatively unimportant matters, and the solution
of that question had been rendered impossible for the time by the
humus-theory, an invention of chemists and agriculturists, which
Treviranus and others had fitted so readily into the doctrine of a
vital force. To Liebig belongs the merit of removing these difficulties
and all the superfluous matter which had gradually gathered round the
subject, and of setting forth distinctly the points which had to be
considered; this was all that was required to ensure a satisfactory
solution of the problem, for former observations had supplied an
abundance of empirical material. But some points of minuter detail were
brought out in the course of his investigations which required new and
comprehensive experiments, and for these a most capable and successful
observer was found between 1840 and 1850 in the person of Boussingault.
But before we go on to give a fuller account of the work of Liebig and
Boussingault, we may mention a circumstance which serves to indicate
the character of the revolution in scientific opinion before and after
1840. An anonymous ‘Friend of science’ had put a prize at the disposal
of the Academy of Göttingen for an answer to the questions, ‘whether
the inorganic elements, which are found in the ashes of plants, are
found in the plants themselves, in cases where they are not supplied
to them from without; and whether these elements are such essential
constituents of the vegetable organism, as to be required for its full
development.’ The first question appears in the present day absurd,
since it implies the possibility of elementary matter coming into
being, and of certain special elements coming into being in the plants
themselves, an idea however not unfamiliar to the nature-philosophy
and the vital force school. It was easy for Wiegman and Polstorff,
the authors of the essay that gained the prize (1842), men of the
new school, to answer the first question in the negative, and indeed
their answer to the second question involved a negative answer to the
first. The investigations made by Wiegman and Polstorff in connection
with the subject of the second question were conducted in a thoroughly
intelligent manner, though they set out from the hypothesis that a
certain quantity of compounds of humic acid, as it was called, must
be present in the food-mixtures. Their experiments, better adapted to
the purpose than any previous ones, showed convincingly that it is
necessary to the normal nutrition of the plant that it should take up
the constituents of the ash; the observers also took into consideration
a number of other questions connected with nutrition, in which however
we may already sec the influence of Liebig’s book which had come out
during their investigations.
This work was the one entitled ‘Die organische chemie in ihrer
Anwendung auf Agricultur und Physiologie,’ which appeared first in
1840 and was afterwards repeatedly reprinted and enlarged. The name of
the author, the first chemist of Germany, raised an expectation that
the questions respecting nutrition would be dealt with otherwise than
they had hitherto been, and this expectation was more than fulfilled
by the novelty and boldness with which Liebig cleared up the most
important points of the theory, seized upon all that was essential
and fundamental, and disregarded the unimportant matter which had
before only served to confuse the question. Moreover, he was able
to rest on long-accepted facts in just those points which were the
most important, and on these he had only to throw the light of his
chemical knowledge to dispel the previous darkness. In accordance with
his main purpose, which was to apply organic chemistry and vegetable
physiology to the service of agriculture, Liebig directed the severity
of his criticism first of all against the humus-theory constructed
by chemists and agriculturists and thoughtlessly adopted by various
physiologists; this was the first thing that must be got rid of, if
the question was to be answered, of what substances does the food
of plants consist, for the humus-theory was at once incorrect, and
the product of a want of reflection which overlooked facts which lay
before men’s eyes. Liebig showed that what was known as humus is not
diminished but constantly increased by vegetation, that the quantity
in existence would not suffice for any length of time for the support
of a vigorous vegetation, and that it is not taken up by plants. This
once established, and Liebig’s calculations left no doubt on the
point, there remained one source only for the carbon of the plant,
namely, the carbon dioxide of the atmosphere, with regard to which it
was shown by a very simple calculation resting on eudiometric results
that its quantity is sufficient to supply the vegetation of the whole
earth for countless generations. It is true that Liebig in his zeal
went much too far, when he found something contradictory in the true
respiration of plants, because it is connected with the elimination of
carbon dioxide, and simply denied its reality. On the other hand the
theoretical significance of the fact established by de Saussure, that
the elements of water are assimilated at the same time as the carbon,
was now for the first time clearly explained. Liebig was better able to
realise the importance of this fact for the theory of nutrition than
de Saussure had been. But these weighty points were not the ones which
attracted most attention with the adherents and opponents of Liebig;
the practical tendency of his book made the discussion, to which it
gave rise especially among chemists and agriculturists, turn rather on
the question of the source of the nitrogen in the substance of plants.
The humus-theory had made the nitrogen like the carbon enter the plant
in the form of organic compounds. De Saussure in his great work of 1804
had named ammonia as a compound of nitrogen which might be taken into
consideration with others, but he arrived at no definite conclusion.
Liebig, from different points of view and in reliance on his own
investigations into the nature of nitrogen and its compounds, arrived
at the result, that ammonia must ultimately be the sole source of the
nitrogen in the plant, and that the ammonia in the atmosphere and in
the soil is quite sufficient to supply vegetation with the requisite
amount of nitrogen just as the carbon dioxide of the atmosphere is
the sole source of the carbon of the plant; and so he concluded that
‘carbon dioxide, ammonia, and water contain in their elements the
requisites for the production of all the substances that are in animals
and plants during their life-time. Carbon dioxide, ammonia, and water
are the ultimate products of the chemical process of their putrefaction
and decay.’
Liebig was less happy, at least as regards his mode of treating the
subject, in his remarks on the necessity and specific importance of
the constituents of the ash to the nutrition of plants. Instead of
insisting on an experimental answer to the question, what constituents
of the ash are absolutely indispensable to the health of one or all
plants, he lost himself in ingenious chemical theories, intended to
show the operation of inorganic bases in fixing vegetable acids, the
extent to which different bases can replace each other, and similar
matters.
