Those who were convinced of the sexuality of plants had endeavoured as

early as the previous century to form some idea with the help of the

microscope of the way in which the pollen effects the formation of the

embryo in the ovule. We may pass over Morland’s and Geoffroy’s very

rude attempts in this direction: Needham (1750), Jussieu, Linnaeus,

Gleichen, and Hedwig imagined that the pollen-grain bursts upon the

stigma, and that the granules it contains make their way downwards

through the style to the ovules, and are there either hatched into

embryos or assist in their production. This way of conceiving the

matter was closely connected with the theory of evolution which then

prevailed, and seemed to find some countenance in the seed-corpuscles

of animals; it was also supported by the observation that pollen-grains

placed under the microscope in water often burst and discharge their

contents in the form of a granular mucilage. It has been already

mentioned that Koelreuter rejected this view; he declared the bursting

of the pollen-grains to be contrary to nature, and considered the oil

which exudes from the grains to be the fertilising substance. This

view was adopted by Joseph Gärtner and Sprengel, but it fell into

disesteem, while that of Needham and Gleichen commanded some assent

some years longer. The next question was, how the granular contents of

the pollen-grain reach the ovules. Accident supplied a starting-point

for further consideration. Amici, who was examining the hairs on the

stigma of Portulaca for another purpose, saw on that occasion (1823)

the pollen-tube emerge from the pollen-grain, and the granular contents

of the latter, commonly known as the fovilla, execute streaming

movements like the well-known movement in Chara. The desire to verify

this remarkable fact, and to discover how the fertilising substance

is absorbed by the stigma, led Brongniart in 1826 to examine a great

number of pollinated stigmas. He succeeded in establishing the fact

that the formation of pollen-tubes is a very frequent occurrence.

The want of perseverance in following out his observation and a

prepossession in favour of Needham’s old theory prevented him from

discovering the course of the pollen-tubes all the way to the ovules;

he supposed, indeed, that after penetrating into the stigma they open

and discharge their granular contents, and he maintained distinctly

that these are analogous to the spermatozoids in animals, and are

the active part of the pollen. But now Amici addressed himself more

earnestly to the question, and in 1830 he not only followed the

pollen-tubes into the ovary, but also observed that one finds its way

into the micropyle of each ovule.

Thus the question was suddenly brought near to its solution, when

observers began to wander from the right path in different directions.

Robert Brown showed in 1831 and 1833 that the grains in the

pollen-masses of Orchids and Asclepiads put forth pollen-tubes as in

other plants, and that fine tubes are found in the ovary of Orchids

in which pollination has taken place; but he was in doubt about the

connection of these tubes with the pollen-grains, and rather inclined

to think that they were formed in the ovary, though possibly in

consequence of the pollination of the stigma. Schleiden went wrong in a

very different way, and by so doing made the question as prominent in

botanical research, as was that of the origin of cells at this time.

He published in 1837 some excellent investigations into the origin and

development of the ovule before fertilisation, certainly the best and

most thorough of the day. He at the same time showed that Brongniart’s

and Brown’s doubts were unfounded, and confirmed the statement of

Amici, that the pollen-tubes make their way from the stigma to the

ovule, which they enter through the micropyle. But he made them

push forward a little too far, for he asserted positively that ‘the

pollen-tube pushes the membrane of the embryo-sac before it, making an

indentation, and its extremity then appears to lie in the embryo-sac.

The extremity of the tube now swells out into a round or oval shape,

and cell-tissue forms from its contents; the lateral organs, one or

two cotyledons, are then produced, the original apical point remaining

more or less free and forming the plumule. The portion of the tube

underneath the embryo and the fold of the embryo-sac which envelopes

it are divided off sooner or later and disappear, so that the embryo

now really lies in the embryo-sac.’ This view, which appears to rest

on direct observation and is illustrated by figures which answer to

the description, corresponds with the old theory of evolution and has

a striking approximation to the ideas of Morland and Geoffroy; and if

it were correct, it would like these imply the necessity of pollination

to the formation of seeds that should contain embryos, but at the same

time it would do away with that which is the essential point in the

sexuality of plants, for the ovule would merely be the spot adapted to

the hatching of the embryo formed from the pollen. Schleiden’s idea

was at once adopted by Wydler, Gelesnow and various other botanists,

and especially by Schacht, but the most eminent microscopists withheld

their assent. Amici was the first who openly opposed the new doctrine;

before the Italian congress of savants at Padua in 1842 he endeavoured

to prove that the embryo is not formed at the end of the pollen-tube,

but from a portion of the ovule which was already in existence before

fertilisation, and that this part is fertilised by the fluid contained

in the pollen-tube. But the choice of a gourd, a plant eminently

unsuitable for his purpose, prevented his discovering the exact

details of the process, and Schleiden did not hesitate to denounce his

assertions in 1845 in the plainest terms. But in the next year (1846)

Amici produced decisive proof for the views which he had maintained;

he showed from the Orchidaceae, which were peculiarly well adapted

for such investigations, not only that Robert Brown’s doubts above

mentioned were without foundation, but, which is the main point, that

a body, the egg-cell, is present in the embryo-sac of the ovule before

the arrival of the pollen-tube, and that this body is excited by the

presence of the pollen-tube to further development, the formation of

the embryo. He gave a connected account on this occasion for the first

time of the whole course of these processes from the pollination of the

stigma to the perfecting of the embryo.

The correctness of the account given by Amici was confirmed in the

following year by von Mohl and Hofmeister, the latter of whom described

in detail the points which were decisive of the question from a

variety of plants, and illustrated them by very beautiful figures in

a more copious work, ‘Die Enstehung des Embryo der Phanerogamen,’

Leipzig, 1849. Tulasne also came forward in opposition to Schleiden’s

theory, being thoroughly convinced that there was no actual contact

of the pollen-tube with the egg-cell, denying indeed the existence of

the egg-cell before fertilisation. Thus a vehement controversy arose

on the subject; a prize offered by the Institute of the Netherlands

at Amsterdam was awarded to an essay of Schacht’s in 1850, which

defended Schleiden’s theory, and illustrated it by a great number of

drawings giving incorrect and indeed inconceivable representations of

the decisive points. Von Mohl says very admirably on this occasion

(‘Botanische Zeitung,’ 1863, Beilage, p. 7): ‘Now that we know that

Schleiden’s doctrine was an illusion, it is instructive, but at the

same time sad, to see how ready men were to accept the false for the

true; some renouncing all observation of their own dressed up the

phantom in theoretical principles; others with the microscope in hand,

but led astray by their preconceptions, believed that they saw what

they could not have seen, and endeavoured to exhibit the correctness of

Schleiden’s notions as raised above all doubt by the aid of hundreds

of figures, which had every thing but truth to recommend them; and

how an academy by rewarding such a work gave fresh confirmation to

an experience which has been repeatedly made good especially in our

own subject during many years past, namely that prize-essays are

little adapted to contribute to the solution of a doubtful question

in science.’ In this case the prize-essay had been refuted before it

appeared by von Mohl, Hofmeister and Tulasne. Schacht adhered all the

more firmly to Schleiden’s theory; after further controversy, in which

other writers of less authority took part, Radlkofer published in 1856

a complete review of the question, which fully confirmed Hofmeister’s

observations, and gave incidentally an account of Schleiden’s views

in the altered form which they had by that time assumed; this account

showed in fact that Schleiden had completely retracted his former

opinions, and in this retractation Schacht was soon after compelled to

follow him, having become acquainted with facts observed in the ovule

of Gladiolus, which were obviously irreconcilable with Schleiden’s

theory.

Hofmeister had from the first directed special attention to the

questions, whether any bodies are found in the pollen-tube which answer

in any way to spermatozoids, and whether any opening can be perceived

at the end of the tube. He found indeed forms in Coniferae in 1851,

which reminded him of the male organs of fertilisation in the higher

Cryptogams; but the pollen-tube was closed both in them and in the

rest of the Phanerogams, in which moreover its outer coat attains to a

considerable thickness. There remained therefore only the hypothesis,

that a fluid substance passes through the walls of the pollen-tube and

of the embryo-sac and effects the fertilisation of the egg-cell; thus

it was not the theory of preformation of the last century, to which

Brongniart still adhered, but the view represented by Koelreuter, which

ultimately proved to be nearer the truth, though it may be said that

all that remained of that view was, that the fertilising substance in

the Phanerogams is a fluid. The granular contents of the pollen-grains,

which were supposed to be spermatozoids, have since been partly found

to be only innocent starch-grains and drops of oil.

8. DISCOVERY OF SEXUALITY IN THE CRYPTOGAMS. 1837-1860.

By the year 1845 no one capable of forming a judgment on the question

any longer doubted the existence of different sexes in Phanerogams.

But it was not so with the Cryptogams, though a number of facts were

acknowledged at this time which seemed to point to the conclusion, that

a moment arrives sooner or later in the course of their development

also, when a sexual act is accomplished. But the question had not

as yet been systematically studied; no experimental investigations

had been made, or observations of such a kind as to demonstrate the

necessity of sexual union.

The great majority of botanists in the second half of the 18th century

had no longer any doubt that the stamens were organs of reproduction,

and they were anxious to prove the existence of similar organs in

the Cryptogams; they rested in this matter on external resemblances

and analogies, which they interpreted in a more or less arbitrary

manner. The obvious external resemblance between the antheridia and

archegonia in Mosses and the sexual organs in the Phanerogams led

Schmidel and Hedwig to consider them to be stamens and ovaries, and the

conjecture was correct, though the true nature of the moss-fruit had

to be learnt in another way. Micheli, Linnaeus and Dillen, trusting

still more to external appearance and with slight knowledge of these

plants, had before this taken the fruit for a male flower, and in

the case of the rest of the Cryptogams the best botanists were only

feeling their way in the dark with no certain experience to guide

them. It is not necessary to give a particular account of the views

which originated in this way; one or two may be mentioned by way of

example. Koelreuter regarded the volva of Mushrooms, Gleditsch and

Hedwig certain tube-like cells in their lamellae, as the male organs

of fertilisation. Gleichen took the stomata, Koelreuter the indusium,

Hedwig even the glandular hairs of Ferns for anthers. It was not yet

suspected that the course of development and the whole morphology of

the Cryptogams could not be so compared with that of the Phanerogams;

correct and incorrect assumptions with regard to the sexual organs

of the Cryptogams were alike devoid of scientific value, being mere

guesses and vague conjectures. Nor was the state of things much better

even in the first years of the 19th century; and if by that time a

number of occasional observations had been made which could afterwards

be turned to scientific account, these were as yet only isolated facts

without scientific connection, and every one was at liberty to concede

or to refuse sexual organs to the Cryptogams generally at his own

discretion. Meanwhile observations gradually accumulated, and towards

1845 it began to be possible by critical examination of them to arrive

at something like a clearer understanding of this part of botany.

The majority of botanists readily accepted Schmidel’s and Hedwig’s

opinion with respect to the Mosses; Vaucher had as early as 1803

maintained that the long-known conjugation of Spirogyra was a sexual

act; Ehrenberg observed in 1820 the conjugation of a Mould, Syzygites;

Bischoff and Mirbel explained the organisation of the antheridia of

the Liverworts in 1845, while Nees von Esenbeck saw the spermatozoids

of Sphagnum in 1822 and Bischoff those of Chara in 1828, though they

were at first taken for Infusoria, an opinion maintained by Unger as

late as 1834. But it was Unger[114], who in 1837, after careful study

of the spermatozoids of the Mosses in 1837, declared them to be the

male organs of fertilisation; in 1844 Nägeli discovered corresponding

forms on the prothallium of Ferns, which had till then been called a

cotyledon, and in 1846 the spermatozoids of Pilularia, the products of

the small spores which Schleiden had explained to be the pollen-grains

of that plant.

These facts were of the highest importance, but little was to be made

of them as long as the female organ in the plants in question, the

Mosses excepted, was unknown, and meanwhile it was only the resemblance

between vegetable and animal spermatozoids which led to the conjecture,

that the one had the same sexual significance as the other.

Light was suddenly thrown upon the subject, when Count Lesczyc-Suminsky

discovered in 1848 on the supposed cotyledon (prothallium) of Ferns

both the antheridia and the peculiar organs, inside which the embryo or

young fern is formed. Though the statements respecting the structure

and development of these female organs and of the embryo were

inaccurate in some important points, yet the place was now indicated

where it might be presumed that the fertilisation by the spermatozoids

takes place; and as the history of the germination of the rest of

the vascular Cryptogams was to some extent known through the earlier

labours of Vaucher and Bischoff, the organs of fructification of these

plants might now be sought, where they are really to be found. But

an erroneous idea respecting the meaning of the small spores of the

Rhizocarps propounded by Schleiden had first to be put out of the way,

and this was done by an appeal to the discovery of Nägeli mentioned

above and by the investigations of Mettenius. Then in 1849 Hofmeister

supplied a connected description of the germination of Pilularia

and Salvinia, in which the decisive points as regards the sexual

act were clearly set forth, and the connection of the spermatozoids

with the fertilisation of the egg-cells in the archegonium was

established. He did the same for Selaginella, which is very unlike the

Rhizocarps and Ferns, and in which the spermatozoids are developed

from smaller spores, and fertilise the egg-cells in archegonia formed

in the prothallium of the large spores. By comparing the processes

of germination in these plants with those of Ferns and Mosses, he

succeeded in throwing entirely new light on the whole of the morphology

of these classes of plants, and thus made it possible for the first

time to compare them with one another and with the Phanerogams, and to

form a right estimate of the sexual act in the Muscineae and Vascular

Cryptogams in its relation to the history of the development of these

plants. Hofmeister arrived at the following conclusion from his

observations in 1849: ‘The prothallium in the vascular Cryptogams is

the morphological equivalent of the leaf-bearing Moss-plant, while the

leafy plant of a Fern, of a Lycopodium and a Rhizocarp answers to the

capsule of the Moss. In Mosses as in Ferns there is an interruption

of the vegetative development by sexual procreation, an alternation

of generations; this takes place in the Vascular Cryptogams very soon

after germination, in the Mosses much later.’ The vast importance

of this discovery to systematic botany has been already noticed.

The conception of these relations developed by Hofmeister was not

less important to the doctrine of the sexuality of plants; it swept

away at one stroke all the old false analogies between Phanerogams

and Cryptogams and brought to light the real agreement; Hofmeister

had detected in the archegonium of the Cryptogams the body which is

developed there, as in the ovule of the Phanerogams, into an embryo

after fertilisation, namely the germinal vesicle or egg-cell. Here

was the point of departure for all further systematic comparison in

the sexual propagation of Cryptogams and Phanerogams. All beside was

of secondary importance, even the fact, that the fertilisation of the

egg-cell in the Cryptogams is not effected by a pollen-tube, but by

spermatozoids. It was now easy to show the corresponding relations of

generation in the other cases which Hofmeister had not yet observed.