It is not requisite for our purpose to follow Liebig in his
applications of his theoretical remarks to agriculture, still less
to occupy ourselves with the sensation and the discussions which his
work excited among practical and theoretical farmers and agricultural
chemists. The scientific value of Liebig’s considerations on the
nutrition of plants stood out in a purer and more definite form for
the vegetable physiologists, who turned their attention chiefly to
the points mentioned above. It is true that Liebig’s work encountered
lively opposition from these men also, and the two foremost
representatives of vegetable physiology at that time, Schleiden and von
Mohl, criticised it unsparingly; this was due partly to the deductive
method adopted by Liebig, to which botanists were unaccustomed in
physiological questions, and partly to the derogatory expressions in
which he indulged against the vegetable physiologists, whom he held
responsible with the botanists generally for all the absurdities
connected with the humus-theory. Von Mohl asked, and justly, whether
de Saussure, Davy, Carl Sprengel, Berzelius and Mulder, the real
founders of the theory, were botanists. But it was unnecessary for von
Mohl, Schleiden and others to feel touched by Liebig’s reproach, at
least so far as it was addressed to professed physiologists, for they
were no more physiologists than Davy, Berzelius or Mulder. Professed
vegetable physiologists, official public representatives of vegetable
physiology there were none, and then as now every one who occupied
himself occasionally with questions of the kind was called a vegetable
physiologist. In this way the contest became a dispute about words,
and Liebig, von Mohl and Schleiden lost an excellent opportunity for
influencing public opinion in favour of the idea, that it was high
time to establish public official representatives of so important a
branch of science, who should devote themselves entirely to it; how
could it be expected that Professors of botany, who were required
by the government and the public to work for the advancement of
systematic botany, phytotomy, and medical botany, to give instruction
in these subjects, and to devote a large portion of their time to the
management of botanic gardens, should do much to promote the study of
vegetable physiology, which demands very considerable acquaintance also
with physics and chemistry? and where were the laboratories and the
instruments for the serious prosecution of this branch of science? But
these questions were not raised, and the old state of things remained
for the time unchanged.
As regards the scientific questions at issue between Liebig and von
Mohl, Schleiden, and various agricultural chemists, the contest was
chiefly about matters of secondary importance, and among these might
be included the objection that Liebig knew scarcely anything of the
anatomy of the plant. The main point was, that he had corrected
mistaken views as to the way in which plants are fed, had refuted
gross errors, had shown what was fundamental and essential and what
was unimportant. Everything that was written on the subject after 1840
shows that he did all this completely; the publications called forth
by the controversy on his book occupied in the main the ground which
Liebig had cleared. Now every body knew all at once what was meant by
the decompositon of carbon dioxide in the green parts of plants, that
the constituents of the ash are not mere seasoning to the vegetation,
and the like; firm ground had been won for all, a number of scientific
truths had become common property for ever; this did not of course make
it less meritorious in others, to test the rest of Liebig’s theories,
or even to correct his great mistake about the respiration of plants,
as was done emphatically by von Mohl.
It would not be consistent with the design of this work to go into all
the details of the discussion excited by the appearance of Liebig’s
book, into questions for instance respecting the first products
of assimilation in plants, and their further transformations by
metabolism. Whether the primary use of the basic mineral constituents
is merely to fix the vegetable acids, whether these acids are the
first products of assimilation, or whether carbo-hydrates are the
immediate result of that process, and similar questions, were for some
time only matter of conjecture, deduction and combination, unsupported
by certain observation obtained by suitable methods; it was not
till after 1860 that new paths were struck out on these subjects,
and important results achieved. More important at the time for the
advance of the science was the further examination of the question
respecting the source of the nitrogen which plants assimilate; it was
the more necessary that this point should be finally settled, because
Liebig’s deductions still gave room for many doubts, and the first
of vegetable physiologists, de Saussure, in his later days made the
mistake of coming forward in opposition to Liebig as a defender of the
humus-theory, maintaining (1842) that ammonia or the nitrates are not
themselves the food-material of plants, but only serve to dissolve
the humus. Others also found it difficult to give up entirely the
old and favourite doctrine of the humus; though von Mohl and others
acknowledged that the carbon of plants is mainly derived from the
atmosphere, yet they thought themselves obliged to assign to the humus,
on account of the nitrogen which it contains, a very important share
in promoting vegetation. Under these circumstances it was extremely
fortunate for physiology that BOUSSINGAULT took up the question. He
had occupied himself before the appearance of Liebig’s book with
experimental and analytical investigations into germination and
vegetation, and specially into the source of nitrogen in plants. His
experiments in vegetation in 1837 and 1838 produced no very decisive
results; but he continued them for some time longer, improving his
methods of observation from year to year; and between the years 1851
and 1855 he succeeded in establishing with all certainty as the result
of many repeated trials, that plants are not capable of assimilating
the free nitrogen of the atmosphere, but that a normal and vigorous
vegetation is produced, when they are supplied with nitrogen from the
nitrates in the soil. It appeared also that plants will flourish in
a soil from which all trace of organic substance has been removed by
heat, if a nitrate is added to the constituents of the ash; this proves
at the same time that the whole of the carbon in such plants is derived
from the carbon dioxide of the atmosphere without the co-operation
of the humus, and that consequently the favourable effect of a soil
rich in humus on vegetation must be due to other causes than those
which were assumed by the humus-theory. We cannot describe the further
services rendered by Boussingault to the theory of nutrition, for this
would take us too much into technical details, and the best and most
important of his results were first given to the world after 1860,
and do not fall therefore within the limits of this history. But it
should be mentioned that Boussingault must be considered the founder
of modern methods of conducting experiments in vegetation. Liebig had
before spoken in terms of sufficient severity of the miserable way
in which experiments on the subject of the nutrition of plants were
managed after de Saussure’s time till later than 1830, but he did not
himself introduce better methods; this was reserved for Boussingault.