Hofmeister’s statements and conclusions respecting Selaginella and

Isoetes were confirmed and some additions made to them by Mettenius in

1850, and in 1851 appeared Hofmeister’s exhaustive work ‘Vergleichende

Untersuchungen,’ in which the mode of production of the embryo in

Coniferae was represented as an intermediate form between those of

Phanerogams and Cryptogams. Further contributions were made to the

knowledge of the subject; Henfrey confirmed Hofmeister’s results in

the case of Ferns; Hofmeister himself and Milde observed in 1852 the

history of fertilisation in Equisetaceae, and the former supplied at

the same time a more complete account of the development of Isoetes; in

1855 he described the decisive points in Botrychium and Mettenius in

1856 those in Ophioglossum.

The processes of development before and after fertilisation were

now cleared up by all these discoveries, but the direct observation

of the act of fertilisation was still wanting. Hofmeister (‘Flora,’

1857, p. 122) describes the state of affairs in the following terms:

‘While numerous investigations had thrown a clear light on the

character of the male and female organs, and on the way in which the

embryo is formed by repeated division of the egg-cell present before

fertilisation, we continued quite in the dark respecting the particular

nature of the fertilisation. Observation and experiment had established

the fact, that the influence of the spermatozoids on the archegonia was

required to produce an embryo in the latter. Female moss-plants[115]

separated from the male, macrospores in the Vascular Cryptogams

separated from the microspores, had in all cases proved unproductive;

but it was not even certainly known to what point in the female organ

the spermatozoids force their way. It is true that Lesczyc and after

him Mercklin had seen the entry of moving spermatozoids into the mouth

of archegonia in Ferns; but Lesczyc’s account of the part which he

supposed them to play there afterwards, was proved to be an illusion.

I had myself observed motionless spermatozoids halfway down the neck

of archegonia of an Equisetum; but nothing was to be learnt of the

manner in which the spermatozoid affects the egg-cell. Then it happened

that in the spring of 1851, being engaged in observing the development

of the organs of vegetation of Ferns, I repeatedly saw spermatozoids

moving about in the basilar cells which enclose the egg-cell in the

archegonia of Ferns, and the majority of them even playing about the

egg-cell. Their movements were put an end to during the observation

by the commencement of changes, which the contents of young vegetable

cells which have been cut open usually experience under the prolonged

influence of water.’ Later observations leave no doubt now that in the

Muscineae and Ferns single spermatozoids force their way into the naked

egg-cell of the archegonium.

The question was first set at rest in the Algae, where the process of

fertilisation could be seen directly and without exposing the objects

to destructive influences. That sexual propagation occurs in the Algae

also had seemed probable, since Decaisne and Thuret in 1845 discovered

organs in species of Fucus, and Nägeli in 1846 in Florideae, which

scarcely admitted of any other explanation. Alexander Braun also had

called attention to the formation of two kinds of spores in a large

number of fresh-water Algae. But as yet there was only conjecture. Then

Thuret proved by experiment in 1854, that in the genus Fucus the large

egg-cells must be fertilised by very small swarming spermatozoids, in

order to set up germination; both organs can be collected separately

and in numbers in this genus, and be brought together at pleasure;

Thuret even succeeded in obtaining hybrids. Pringsheim first observed

in 1855 the formation of spermatozoids in the little horns of Vaucheria

and established the fact that spores capable of germination are not

formed unless the spermatozoids approach the egg-cell. To Thuret’s

statements he added the very important one, that the remains of

spermatozoids may be recognised on the surface of the contents of

the fertilised egg-cell of Fucus, which is already surrounded by a

membrane. About the same time Cohn published his observations on

Sphaeroplea annulina, which confirmed the fact of the approach of

the spermatozoids to the egg-cells, which consequently, as in Fucus

and Vaucheria, form a cell-wall and are rendered capable of further

development.

Still the decisive observation had not yet been made; no one had

yet seen how the two fertilising elements behaved at the moment of

fertilisation. Pringsheim had the good fortune to make this observation

in one of the commonest of fresh water Algae, Oedogonium. There he saw

the moving spermatozoid first come into contact with the protoplasmatic

substance of the egg-cell, and then force its way into it, blend with

it and dissolve. And thus the first observation was made, which proved

decisively that a real intermixture takes place of the male and female

elements of fertilisation; this important fact was confirmed by De Bary

in the same year.

Now that it was once established, that fertilisation in Cryptogams

consists in the blending together of two naked bodies of protoplasm,

the spermatozoid and the egg-cell, it was reasonable to conclude that

conjugation in Spirogyra and generally in Conjugatae, was an act of

fertilisation, only in this case the two fertilisation-elements are

not of different size and shape, but similar in appearance. To this

conclusion De Bary arrived in 1858 in his monograph of the Conjugatae.

This extension of the idea of fertilisation to cases in which the

uniting cells are to outward appearance alike, was of special value

to the theory of sexuality, as was seen in the sequel, when other

forms of fertilisation were observed which made it necessary still

further to extend the idea of sexuality. In 1858 Pringsheim discovered

arrangements for fertilisation in another group of Algae, the

Saprolegnieae, which to outward appearance at least departed widely

from those hitherto known in the lower plants.

Thus between the years 1850 and 1860 a number of fundamental facts

were discovered, and were afterwards confirmed and extended by fresh

observations in the course of the following years. It does not fall

within the limits of this work to notice the many discoveries that

were made in this part of botanical science after 1860; we will only

remark, that between 1860 and 1870 the processes of fructification were

observed by Thuret and Bornet in Florideae, and especially by De Bary

and his pupils in Fungi, in some of which very peculiar forms were

brought to light. No doubt any longer exists that difference of sex

prevails generally in the Thallophytes also, though it is still an open

question, whether it may not be wanting in some of the very simplest

and smallest kinds.

One of the most important results of these investigations is

obviously the striking resemblance between many of the processes of

fertilisation in Cryptogams and in the lower animals; here is another

confirmation of the fact, often brought out in other ways by modern

zoological and botanical research, that the points of resemblance in

the vegetable and animal kingdoms appear most plainly, if we compare

together the simplest forms to be found in both; we have in this

fact a plain proof also, that both kingdoms have been developed from

like common elements, as the theory of descent implies. With respect

to the true nature of fertilisation itself, which is evidently a

similar process in the main in animals and plants, we can only say at

present, that it amounts in all cases to a material blending together

of the contents of two cells, neither of which is capable of further

development by itself, while the product of the combination is not only

capable of such development, but unites in itself the characteristics

of the two parent forms and transmits them to its descendants. That

fertilisation is not the intimate union of two bodies possessing a

definite form, but that the male fertilising substance at least may

be a simple fluid, appears to be distinctly shown by the process in

Phanerogams; and we may assume, that in Cryptogams also, the sexual act

is not affected by the form of the fertilisation-elements, though a

certain shape and power of movement is necessary for the conveyance of

the fertilising substance to that which is to be fertilised.

CHAPTER II.

HISTORY OF THE THEORY OF THE NUTRITION OF PLANTS.

1583-1860.

That plants take up certain substances from their environment for the

purpose of building up their own structures could not be a matter of

doubt even in the earliest times; it was also obvious, that movements

of the nutrient material must be connected with this proceeding. But it

was not so easy to say, what was the nature of this food of plants, in

what manner it finds its way into and is distributed in them, and what

are the forces employed; it was even for a long time undecided, whether

the food taken up from without suffers any change inside the plant,

before it is applied to purposes of growth. Such were the questions

which had engaged the attention of Aristotle, and which formed the

chief subject of Cesalpino’s physiological meditations.

But the questions respecting the nutrition of plants acquired a much

more definite shape in the latter half of the 17th century, when the

various phenomena of vegetation began to be more closely observed, and

some attempt was made to understand their relations to the outer world.

Malpighi, the founder of phytotomy, was the first who undertook to

explain the share which belongs to the different organs of the plant

in the whole work of nutrition; guided by analogy, he perceived that

the green leaves are the organs which prepare the food, and that the

material so prepared by them passes into all parts of the plant, there

to be stored up or employed for purposes of growth. But this gave no

insight into the nature of the substances from which plants prepare

their food. On this point Mariotte endeavoured to give such information

as could be obtained from the chemistry of his day; and he has the

merit of having shown, in opposition to the old Aristotelian notion,

that plants convert the food-material which they derive from the ground

into new chemical combinations, while the earth and the water supply

the same elements of nutrition to the most different kinds of plants.

It could not escape the notice of physiologists even of that time,

that the water which plants take up from the ground introduces into

them but very small quantities of matter in solution. Van Helmont in

the first half of the 17th century had shown this by an experiment,

the results of which, however, led him to think that plants were able

to produce both the combustible and incombustible parts of their

substance from water. Hales at the beginning of the 18th century formed

a different opinion, being led by the evolution of the gases in the dry

distillation of plants to conclude, that a considerable part of their

substance was absorbed in a gaseous form from the atmosphere.

The views propounded by Malpighi, Mariotte, and Hales contained the

most important elements of a theory of the nutrition of plants; fully

understood they would have taught that one part of the food of plants

comes from the earth and the water, and another part from the air;

that the leaves change the materials thus obtained in such a manner as

to produce from them the substance of plants and to apply this to the

purposes of growth; but the ideas were not combined in this way, for

during some years after their time botanists were chiefly engaged in

observations on the movement of the sap in plants, and they arrived

even on this point at very obscure and even contradictory results,

because they overlooked the function of the leaves which had already

been recognised by Malpighi. All insight not only into the chemical

processes in the nutrition of plants, but also into the mechanical laws

of the movement of the sap, and generally into the whole internal

economy of plants, depends on a knowledge of the fact, that it is only

the cells which contain chlorophyll, and therefore in the higher plants

the leaves chiefly as consisting largely of such cells, which have the

power of converting the gaseous food supplied by the atmosphere into

the substance of the plant with the aid of the materials taken up from

the soil. This fact is of fundamental importance to the whole theory of

the nutrition of plants; it is only by a knowledge of it that we can

explain the movement of material connected with nutrition and growth,

the dependence of vegetation on light, and to a great extent also the

function of the roots.

But this principle could not be discovered till the new chemical

system founded by Lavoisier took the place of the old phlogistic

chemistry, and it is remarkable that the discoveries, which laid the

foundation of modern chemistry in the period between 1760 and 1780,

contributed essentially to the establishment at the same time of the

modern doctrine of the nutrition of plants. Ingen-Houss, in reliance

on Lavoisier’s antiphlogistic views on the composition of air, water,

and the mineral acids, succeeded in proving that all parts of plants

are continually absorbing oxygen and forming carbon dioxide, but that

the green organs at the same time under the influence of light absorb

carbon dioxide and exhale oxygen; and as early as 1796 he considered

it probable that plants obtain the whole mass of their carbon from

the carbon dioxide of the atmosphere. Soon after (1804) de Saussure

proved, that plants, while they decompose carbon dioxide, increase in

weight by a greater amount than that of the carbon which they retain,

and that this is to be explained by the fact that they at the same

time fix the elements of water. He likewise showed that the small

quantities of saline compounds, which plants take up from the soil, are

a necessary part of their food, and that it was at least probable, that

the nitrogen of the atmosphere does not contribute to the formation of

nitrogenous substances in plants. Senebier had before insisted on the

fact, that the decomposition of carbon dioxide under the influence of

light only takes place in green organs.

Thus the most important points in the nutrition of plants were

discovered by Ingen-Houss, Senebier and de Saussure. But, as often

happens in the case of discoveries of such magnitude, their ideas

were for a long time exposed to great misunderstanding. They were

better appreciated in France than in any other country; Dutrochet and

De Candolle were able to see the importance of the interchange of

gases in the green organs to the general nutrition and respiration;

but others, and especially German botanists, were not content with

these simple chemical processes as the foundation of the whole system

of nutrition and consequently of the whole life of the plant; the

theory of the vital force, which was elaborated in connection with the

nature-philosophy during the first years of the 19th century, and was

generally accepted by philosophers and physiologists, chemists and

physicists, preferred to supply the plant with a mysterious substance

for its food, which had its source in the life itself and which it

called humus. The most obvious considerations, which must at once have

shown that this humus-theory was absurd, were entirely overlooked;

and thus in the face of de Saussure’s results the food of plants was

once more referred entirely to the soil and the roots, as it was in

the earliest times; one of the consequences of this humus-theory in

combination with the vital force was that the ash-constituents of

plants were supposed to be merely accidental admixtures or stimulants,

or to be directly produced in the plant by the vital force.

In the period between 1820 and 1840 the reaction set in from different

quarters against the theory of vital force; chemists succeeded in

producing by artificial means certain organic compounds, which had

hitherto been regarded as products of that force; Dutrochet discovered

in endosmose a process, which served to refer various vital phenomena

in plants to physico-mechanical principles; de Saussure and others

showed that the heat of plants is a product of respiration, and by

1840 the earlier theory of a vital force might be looked upon as

antiquated and obsolete. It remained to restore to their rights the

observations of Ingen-Houss and de Saussure, which under the influence

of that theory and of the notions respecting the humus had been so

utterly misconstrued. Liebig set aside the humus-theory in 1840,

and referred the carbon of plants entirely to the carbon dioxide

of the atmosphere, and their nitrogenous contents to ammonia and

its derivatives; he claimed the components of the ash as essential

factors in the nutrition, and taking his stand on the general laws of

chemistry endeavoured to obtain chiefly by the method of deduction an

insight into the chemical processes of assimilation and metabolism.

The whole theoretical value of the facts discovered by Ingen-Houss,

Senebier and de Saussure was first made apparent by the connection

which Liebig succeeded in establishing between the phenomena of

nutrition. The doctrine of nutrition burst suddenly into new life;

firm ground was gained, and the botanist, no longer distracted by

the difficulties raised by the vital force but resting on physical

and chemical principles, might now resume the task of investigation.

Oxygen-respiration denied by Liebig was first of all re-established

by von Mohl and others. Liebig’s views on the source of nitrogen in

plants and on the importance of the ash-constituents rested chiefly on

general considerations and observations and on calculation, and had now

to be tested by systematic investigation and especially by experiments

on vegetation in individual plants. And here the place of honour must

be assigned to Boussingault, who pursued the path of pure induction

as contrasted with Liebig’s deductive mode of proceeding, gradually

improved the methods for experimenting on vegetation, and soon

succeeded in so producing plants in a purely mineral soil free from all

humus, that he finally settled the question of the derivation of the

carbon from the atmosphere and of the source of the nitrogen also.

He showed from the plants thus artificially nourished, and with due

consideration of the many sources of error which beset the question,

that the uncombined nitrogen of the atmosphere does not contribute to

the nutrition of plants, but that a normal increase in the nitrogenous

substances in a plant takes place when the roots take up nitrates as

well as the necessary constituents of the ash.