One instance may be given; those who desired to decide the question of
the humus by experiment, such as Hartig in conjunction with Liebig and
others, generally adopted the plan of supplying plants with compounds
of humus-acid, and seeing what would be the result. Boussingault did
as Columbus with the egg; he simply made plants supply themselves
with food in a soil artificially deprived of all trace of humus and
containing a mixture of food-material, in order to prove beyond
question that they do not need humus.
In Germany also Prince Salm-Horstmar made similar experiments to
those of Boussingault; he occupied himself chiefly in determining the
relative importance of the acids and bases of the ash in the nutrition
of plants, whether any and which of them are indispensable; these are
questions which approached their solution only after 1860, and some are
not yet decided.
The establishment of the facts, that plants containing chlorophyll
derive the whole of their carbon from the carbon dioxide of the
atmosphere, and that the latter is also the original source of the
carbon in plants and animals which do not contain chlorophyll; further
that the nitrogen which plants assimilate is derived from ammoniacal
salts or nitrates, and that the alkalies, alkaline earths in the form
of sulphates and phosphates, are indispensable ingredients in the food
of plants, must be considered to be the great results of the labour
bestowed on the theory of nutrition in the period from 1840 to 1860,
while the way was also prepared for many points, which were afterwards
of the first importance in the enquiry.
On the other hand the advance made in the theory of the movement of
the sap from the time of Dutrochet till nearly 1860 was so small as
to be scarcely worth mentioning; yet it was an advance, that the
physiological value of the doctrine of endosmose was more and more
highly estimated, and that more solid proofs of the theory itself
and a more exact acquaintance with osmotic processes were making it
possible to explain more of the details of the movement of material
in the plant, though the whole question was far from being finally
settled. One discovery must be specially mentioned, the establishment
by Hofmeister in 1857 of the fact, that the phenomenon observed for
centuries in the grape-vine and other trees, and more recently in Agave
and in many tropical climbing plants, known by the name of bleeding or
weeping and supposed to be confined to certain periods of vegetation,
not only occurs in all plants with true woody cells, but may be
produced in them at all times by suitable means. The knowledge of this
fact was an aid to the investigation of the cause of the weeping.
The theory of the descending sap was in the least advanced condition
during this period; appeal was still made to experiments of the kind
which Malpighi, Du Hamel, and Cotta had made, and which in reality show
nothing more than that in dicotyledonous woody plants a food elaborated
in the leaves is carried downwards through the cortex. As soon as it
was understood, that all organic substance originates in the leaves, a
fact which no one could doubt after 1840, no experiment was required
to prove that the formative matter necessary for the growth of the
roots, buds, and fruit, must be conducted to those parts from the
leaves. It could no longer be a question whether such a movement of
assimilated material takes place; it remained only to consider what are
the conducting tissues, and what is the nature of the substances which
are produced in the leaves and conducted to the rest of the organs.
Both questions in accordance with the organisation of the plant could
be properly answered only by microchemical methods, and these were not
adopted and further developed till after 1857. We have already said
that nothing certain was known even as late as 1860 about the chemical
combinations formed by assimilation in the leaves; De Candolle supposed
that the primary formative sap was a gum-like substance, from which
the rest of the various vegetable substances were secreted in the
different tissues. Theodor Hartig, who had done good service between
1850 and 1860 by his investigations into the starch in the wood of
trees and into proteid in seeds, by the discovery of sieve-tubes, by
observations on the amount of water in woods at different times of the
year, and by other contributions to botanical science, also occupied
himself with the subject of the descending sap, which he conceived of
as a formless primary mucilage, from which, as from De Candolle’s gum,
the various substances in the plant were deposited as it travelled
through the plant. He says (‘Botanische Zeitung’ for 1858, p. 341),
‘The crude sap is changed in the leaves into primitive formative sap,’
and ‘the formation of solid reserve-material (from this) involves
the elimination of large quantities of watery fluid.’ The occasional
remarks of vegetable physiologists of all sorts between 1840 and
1860 prove, that similar ideas respecting the formation of a primary
mucilage of this kind in the leaves were generally entertained.
CHAPTER III.
HISTORY OF THE DOCTRINE OF THE MOVEMENTS OF PLANTS (PHYTODYNAMICS).
It will scarcely be doubted at the present day, that the mechanical
laws of growth, of geotropic and heliotropic curvatures, of the
various kinds of periodic movements, of the twining of tendrils
and climbing plants, and of movements dependent on irritation, may
be referred to a common principle, and that in all these movements
besides the elasticity of the cell-walls the still unknown qualities
of the protoplasm play the most important part, and that consequently
the ‘streamings’ of the protoplasm, the movements of swarmspores and
similar occurrences must be ranked with these phytodynamical phenomena.
From this point of view phytodynamics would appear to be one of the
most important foundations of vegetable physiology. The recognition of
this fact is however of very recent date, and to imagine that such a
conception of the movements of plants was present to the minds of the
early physiologists would be to attribute to the past ideas to which
it was entirely a stranger. These movements were scarcely noticed even
as curiosities in former ages, and it was not till the end of the 17th
century that some attention began to be paid to them; and very slow
progress was made at a later time in disentangling the relations which
come under consideration and which are some of them very complicated,
in determining the dependence of the phenomena on external influences,
and explaining to some extent their mechanical conditions.
Single movements of parts of plants are noticed in a cursory manner by
some early writers. Varro was the first who mentioned the heliotropic
movements of the stalks of many flowers, which he says were for that
reason called heliotropic flowers; in the following century Pliny
says that the leaves of clover close when bad weather is approaching;
Albertus Magnus in the 13th century, Valerius Cordus and Garcias del
Huerto in the 16th, thought the daily periodical movements of the
pinnate leaves of some Leguminosae worth recording; Cesalpino noticed
the movements of tendrils and climbing plants, and was surprised to
see that the latter to some extent seek for their supports. These were
every-day phenomena, but the striking sensitiveness of the leaves
of Mimosa pudica introduced from America could not fail to attract
attention, and so we find an essay on the causes of it in Robert
Hooke’s ‘Micrographia’ of 1667. The irritability of the stamens of
Centaurea had been already mentioned by Borelli in 1653.