With the exception of some doubts which still remained respecting the

necessity of certain constituents of the ash, such as sodium, chlorine

and silicic acid, the source of the materials which take a part in

the chemistry of the nutrition of plants was known before 1860; but

the knowledge obtained with regard to processes in the interior of

the plant, the origination of organic substances in the processes of

assimilation, and the further changes which they undergo was still

fragmentary and uncertain, and led to no general and conclusive results.

1. CESALPINO.

Aristotle had sought to determine the nature of the materials which

plants take up as food, and had laid down the proposition, that the

food of all organisms is not simple but composed of various substances.

This view was correct, but he united with it the erroneous notion,

that the food of plants is elaborated beforehand in the earth, as in a

stomach, and is made applicable to purposes of growth, so as to exclude

the necessity of any separation of excrements in the plant; this error

was refuted by Jung, as we shall see, but nevertheless it continued to

live as late as into the 18th century, and ultimately quite spoilt Du

Hamel’s theory of nutrition.

Cesalpino, whom we have learnt to regard as a faithful and gifted

disciple of Aristotle, directed his speculations to the mechanical

rather than to the chemical side of the question, and chiefly tried to

explain the movement of the nutrient sap in plants. He had a larger

stock of material drawn from experience at his disposition than his

master, and it is instructive therefore to make a nearer acquaintance

with his views, because they show how far the old philosophy was in a

condition to turn better empirical knowledge than Aristotle possessed

to a satisfactory use; they will also show that Cesalpino’s first

essays led him to views which can no longer be said to be strictly

Aristotelian.

In the second chapter of the first book of the work from which we have

already quoted, ‘De plantis libri XVI,’ 1583, he raises the question,

in what way the food of plants is taken in and their nutrition

accomplished. In animals we see the food conveyed from the veins to the

heart, which is the laboratory of the warmth of the body, and after it

has been finally perfected there, spread abroad through the arteries

into all parts of the body; and this is effected by the operation of

the force (spiritus) which is generated in the heart from the food.

In plants on the contrary we see no veins, or other channels, nor do

we feel any warmth in them, so that it is difficult to understand

how trees grow to so great a size, since they seem to have much less

natural heat than animals. Cesalpino explains this enigma by saying,

that animals require much food for maintaining the activity of the

senses and the movements of their organs. The larger quantity of animal

food also requires larger receptacles, namely the veins. Plants on the

other hand need less food, because this is only used for purposes of

nutrition, or to a very small extent for the production of internal

heat as well, and therefore they grow more vigorously and bear more

fruit than animals. At the same time plants are not without internal

heat, though it cannot be perceived by the touch because all objects

seem cold to us, which are less warm than our organ of feeling. That

plants moreover have veins, though only narrow ones in accordance with

the small mass of their food, is shown by those which yield a milky

juice, such as Euphorbia and Ficus, which when cut bleed like the

flesh of animals; Cesalpino adds ‘and this is very frequent also in

the vine,’ which shows that he made no distinction between milky juice

and the exuding water of the weeping vine-stock. These narrow veins

cannot be seen on account of their fineness; but in every stem and in

every root things may be discerned which like nerves in animals can be

split longitudinally and are called the nerves of the plant, or also

certain thicker things, such as those which branch in most leaves and

are there called veins. These should be considered as food-passages

and as answering to the veins in animals; but plants have no main vein

like the vena cava in animals, but many fine veins pass from the root

to the heart of the plant (cor, root-neck, see above, Book I. chap. 2),

and ascend from it into the stem; for it was not necessary that the

food should be collected in a common receptacle in plants, as it is in

the heart in animals, where this is necessary for the production of

the spiritus, but it was sufficient that the fluid in plants should be

changed by contact with the medulla cordis (in the root-neck), as it is

changed in animals in the marrow of the brain or in the liver; and in

these organs the veins are very narrow, as they are in plants.

Since plants have no sense-perception, they cannot seek their food

like animals, but they draw up the moisture from the ground into

themselves in a way of their own; but it is not easy to see how this

takes place. Cesalpino, in trying to explain this, gives us a glimpse

into the physics of the day, and we observe also to our surprise an

attempt made to explain phenomena in living creatures by physical

laws, a step beyond the limits of Aristotelian modes of thought and in

the right direction. It is not the ratio similitudinis, which draws

iron to the magnet, that can cause the attraction of the juice by the

roots, for then the smaller would be drawn to the larger; and if the

attraction of the fluid of the earth by the roots were the same thing

as the attraction of the iron by the magnet, the moisture of the earth

would draw out the juice from the plant, which is just what does not

happen. Nor can it be the ratio vacui; for since not moisture only

but air also is contained in the earth, the plant would be filled not

with juice but with air. But Cesalpino hits upon a third kind of cause

by which juices may be drawn into the plant. Do not many dry things,

he says, in accordance with their nature attract moisture, as linen,

sponge and powder, while others repel it, as the feathers of many birds

and the herb Adiantum, which are not wetted even when dipped in water;

but the former absorb much water, because they have more in common with

it than with air; of this kind Cesalpino thinks those parts of plants

must be, which the nourishing soul employs to take in food. Therefore

these organs are not traversed by a continuous canal such as the veins

in animals, but formed like the nerves of a fibrous substance; and thus

the power of suction (bibula natura) conveys the moisture continually

to the place, where the principle of internal heat is placed, just as

may be seen in the flame of a lantern, to which the wick continually

conducts the oil. The absorption of the moisture is also increased

by the outer warmth, for which reason plants grow more vigorously in

spring and summer.

That Cesalpino had no suspicion of the use of the leaves in the

nutrition of plants appears incontestably from his repeating the

Aristotelian idea, that the leaves are only for the protection of young

shoots and fruits from air and sun-light; this idea is no result of

speculation, but came simply from observing a vineyard in a hot country.

2. FIRST INDUCTIVE EXPERIMENTS AND OPENING OF NEW POINTS OF VIEW IN THE

HISTORY OF THE THEORY OF THE NUTRITION OF PLANTS.

All that Aristotle and his school, Cesalpino not excepted, are able to

tell us about the phenomena of vegetable life, was the result of the

most every-day observations, none of which were critically and exactly

tested to ascertain their actual correctness, while the larger part

of their physiological axioms were not derived from observations on

plants at all, but from philosophical principles, and especially from

analogies taken from the animal world.

The first step towards a scientific treatment of the doctrine of

nutrition was an enlargement and critical examination of the materials

to be gained from experience; nor were any difficult observations or

experiments needed to discover contradictions between the truths of

nature and the old philosophy; all that was necessary was to look into

things more closely and to judge of them with less prejudice.

In this way Jung was led to oppose one important point of the

Aristotelian account of nutrition. In the second fragment of his work

‘De plantis doxoscopiae physicae minores’ is to be found a remark,

which is evidently directed against the notion that plants receive

their food already elaborated from the earth, and therefore give

off no excrements[116]. Plants, says Jung in accord with Aristotle,

appear not to need a thinking soul (anima intelligente), which would

be able to distinguish wholesome from unwholesome food, and Aristotle

therefore provided them with food which had already been perfectly

prepared in the earth. But Jung takes another view founded on actual

observation. It is very possible, he says, that the openings in the

roots which take in liquid matter are so organised, that they do not

allow every kind of juice to enter, and who can say that plants have

the peculiarity of only absorbing what is useful to them, for like

all other living creatures they have their excreta, which are exhaled

through the leaves, flowers, and fruits. But among these he reckons the

resins and other exuding liquids, and says that it is possible after

all that a large part of the juices of plants escapes by imperceptible

evaporation, as happens in animals.