1. We meet with the first speculations on the subject at the end of
the 17th century. Ray in his ‘Historia Plantarum’ (1693) commences
his general considerations on the nature of the plant with a succinct
account of phytodynamical phenomena, and introduces the whole by a
sentence of Jung: ‘Planta est corpus vivens non sentiens,’ etc. Though
Ray, like Cesalpino, seems to believe in the Aristotelian soul of
plants, yet he does on the whole endeavour to explain the movements
which he describes by physical and mechanical laws; he thinks that
the irritability of Mimosa in particular is not due to sensation,
but to known physical causes; the movement of the leaf when it is
touched is caused by a contraction, which again is due to a withering
or relaxation of its parts. He endeavours to apply the knowledge
of his time to the explanation of the mechanical process: leaves,
he says, remain tense only because the loss by evaporation is kept
constantly supplied by the water that flows to them from the stem;
if then in consequence of a touch the sap-passages of the leaves are
pressed together, the supply of water is not sufficient to prevent
their becoming relaxed. Ray mixes up together the movements from
irritability and the daily periodical movements, as was done till
recent times; the latter, he says, occur not only in the leaves of
Leguminosae, but in almost all similar pinnate leaves, and with these
periodical movements of leaves he places also the periodical opening
and closing of the flowers of Calendula, Cichorium, Convolvulus, and
others. That these last movements are due to changes of temperature
appeared to him to be proved by an experiment of Jacob Cornutus on
flowers of Anemone, which, when cut off and placed in a well-closed
box in a warm place, opened at an unusual time if the flower stalk
only was dipped in warm water. This fact, afterwards forgotten and
discovered again a few years ago, of the dependence of the movements of
flowers on changes of temperature, was applied by Ray to explain the
periodical movements of leaves, which, to use his own expression, fold
themselves together as the cold of night draws on, and open again with
the day, and as he thought that these movements are of the same kind
as the movements of irritability in Mimoseae, he tries to explain how
cooling has the same effect as a touch. It was natural in the existing
state of science to assume that changes of temperature were the first
causes of various movements, for a thrust was at that time almost the
only recognised cause of motion. Hence Ray explained the movements
of growing stems which are now called heliotropic by a difference of
temperature on the opposite sides. A certain Dr. Sharroc had observed
the stem of a plant on which he was experimenting grow towards that
part of a window, where the air found free entrance through an
opening; from this circumstance, and from the rapid elongation of the
stems of plants growing under cover, which he ascribed to the higher
temperature, Ray concluded that cold air hinders the growth of the side
of a stem on which it falls, and that this side must become concave.
Thus Ray used the etiolation of plants grown under cover to explain
their heliotropic curvatures, as De Candolle did one hundred and
forty years later, only with this difference, that he described the
rapidity with which forced plants shoot up to the higher temperature,
De Candolle to want of light. On the other hand Ray knew perfectly well
that the green colour of leaves is not produced by the access of air
but by the light, for, as he says, plants become green under glass, and
not under an opaque cover; and if they become less green under glass
than in the open air, this is because the glass absorbs certain rays of
light and reflects others. Ray however, like almost all later observers
till quite recent times, did not keep the elongation and bleaching of
etiolated plants sufficiently distinct; his account of this phenomenon
is spoilt by the presence of much that is obscure.
It has been justly observed by other writers on botanical subjects
that no notice is usually taken of one of the most remarkable of the
phenomena of which we are here speaking, because, being a matter of
every-day occurrence, it is simply accepted as something obviously in
accordance with the nature of things; this is the fact, that the main
stems of plants grow vertically upwards and their main roots downwards.
To the French academician Dodart, whom we have already encountered
in the history of the theory of nutrition, is due the great merit of
being the first to find this apparently simple phenomenon really very
remarkable; he convinced himself by experiments on germinating plants,
that these vertical positions are caused by curvatures, and endeavoured
to discover the physical reason why the main roots if placed in an
abnormal position escape from it by curving in the downward direction,
and the main stems in the upward direction, till they both reach the
vertical line. It was a matter of minor importance that his mechanical
explanation, which supposed that the fibres of the roots contract on
the moister side and those of the stem on the same side lengthen, was
quite unsatisfactory; it was much more important that these remarkable
phenomena were made the subject of scientific enquiry, and we find that
various observers soon after directed their attention to them, and
exercised their acuteness in attempts at explaining them; to these
attempts we shall return in a future page.
A still more universal phenomenon than the vertical growth of stems
and roots is the growth of plants generally, and it required as
much or even more of the spirit of enquiry to propose the question,
whether this growth can be explained by mechanical laws, and what that
explanation is. Mariotte touched on this question in 1679, but only
incidentally, and supposed that the stretching of the pith, which meant
at that time the whole of the parenchymatous tissue, was the cause of
the growth of the parts of plants. This idea might have had its origin
in the Aristotelian notion that the pith is the seat of the vegetable
soul, but Mariotte endeavoured to give physical reasons for it. Hales
in his ‘Statical Essays’ of 1727 went much more minutely into the
question of the growth of plants. Following the train of thought in his
doctrine of the nutrition of plants, he introduces his observations on
their growth with the remark, that plants consist of sulphur, volatile
salts, earth, water, and air, the first four of which attract one
another, and therefore form the solid, inert part of the substance of
plants; the air behaves in a similar manner as long as it is kept by
the other substances in a solid condition; but as soon as it is set at
liberty it is capable of expansion. On this power of expansion in the
air, by which the juices of plants are quickened and strengthened, he
builds his mechanical theory of growth, according to which the plastic
parts of the plant assume a state of tension, and as the air enters
into combination with other substances and so becomes fixed, warmth and
movement are excited, and these make the particles of sap assume by
degrees a form and shape. These principles supplied his starting-point.