According to Aristotle’s view the plant itself was quite passive in

the work of nutrition; since food was offered to it which had been

already prepared for it in the earth, growth was to some extent merely

a process of crystallisation unaccompanied by chemical change. In

pointing to the formation of excreta Jung on the contrary ascribed a

chemical activity to the plant, and by supposing that the organisation

of the root was such as to prevent the entrance of certain matters

and to favour that of others, he made the plant co-operate in its own

nourishment, though he did not assume that it needed a thinking soul

for this purpose.

Johann Baptist van Helmont[117], physician and chemist, and a

contemporary of Jung, took up a position still more decidedly opposed

to Aristotelian doctrines. He rejected the four elements of that

philosophy, and regarding water as a chief constituent of all things

he considered that the whole substance of plants, the mineral parts

(the ash) as well as the combustible, was formed from water. Thus while

Aristotle made the component parts of plants be introduced into them by

water in a state ready for use, Van Helmont, on the contrary, ascribed

to the plant the power of producing all kinds of material from water.

It would scarcely have been necessary to mention this resistance to old

dogmas, originating as it did in the notions of the alchemists, if Van

Helmont had not made an attempt to establish his views by experiment;

this was the first experiment in vegetation undertaken for a scientific

purpose of which we have any information, and it was repeatedly

quoted by many later physiologists, and employed in support of their

theories. He placed in a pot a certain quantity of earth, which when

highly dried weighed two hundred pounds; a willow-branch weighing five

pounds was set in this pot, which was protected by a cover from dust,

and daily watered with rain-water. In five years’ time the willow had

grown to be large and strong, and had increased in weight by a hundred

and sixty-four pounds, though the earth in the pot, when once more

dried, only showed a loss of two ounces. Van Helmont concluded from

this experiment that the considerable increase of weight in the plant

had been gained entirely at the cost of the water, and consequently

that all the materials in the plant, though distinct from water,

nevertheless come from it.

These objections to Aristotelian teaching on the part of Jung and Van

Helmont remained isolated and unproductive, but an incentive to new

investigations in vegetable physiology was supplied from a different

quarter, and its influence lasted till far into the 18th century. This

was the suggestion, that not only does a nutrient sap taken up by

the roots ascend to the leaves and fruits of plants, but that there

is also a movement of the same sap in the opposite direction in the

rind. But this idea assumed from the first two different forms. Some

botanists, evidently resting on the analogy of the circulation of the

blood in animals, supposed that there was also an actual circulation of

the sap in plants; others on the contrary were content with supposing

that while the watery sap absorbed by the roots rises in the wood, an

elaborated sap capable of ministering to growth moves in the rind, the

laticiferous vessels, and the resin-ducts. The two views were at a

later time repeatedly confounded together, and those who refuted the

first believed that they had refuted the other also. It appears that a

physician from Breslau, Johann Daniel Major[118], Professor in Kiel,

first gave expression to the opinion, that there is a circulation of

the nourishing substance in plants as in animals; and from this time

to the end of the 18th century the circulation of the juices of plants

was a favourite subject of discussion, but more often chosen by the

impugners of the doctrine than by its defenders.

The better form of the idea, namely, that there is a return-movement

of material towards the root, combined with the view, that the leaves

are the organs which produce the substances required for growth from

the crude material supplied to them, was expressed by Malpighi as early

as 1771 in the shape of a well-considered theory. In his ‘Anatomes

plantarum idea’ of that year he devotes the last pages to a short

account of the theory of nutrition, as he understood it. He regarded

the fibrous constituents of the wood as the organs for conducting the

sap taken up by the roots, and the vessels as air-passages, which he

named tracheae on account of their resemblance to the tracheae of

insects. He was in doubt whether the air came from the earth through

the roots, or from the atmosphere through the leaves, for he had

never succeeded in finding openings for the entrance of air in the

roots or the leaves; but he thought it more probable that the air is

absorbed by the roots, because they are well supplied with tracheae,

and air has besides a tendency to ascend. Beside these fluid-conducting

fibres and air-conducting tracheae in the wood he called attention

to the existence of special vessels, which conduct peculiar juices

in many plants, as the laticiferous vessels, gum-passages, and

turpentine-canals.

Respecting the movement of the juices, he notices that the direction

may be reversed, because shoots planted upside down send out roots

into the earth from what is organically their upper end, and grow into

trees; and though they do not grow vigorously, yet the experiment

proves that the movement of the sap in them is in the reverse

direction.

After these preliminary remarks he proceeds to prove, that it is in

the leaves that the crude juices of nutrition undergo the change which

fits them for the maintenance of growth. The way in which Malpighi

arrives at this view is as simple as it is original. He considers

the cotyledons of young plants to be genuine leaves (in leguminibus

seminalis caro, quae folium est conglobatum), as is shown in the

gourd, where the cotyledons grow into large green leaves. Liquid is

conveyed to them through the radicle, and a portion of the substances

which they contain passes from them into the plumule to make it grow,

which it will not do if the cotyledons are removed; hence he concludes

that all other leaves also are intended to elaborate (excoquere) the

nutritive juice contained in their cells, which the woody fibres have

conveyed to them. The liquids mingled together in their long passage

through the network of fibres are changed in the leaves by the power

of the sun’s rays, and blended with the sap before contained in their

cells, and thus a new combination of the constituent parts is effected,

transpiration proceeding at the same time; he compares the whole

process with that which goes on in the blood of animals.

We see that Malpighi’s view of the function of the leaves in nutrition

approaches very closely to the truth, as closely indeed as was at

all possible in the existing condition of chemical knowledge. He was

induced by the results of anatomical investigation to carry this view

farther and indeed correctly; he supposed that the parenchymatous

tissue of the rind acts in the same way as the leaves; but he went a

step too far in assigning the function of the leaves to the colourless

parenchyma also, which only serves for the storing up of assimilated

matter. He says we must ascribe a character similar to that of the

leaf-cells to the corresponding cells in the rind and to those also

which lie transversely in the wood (the medullary and cortical rays),

and that it is not unreasonable to conclude that the food of the plant

is elaborated and stored up in these cells. As he makes no sharp

distinction between elaboration and mere storing up, he ascribes the

function of the leaves to the parenchyma of fleshy fruits also and to

the scales of bulbs; he concludes from the exudations from stumps of

trees and from the cut surfaces of other parts of plants, that they are

filled with reserve-matter (asservato humore turgent).

Thus the essential points in Malpighi’s theory of nutrition in the year

1671 were, that the vessels of the wood are primarily air-conducting

organs, that the leaves elaborate the crude sap for purposes of

growth, that the sap so elaborated is stored up in different parts of

the plant, and that the fibrous elements of the wood convey upwards

to the leaves the crude materials of nutrition which are absorbed by

the roots. No mention is made of a circulation of juices, comparable

to the circulation of the blood, though this idea was in later times

often imputed to him; and we find by his later remarks, that while he

was in no doubt as to the elementary organs which convey the ascending

sap, he confined himself to conjecture with respect to the way by

which the sap elaborated in the cell-tissue of the leaves, rind and

parenchyma generally is carried on its further course. But he was in

no doubt about the direction of that course; he believed that this sap

forces itself downwards through the stem into the roots, and upwards

in the branches above the leaves and so into the fruit. Thus Malpighi

had formed a more correct idea of the movement of assimilated matter

than the majority of his successors who introduced the very unsuitable

expression, ‘descending sap.’ He further thought it probable that the

elaborated sap passes through the bast-bundles[119], but without a

continuous flux and reflux (absque perenni et considerabili fluxu et

refluxu); that it rests to some extent in the laticiferous vessels,

but that it is also driven sometimes, when occasion requires, by

transpiration and external causes into the higher parts of the plant,

where it is the means of maintaining growth and nutrition. These later

remarks also are better than much that was said about the movement of

the sap in the 18th and even in the 19th century, and at all events

they prove that to speak of Malpighi as a defender of the circulation

of the sap in Major’s sense, as was often done in later times, was an

entire misunderstanding of his views.

Malpighi published his theory in a brief and connected form in 1671; it

appeared again further worked out in detail in the fuller edition of

the Phytotomy in 1674; he attributed a special value to his discovery,

that plants require air to breathe as much as animals, and that the

vessels of the wood answer in function to the tracheae in insects and

to the lungs in other animals; he recurs also several times to the

importance of leaves as organs for the elaboration of the food.

If we compare Malpighi’s theory of the nutrition of plants with the

views of his predecessors, we cannot help seeing, that it was an

entirely new creation, in which Aristotelian doctrines had no share.

If his successors had apprehended the important and essential points

in his doctrine and had striven by experimenting on living plants

to support and illustrate them by new facts, we should have been

spared many erroneous notions which established themselves in the

theory, and made it a perfect chaos of misconceptions. That particular

misconception, which we have already mentioned more than once,

namely, that Malpighi, like Major and Perrault after him, assumed a

continuous circulation of the juices of the plant, necessarily involved

an incorrect idea of the function of the leaves; that function was

by many later writers either quite neglected, or sought for chiefly

in transpiration, the chemical activity of the leaves being quite

overlooked.

Malpighi’s theory can hardly be said to take into consideration the

chemical nature of the food of plants; it is chiefly occupied with the

relation of the organs to the main points in the nutritive process;

its foundations are for the most part laid in the anatomy of the

plant. Grew, who in all essential points adopted Malpighi’s views,

but without doing much to advance them by his lengthy discussions on

particular questions, made some attempt to extend the knowledge of

the chemistry of the subject; but his notions were entirely borrowed

from the corpuscular theory of Descartes, and he may be said to have

constructed his own chemical processes; the consequence was that he

usually overlooked the points that were of fundamental importance, and

brought nothing to light that could assist the further development of

the theory of nutrition. But there is another writer, whose name is in

the present day known to few in the history of vegetable physiology,

but whose ideas on the chemistry of plants are of great interest. This

writer is MARIOTTE[120], the discoverer of the well-known law of gases,

one of the greatest physicists of the latter half of the 17th century,

who also enriched the physiology of the human body with some valuable

discoveries. We have a tolerably copious treatise of Mariotte’s in the

form of a letter to a M. Lantin in the year 1679, to be found in the

‘Œuvres de Mariotte,’ Leyden, 1717, under the title, ‘Sur le sujet

des plantes.’ It is highly instructive to gather from this letter the

ideas of one of the most famous and ablest of the natural philosophers

of that day on chemical processes and conditions in the nutrition of

plants, a few years after the appearance of Malpighi’s great work and

about the time that Grew’s Phytotomy was being published. It is to be

expected that Mariotte should give but an incidental and superficial

attention to the more delicate structure of plants; but we are

compensated for this by his making us acquainted with everything

fundamentally important and new which could at that time be said on

the chemistry of the food of plants. Speaking of the ‘elements’ or

‘principles’ of plants, Mariotte propounds three hypotheses. The first

is, that there are many immediate principles (principes grossiers et

visibles, evidently what we should call proximate constituents) in

plants, such as water, sulphur or oil, common salt, nitre, volatile

salt or ammonia, certain earths, etc.; and that each of these immediate

constituents is a compound of three or four more simple principles,

which have united together into one body; nitre for instance has its

‘phlegma’ or tasteless water, its ‘spiritus,’ its fixed salt, and other

things; common salt in the same way has the like constituents, and it

may be assumed with much probability, that these more simple principles

also are compounds of parts that differ among themselves, but are too

small to be distinguished by any artificial means as to figure or any

other characters. Having shown how certain principles unite together,

he goes on to say, that he is unwilling to ascribe to them any sort

of consciousness (connaissance) by which they seek to unite together;

but he thinks that they are endowed with a natural disposition to

move towards one another, and to unite closely as soon as they touch

one another; though it is very difficult to define the nature of this

disposition, it is enough to know that there are many instances of such

movements to be found in nature; thus heavy bodies move towards the

centre of the earth, and iron to the magnet; nor are these movements

more difficult to conceive, than that of the planets in their courses

or of the sun round its axis, or that of the heart in a living animal.

With this first hypothesis Mariotte places himself, in opposition to

the Aristotelian doctrine with its entelechies and final causes which

prevailed at that time among botanists and physiologists, upon the

firm ground of modern science with its atoms, and its assumption of

necessarily active forces of attraction.

Mariotte’s second hypothesis more specially concerns the chemical

nature of plants; he supposes that several of his principes grossiers

are contained in every plant, and he endeavours first to explain their

source; the motes in the air, he says, which when burnt by lightning

smell of sulphur, are carried by rain into the earth, and parts of

them are taken up into the plant. Moreover distillation in all plants

produces a water, which the chemists call phlegma, and also acids and

ammonia, and if the residuum is burnt there remains an ash, from which

we obtain an earth which is without taste and insoluble in water,

and fixed salts; these salts differ from one another according as

they are mixed with more or less acid and ammoniacal spirit or other

unknown principles, which the fire could not volatilise. It is not to

be wondered at that these principles are found in plants, since they

derive their food from the earth which contains them. We see how great

has been the advance since the time when Van Helmont believed that he

had proved by his experiment, that all the materials in plants come

from pure water.

It remained to confront one view of the source of the substances in

plants, which was also drawn from the treasure-house of Aristotelian

conceptions, and was still in vogue. It was supposed that the very

materials of which the plant is composed were contained in their own

form in the earth, and had only to be taken up by the roots. Aristotle

had himself said: ‘Everything feeds on that of which it consists, and

everything feeds on more than one thing; whatever appears to feed only

on one thing, as the plant on water, feeds on more than one thing, for

earth in the case of the plant is mixed with the water; therefore the

country-people water plants with mixtures of things.’ This passage

might leave some doubt about Aristotle’s view, if we did not find the

following: ‘As many savours as there are in the rinds of fruits, so

many it is plain prevail also in the earth. Therefore also many of

the old philosophers said, that the water is of as many kinds as the

ground through which it runs[121].’ These passages taken with those

quoted above show that Aristotle made the substances required for the

growth of plants reach them from the earth ready elaborated, as has

been before observed; and this view, still maintained in Mariotte’s

time, may yet be met with among those who are ignorant of physiology.

It is interesting then to see, how vigorously Mariotte exposes the

incorrectness and absurdity of this idea, though he has no new

discovery to help him. In his third hypothesis he maintains, that

the salts, earths, oils, and other things, which different species

of plants yield by distillation, are always the same, and that the

differences are due entirely to the way in which these principes

grossiers and their simplest parts are united together or separated,

and he proves it thus: If a bonchretien pear is grafted on a wild one,

the same sap, which in the wild plant produces indifferent pears,

produces good and well-flavoured pears on the graft; and if this

graft has a scion from the wild pear again grafted on it, the latter

will bear indifferent fruit. This shows that the same sap in the stem

assumes different qualities in each graft. But still more forcible is

his proof of the fact, that plants do not take their substance direct

from the earth, but produce it themselves by chemical processes. Take

a pot, he says, with seven to eight pounds of earth and grow in it any

plant you like; the plant will find in this earth and in the rain-water

which has fallen on it all the principles of which it is composed in

its mature state. You may put three or four thousand different kinds of

plants in this earth; if the salts, oils, earths were different in each

species of plant, all these principles must be contained in the small

quantity of earth and rain-water which falls upon it in the course of

three or four months, which is impossible; for each of these plants

would yield in the mature state a dram of fixed salt at least and two

drams of earth, and all these principles together with those which are

mixed with the water would weigh at least from two to three ounces, and

this multiplied by four thousand, the number of the species of plants,

would give a weight of five hundred pounds.