To get a clearer idea of the way in which the growth of the parts
of plants proceeds, he made equi-distant punctures in young stalks
and leaves, and found that the intervals between them increased by
growth more in the younger intervening parts than in the older. In the
course of these observations he is particularly struck by the great
longitudinal extension which accompanies growth, because, as he says,
the vessels still continue hollow, as a glass tube when drawn out to
its utmost extent retains its canal. He finds Borelli’s idea confirmed,
that the young shoot grows by the extension in length of the moisture
in the spongy pith; and he endeavours to explain the fact that the
growing shoot does not extend equally in the transverse direction, and
so become spherically rounded off like an apple, from the nature of the
structure of the cell-tissue. That the air enclosed in the tissue and
the sap with it presses into the shoot with sufficient force to produce
so great an extension, he thinks is proved by his experiments, which
show him the great force with which the water rises in the bleeding
vine, and forces itself into swelling peas; it is known, he says, that
water acts with great force when it is heated in a vessel, for water
can be driven into the air by heat; the sap in plants is composed of
water, air, and other active ingredients, and makes its way with great
force into the tubes and cells, when it is heated by the sun.
2. The course of the 18th century gradually increased the number
of the phytodynamical phenomena, to which physiologists paid more
or less attention, and repeated attempts were made to explain them
on mechanical principles. These attempts were for the most part
unsatisfactory, because movements distinct in kind from one another
were mixed up together, their dependence on external influences was not
distinctly perceived, and the knowledge of the anatomical structure of
the parts which exhibited the movements was, owing to the decline of
phytotomy, extremely imperfect. Moisture and warmth played the chief
part in these explanations, but their mode of operation was expressed
in the most general terms; the mechanical processes in plants were
described much in the way in which a person with very indefinite ideas
as to the nature of steam and the construction of the inside of a
steam-engine might speak of its movements. The majority of writers,
in accordance with the tendencies of the age, professed their desire
to refer the phenomena of life in plants not to an unknown principle
called the soul, but to mechanical and physical causes; but they did
not apply their minds to the examination of these phenomena with that
strenuous effort, which in this subject especially could alone lead to
a complete and satisfactory explanation of them.
Linnaeus studied the periodical movements of flowers in 1751 and those
of leaves in 1755, but a mechanical explanation of them was not to be
expected from him; he contented himself with pointing out the external
conditions of these phenomena in many species, with classifying them,
and giving the periodical movements a new name by calling the positions
assumed by night the sleep of plants; nor did he use the word at all in
a metaphorical sense, for he saw in this sleep of plants a phenomenon
entirely analogous to sleep in animals. That the sleep-movements were
not capricious but due to external influences was with him a necessary
consequence from the nature and idea of the plant, which was that of
a living and growing being, only without sensation. But it should be
mentioned that he stated correctly that the movements connected with
the sleep of plants are not caused by changes of temperature, or not
by these only, but by change of light, since they take place in the
uniform temperature of a conservatory.
Linnaeus’ account of these kinds of movement was only formal, it is
true, but still it was well-arranged and clear; the treatment of the
same and other movements by his contemporary Bonnet was quite the
reverse. It is scarcely possible to imagine anything more shapeless,
such an utter confusion of things entirely different from one another,
as is to be found in Bonnet’s experiments and reflections on the
various movements of leaves and stems in his work on the function
of leaves, published in 1754; geotropic and heliotropic curvatures,
nutations and periodic movements, are all run one into another; a
person who understands something of the subject may find here and
there single things in his experiments that may be turned to account,
but he was himself unable to make any use of them. He set out with a
preconception which prevented him from the first from understanding
what his experiments showed him; it was his object to prove from a
multitude of instances, that stalks and leaves so curve, twist and turn
in all cases, that the under sides of the leaves are directed towards
the ground, in order that they may be able to suck up the dew, which
according to his theory is the chief nutriment of plants and rises from
the ground. It is no great merit in him, that amid all this confusion
a correct observation here and there forced itself upon him, as for
instance that organs, chiefly such as are young and ductile, if they
are put out of their natural position, endeavour to recover it by
bending and twisting. On the other hand his conclusions with regard to
the mechanical causes of these movements are utterly inane; the least
skill in judging of the results of his experiments must have led him to
very different ideas; warmth and moisture, he says, appear to be the
natural causes of movement, but warmth is more effective than moisture,
and the warmth of the sun more effective than that of the air. This
explanation is unsuitable to just those cases which he chiefly studied,
the geotropic and heliotropic curvatures. In one point only he arrived
ultimately at a right judgment, namely that the great lengthening of
the stem, the small size attained by the leaves and the want of colour
in plants grown under cover, are caused by partial or entire absence of
light; Ray however had shown this before as regards the colour.
Though Du Hamel, like many later writers, treated Bonnet’s
investigations, uncritical as they were and without plan, with great
respect, he gave himself a much better account of the various movements
of plants. In the sixth chapter of the fourth book of his ‘Physique
des arbres,’ 1758, he discussed all the phenomena of the kind that
were known to him under the title: ‘On the direction of stem and
roots, and on the nutation of the parts of plants.’ Under the head
of upright or oblique direction of the stem and roots, he speaks of
geotropic, heliotropic, and some other curvatures; then follows a
chapter on etiolation, and under the title, ‘Movements of plants, which
approximate to some extent to the voluntary movements of animals,’ he
enquires into the periodical and sensitive movements of the leaves of
Mimosa; he winds up with a short account of Linnaeus’ flower-clock, and
of the hygroscopic movements of the valves of fruits. The movements
of tendrils and climbing stems, of which Du Hamel seems to have known
little, are not mentioned in this connection; but they are noticed in
a former chapter with hairs, thorns and similar things,—a plan which
Cesalpino also adopted. If this way of dealing with the different
movements of plants is to be taken as a classification of them, it
was a very unsatisfactory one; for it separates like things, and
brings together things unlike; still it is an improvement on Bonnet’s
arrangement, while the author gives us also some new and valuable
observations. He may claim to be the first who made heliotropic
curvature depend on light, and it is a significant fact that he got
this conclusion from Bonnet’s experiments. After examining, like Hales,
into the distribution of growth in shoots, and discovering that this
ceases with the commencement of lignification, he proposed to himself
the question: at what spots does the lengthening of the roots take
place, and he found from suitable experiments that every root-fibre
grows only at its terminal portion, which is a few lines in length,
and that no other part of it increases in length. In the chapter on
the direction of the parts of plants he examines the explanations
which had been given of heliotropic curvatures. Astruc and de la Hire
had supposed the weight of the descending sap to be the cause of the
downward curvature of the roots, and the lighter vapours which ascend
in the tissue to be the cause of the upward curvature of the stem;
Bazin on the contrary attributed the geotropism of the roots to the
moisture in the earth. Du Hamel undertook to determine whether the
moisture, the low temperature, or the absence of light in the earth
made the roots curve downwards, and he was obliged by the result of
his experiments to deny that they do. But he was unfortunate in his
own explanation of the movements which we should now call geotropic,
heliotropic and periodic, for he came to the conclusion that the
‘direction of the vapours’ inside the vessels of the plant and round
about the plant has more to do with producing these movements than
any other causes, and that if warmth and light appear to influence
them, it is perhaps only because they produce vapours or communicate a
definite movement to them. As regards the movements of the leaves of
Mimosa, Du Hamel repeated the experiment made by Mairan in 1729, in
which the periodic movement continued even in constant darkness; he
found that this was so, and concluded that the periodic movements of
Mimosa do not essentially depend on temperature and changes of light;
Hill had determined in 1757 that the alternation of day and night was
the cause of the movements connected with the sleep of plants, because
he found that darkness artificially produced in the day-time made the
plants assume the nocturnal position; but Zinn in 1759 came to the same
conclusion as Mairan and Du Hamel. It was not till long after that the
question was to some extent cleared up by Dutrochet. Du Hamel thought
it necessary to give a formal refutation of the opinion expressed by
Tournefort, that the movements of plants are produced by muscles, and
to show that Tournefort’s vegetable muscles are hygroscopic fibres.
We have to mention in conclusion, that Du Hamel was the first who
observed that the two branches of a vine-tendril twine in opposite
directions round a support that happens to be between them; he also
appears to have been the first who compared the irritability of the
stamens of Opuntia and Berberis with that of Mimosa-leaves; the
stamens of Berberis were afterwards examined by Covolo in 1764, by
Koelreuter 1788, by Smith in 1790, and by others, but without leading
to any discoveries respecting the nature of the irritability. Dal
Covolo’s famous essay on the stamens of the Cynareae (1764) produced
no absolutely final result, but it contained some particulars which
threw light on the mechanical laws of these movements of irritability.
Koelreuter, who studied these objects in 1766, thought less of
discovering a mechanical explanation of them, than of finding arguments
in the irritability of the stamens for the necessity of insects to
pollination. An entirely new kind of movement was discovered by Corti
in 1772 in the cells of Chara, which is now known as the circulation
of the protoplasm; this form of movement in plants appeared at first
to bear no resemblance whatever to the phytodynamic processes then
known, and it was not brought into connection with them till a long
time after; on the contrary an erroneous idea soon began to prevail,
that it was a real rotation of the sap, as understood by the early
physiologists; this idea held its ground till far into the 19th
century, and being combined with mistaken notions respecting the
movements of latex, was developed by Schultz-Schultzenstein into the
doctrine of the circulation of the vital sap. For a time indeed Corti’s
discovery was forgotten, and had to be reproduced by Treviranus in
1811. A somewhat similar fortune attended the discovery of the movement
of the Oscillatorieae by Adanson in 1767, which misled Vaucher into
pronouncing them to be animals.
3. Imperfect as were the theoretical efforts of the 18th century in
this branch of botanical study, yet they aimed at tracing the various
movements back to the play of physical forces. But in the closing
years of the century another order of ideas, injurious to the healthy
progress of science, made its appearance in this, as in other parts of
botany and zoology. Even the majority of those who had no sympathy with
the nature-philosophy and its phraseology, believed that there was in
organised bodies something of a special and peculiar nature; because
the attempts made to explain the phenomena of life by mechanical laws
were on the whole unsatisfactory, all such explanations were looked
upon as impossible and even absurd, while it was forgotten that the
vital force, which was to explain everything, was a mere word for
everything that could not be explained in the life of organisms. This
vital force was personified, and seemed to assume a really tangible
form in the movements of plants. But the moment that a phenomenon was
handed over to this force, all further investigation was abandoned;
the idea with regard to phytodynamical phenomena especially was that
of the peasant, who could only explain the movement of the locomotive
by supposing that there was a horse shut up in it. Moreover the
knowledge of the inner structure of plants was at its lowest point
at the end of the 18th century; the spiral threads which could be
unwound were the only structural element whose form was to some extent
understood, and their hygroscopic movements were supposed to be due
to a combination of the pulsations of the vital force with the spiral
tendency of the plant. At the same time whole bundles of vessels were
taken for spiral fibres, or were supposed to consist of them, and
these were thought to be vegetable muscles, which contract under the
influence of various kinds of irritation, and so cause the movements
in the organs of plants; but it was forgotten that in the organs which
exhibit the most striking movements, such as sensitive leaves and
leaves that suffer periodical changes of position, these ‘muscles’
occupy a central position which unfits them for the function ascribed
to them. It would be unprofitable and wearisome to give many examples
of what is here stated, though many might easily be collected; it will
suffice to quote some sentences only from Link’s ‘Grundlehren der
Anatomie und Physiologie’ of 1807; they are particularly instructive,
because Link declared against the nature-philosophy and professed to
be on the side of inductive science. Under the head of movements of
plants, he discussed geotropic curvatures and other movements in the
superficial manner of the time and only to come to the conclusion, that
the direction of growth of stems and roots is caused by a polarity of
a definite kind in every plant, from which we may argue, he says, ‘to
higher connections of our planet in the world of space.’ He says again,
‘that it is natural to conjecture that light is the cause of the sleep
of plants,’ and then gives the contradictory statements of Hill, Zinn,
and De Candolle, all jumbled together into an inextricable tangle in a
fashion which sets all maxims of reasonable discussion at defiance. He
then puts aside all attempts at mechanical explanation with the remark,
that plants observe their regular times of sleep even when kept in the
dark and at a low temperature, for this evident habituation is one of
the most important marks of vitality. He is led to similar results by
Desfontaine’s observation, that a Mimosa, exposed to the shaking of
a wheeled vehicle, closes at first but then opens again. Speaking of
the rapid oscillations of the leaves of Hedysarum gyrans and similar
movements, he rejects Percival’s idea of a will in plants, but says
that the attempts to derive them from mechanical or chemical causes has
only led to solemn trifling.
It is plain that men who could print such remarks as these and still
worse than these, could not possibly effect anything in the province
of botany which we are considering. The broad and shallow stream of
such opinions as these flowed on till later even than 1830, but it
ran dry at last when its supplies were cut off by the effect of new
discoveries, and scientific investigation again gained the upper hand.
Some calmer thinkers, who could not rest content with empty words, had
meanwhile been pursuing the path trodden by Ray, Dodart, Hales, and
Du Hamel, and by experiment and earnest reflection had brought new
facts to light, which were at least calculated to pave the way for the
mechanical explanation of phytodynamical phenomena. Senebier in his
‘Physiologic végétale’ (1700) had described some minute researches
which he had made into the subject of etiolation; and though he made
the great mistake of attributing the want of colour in the leaves
and the excessive elongation of the stems to the decomposition of
carbon dioxide which does not take place in the dark, yet he gave an
example of genuine scientific investigation and again expressed its
true spirit, when he said that the Linnaean phrase, ‘the sleep of
plants,’ was unsuitable, because the sleeping leaves are not relaxed,
but continue as stiff as in the day-time. De Candolle also, like
Senebier, experimented in 1806 on the influence of light on vegetation,
and succeeded in proving that the daily period of leaves may be
reversed by artificial illumination; he was, as we have said above,
an adherent of the theory of a vital force, but only made use of it
when physical explanations failed him. The same year, (1806) is the
date of a brilliant discovery, which was extremely inconvenient to the
thorough-going adherents of the nature-philosophy and the vital force,
and did much to bring the scientific study of the movements of plants
back to the right path. ANDREW KNIGHT[137] showed by experiment that
the vertical growth of stems and primary roots is due to gravitation;
he attached germinating plants to a rapidly revolving wheel, and thus
exposed them to the centrifugal force, either alone or combined with
gravitation; the radicles, which normally follow gravitation, here
took the direction of the centrifugal force, while the stems assumed
the opposite direction. The next question was, why gravitation makes
the roots and stems take exactly opposite directions, why, that is,
in a plant placed in a horizontal direction, the root-end curves
downwards and the stem upwards. Knight supposed that the root, being
of a semi-fluid consistence, is bent downwards by its own weight,
while the nutrient sap in the stem moves to the underside and causes
stronger growth there, until by means of the curvature so produced the
stem assumes the upright position. Here too, as in Dodart’s case, it
was no great misfortune that the explanation proved afterwards to be
insufficient; it served at the time to explain as much as was then
known of the matter. The spirit of true scientific research displayed
in Knight’s explanation of geotropism was expressed in many other
contributions which he made to vegetable physiology; two only must be
mentioned here. He showed in 1811 that under suitable conditions roots
are diverted from the vertical direction by moist earth, an observation
which was confirmed by Johnson in 1828 and afterwards forgotten. More
attention was excited by his discovery in 1812, that the tendrils
of Vitis and Ampelopsis are negatively heliotropical, that is, that
they turn away from the source of light. A few other cases of this
kind of heliotropism have since been discovered, and they are highly
interesting, because they teach that there is the same opposition in
the relations of plants to light as in their relations to gravitation.
Knight possessed some of the direct and bold reasoning power of his
countryman Hales; he defied the vital force, and was always ready with
a mechanical explanation, if it was at all possible to find one. Thus
he explained the twining of tendrils by supposing that the pressure of
the support drives the juices to the opposite side, which consequently
grows more vigorously and causes the curvature, which makes the tendril
wind round the support. This theory was at all events better than the
one which von Mohl sought to put in its place in 1827, and no better
one was suggested till very recently. Much the same may be said of
Knight’s explanation of geotropic curvatures; it is true that Johnson
showed in 1828 that the ends of roots as they curve downwards set in
motion a heavier weight than themselves, and therefore do not simply
sink down, and Pinot in 1829, that they force their way even into
quicksilver, and that consequently Knight’s theory, at least as regards
the roots, is unsatisfactory; but no better theory has yet been found,
and his view also of the progress in the upward curvature of the stem
has not given place to any one that can be said to be more generally
accepted.
It was the commonly received opinion till after 1820 that the
movements of the parts of plants are produced by the spiral vessels,
or, which meant the same thing in those days, by the vascular bundles.
It was an important event therefore when Dutrochet proved in 1822,
that the movements of the leaves of Mimosa were due to the alternate
expansion of the antagonistic masses of parenchyma in the pulvinus or
cushion of succulent tissue found at the articulation, and that the
central vascular bundle follows passively their curvatures. Lindsay had
indeed arrived at the same conclusion from similar experiments as early
as 1790, but his unprinted essay on the subject was first produced by
Burnett and Mayo in 1827. Meanwhile Dutrochet had also found that light
influences the movements of the leaves in different ways; alternation
of light and darkness excites them to motion, while leaves which have
become rigid in continued darkness are restored by light to their
normal condition of sensitiveness.
Much attention was bestowed in the period between 1820 and 1830 on
various questions connected with the movements of the organs of plants.
In 1826 the faculty of medicine in Tübingen offered a prize for an
essay on the peculiar nature of tendrils and climbing plants, which
was intended to bring into discussion all the points which required
to be cleared up before a more thorough understanding of the whole
subject could be obtained. The two essays which gained the prize were
published in 1827. One was by Palm, the other by von Mohl, both of very
different value. Palm’s essay is a good and careful college-exercise;
but there is nothing of this character in von Mohl’s. The skill
of the composition, the exact knowledge of the literature of the
subject, the wealth of personal experience, the searching criticism,
the prominence given to all that is fundamental and important, the
feeling of certainty and superiority which the book inspires, all
unite to make the reader forget that it is not the work of a mature
and professed naturalist, but of a student of two-and-twenty years
of age. This academical prize-essay on the structure and twining of
tendrils and climbing-plants was one of von Mohl’s best works, and
altogether the best that appeared on the subject before Darwin wrote
upon it in 1865; at the same time it must be said that von Mohl did
not explain the exact mechanical processes in the tissues, for he
assumed a sensitiveness in both cases which causes the winding round
the support, and thought that this sensitiveness must be conceived of
‘dynamically’ and not ‘mechanically.’ Nevertheless von Mohl conducted
his investigation up to this point according to strict rules of
inductive science, and studied the facts which were capable of being
established by observation and experiment with an exactness such as
had not yet been applied to any case of movement in plants. It was
a genuine production of its author, strictly inductive up to the
point at which deduction became necessary. Von Mohl pointed out in it
essential differences in the behaviour of tendrils and climbing plants,
and the corresponding distinction between the organs which have to
be considered in each case, and he made the important discovery that
contact with the support acts as a stimulus on the tendril, though
he was wrong in supposing that the climbing stem also is similarly
affected. He at once assented to Dutrochet’s new view, that it is not
the vascular bundles but the layers of parenchyma which produce the
movements. He distinctly rejected the notion constantly repeated,
though with some hesitation, since the time of Cesalpino, that tendrils
and climbing-plants ‘seem to seek for’ their supports, as also the idea
which many had adopted without reflection from Grew, that the varying
direction of a climbing-stem is due to the varying influence of the
course of the sun and moon, and showed that the movements of nutation
in the stem are sufficient to explain the apparent seeking for the
support; it is true that he did not fully explain the corresponding
phenomena in tendrils, but he saw enough to set aside the old ideas.
We must not here go further into his many, and for the most part
excellent, observations; some of course had afterwards to be corrected,
but the important point was, that his full investigation of the
subject showed how such phenomena must be studied, if we are to arrive
at a strictly mechanical explanation of them.
If von Mohl had attempted to give a mechanical explanation of the
processes in the tissue of twining organs he must necessarily
have failed from ignorance of the agency of diffusion, which must
certainly be taken into consideration. This agency was not discovered
by Dutrochet till the year (1826) in which von Mohl undertook his
investigation, and some time elapsed before it was sufficiently
understood to be successfully applied to the explanation of phenomena
in vegetation. Dutrochet did indeed attempt so to apply his theory in
1828, and showed that changes in the turgidity of tissue are produced
by endosmose and exosmose, and consequently that a new mechanical
method of explanation had been discovered for processes which had been
usually referred to a supposed vital principle; but in his later and
more detailed researches into geotropism, heliotropism, periodical
movements and movements of irritability, which he collected together
in his ‘Mémoires’ of 1837, he fell into two different mistakes: he
assumed conditions of size and stratification in cells which do not
actually exist, for the purpose of explaining very various kinds of
curvature by endosmose, and he was not satisfied with endosmose in
the parenchyma; he postulated changes in the vascular bundles also,
which were supposed to be produced by the influence of the oxygen
in a way which he did not explain. Thus there were blots in his
explanation of separate processes, and his mechanical theories remained
unsatisfactory; but it is worthy of recognition and was most important
for the development of phytodynamics, that he was thoroughly in earnest
in his purpose of explaining every movement in plants by mechanical
laws. Even the opponents of such explanations were obliged to go deeply
into mechanical relations in order to refute him, and no one could any
longer be imposed upon by the simple assertion that all depends on the
vital force; so devoted a partisan of vital force as Treviranus had to
deal with endosmose as an established principle. Moreover Dutrochet’s
copious investigations presented such an abundance of interesting
observations, delicate combinations, and suggestive considerations,
that the study of them is still instructive and indeed indispensable to
any one who is occupied with such researches. Comparison of his papers
in the ‘Mémoires’ of 1837 with what was before known on the mechanical
laws of the movements of plants leaves us in no doubt that energetic
mental effort had taken the place of the old complacent absence of
thought.
Still no single movement had as yet been fully explained on mechanical
principles; but by the year 1840 clearer views had been attained
on the whole subject; the co-operation of external agencies was in
substance recognised, and the different forms of movement were better
distinguished, though much still remained to be done in this direction;
and as regards the mechanical changes in the tissue of the parts
capable of movement, a factor had been given in endosmose which must be
taken into account, though it might be necessary to seek a different
mode of applying it.