These arguments like those of Jung, and in the main also those of

Malpighi, rested on facts which were on the whole as well known in

ancient times as in the 17th century; but no one had before given heed

to considerations, which were in themselves quite sufficient to do

away with the Aristotelian teaching on the subject of the nutrition of

plants.

In the second part of his letter Mariotte discusses the phenomena of

vegetation which depend on nutrition; he compares the endosperm in the

seed with the yolk of the egg in animals, and the entrance of the water

into the roots with its rising in capillary tubes; he takes the milky

juice to be the nutrient sap and compares it with arterial blood, the

other watery juices answering to venous blood. He says something quite

new about the pressure of the sap; he notices the high pressure at

which the sap stands in plants, and concludes from it that there must

be contrivances in them, which allow of the ingress of the water but

not of its egress. The existence of the pressure is well demonstrated

by the outflow from plants which contain milky juice when they are

wounded, and is compared with the pressure on the blood in the veins.

Equally striking is his further conclusion, that the pressure of the

sap expands the roots, branches, and leaves, and so contributes to

their growth. The sap, he adds, would not be able to remain at this

pressure, if it did not enter by pores, which forbid its return.

In these remarks lay the first germs of speculation on the growth

of plants, such as we shall meet with in Hales also in a somewhat

different form, but in the backward state in which phytotomy then was

they could not at present be further developed; we shall recur to them

further on, though in a different connection.

Mariotte concluded that the primary sap finds its way into the plant

through the leaves as well as through the roots from the fact, that if

a branch is taken from a tree, and one of its smaller branches kept in

water, another will remain fresh for some days; the conclusion was not

quite justified, as the future showed. His remarks on the necessity of

sun-light to nutrition, on the ripening of fruit, and other matters,

rests on very imperfect experience and need not be noticed.

The characteristic and the important point in Mariotte’s theory of

nutrition is the marked contrast between his point of view in natural

science and the Aristotelian and scholastic doctrines still widely

diffused, and thus he is led to declare war also against Aristotle’s

vegetable soul. He connects his remarks on this point with a fact which

excites his astonishment, namely that every species of plant reproduces

its properties so exactly; no explanation of this fact, he says, is

gained by the assumption of a vegetable soul, of which no one knows

what it is. He declares as decidedly against the theory of evolution,

also much in vogue in his day. In opposition to the notion that all

future generations are shut up one inside another in the seeds of a

plant, he thinks it much more probable that the seeds only contain the

essential substances, and that their influence on the crude sap brings

about the successive formation of the rest of the constituents of the

plant, a view which we may still allow to be correct. He regards the

whole process of nutrition and life in plants as a play of physical

forces, as the combination and separation of simple substances, but

he believes at the same time that he can prove the commonly received

doctrine of spontaneous generation to be a necessary conclusion from

this view. On this point he went wrong from want of sufficient and

well-sifted experience, for he regarded it as a proof of generatio

spontanea that numerous plants spring up from the soil thrown out from

ditches and swamps that have been laid dry. ‘We may therefore suppose,’

he says, ‘that there are in the air, in the water, and in the earth

an infinite number of minute bodies so fashioned that two or three

uniting together may make the beginning of a plant, and represent the

seed of such a plant, if they find a soil favourable to their growth.

But it is not probable that this little complex body contains already

all the branches, leaves, fruits, and seeds of this plant, and still

less that this seed contains all the branches, leaves, flowers, etc.,

which proceed ad infinitum from the first germination.’ The contrary

he thinks is proved by the fact, that a rose-bush which has lost its

leaves in the winter may produce in the next year nothing but leafy

shoots from its flower buds, which shows that the blossoms were not

previously formed in those buds, and that a similar conclusion is to be

drawn from another fact, that the seeds of one and the same fruit-tree

or of a melon produce descendants that differ from one another by

variation; here we have an argument against the theory of evolution

much more to the purpose than the greater part of those which were

alleged against it before Koelreuter obtained his hybrids.

Other prejudices also of his day were opposed by Mariotte, and on

good grounds; the medicinal effects, commonly known as the ‘virtutes’

of plants, played an important part in the botany, and still more in

the medicine and chemistry of that time. He rejects the old theory of

heat and cold, moisture and dryness, things supposed to be essentially

immanent qualities of the substance of plants and used to explain

their medicinal effects, and pointing to the fact, that poisonous

plants grow in the same soil as harmless ones and side by side with

them, he concludes, as he had before concluded, that different plants

do not derive their peculiar constituents immediately from the soil,

but that they form them themselves by separation and combination of

the common principles. Finally he declared against one of the grossest

errors which had come down from the previous century, the ‘signatura

plantarum,’ which supposed that the medicinal properties of plants

could be deduced from their external features, and especially from

resemblances between their organs and the organs of the human body.

Mariotte insists that the medicinal properties of plants are to be

ascertained by trying them on sick people.

Mariotte’s letter, the most important parts of which have here been

given, presents us with a lively picture of the views which prevailed

in the second half of the 17th century respecting the life of plants;

it shows at the same time how an eminent investigator of nature,

adopting the principles of a more modern philosophy and knowing how

to make a skilful use of the facts that were known to him, was led

to oppose antiquated error, the result of prepossessions and want of

reflection. If we combine the views of Malpighi on the internal economy

of the plant, derived chiefly from its anatomy, with the chemical and

physical disquisitions of Mariotte, we have an entirely new theory of

the nutrition of plants, not only antagonistic to the Aristotelian

doctrine, but distinguished from it by a much greater wealth of ideas

and by more sagacious combinations.

These two men had in truth discovered all the principles of vegetable

life and nutrition, which could have been discovered in the existing

condition of phytotomy and chemistry; Mariotte especially had succeeded

in applying the very best that was to be obtained from the uncertain

chemical knowledge of his day to the explanation of the phenomena of

vegetation. Chemistry was at that time beginning to set herself free

from the notions of the medical science, the iatro-chemistry of a

former age, only to throw herself into the arms of the theory of the

phlogiston; and how little she could contribute to the explanation of

the processes of nutrition in plants, how little the methods then in

use were adapted to the examination of organised bodies, may be learnt

from a little book published in 1676 and again in 1679, ‘Mémoires pour

servir à l’histoire des plantes,’ which appeared indeed in Dodart’s

name, but which was compiled and approved by the body of members of

the Academy of Paris. It contains no results of investigation, but

a detailed scheme for researches into botanical science, and more

particularly into the chemical part of it. There we read, that plants

must be burnt slowly, in order that the destroying and transmuting

power of the fire may have less effect; the ‘virtutes plantarum’ play

an important part in the chemical examination of plants, and blood

was mixed with their juices, in order to discover their properties.

A writer named Dedu in a treatise, ‘De l’âme des plantes’ (1685)

derived the generation and growth of plants from the fermentation and

effervescence of the acids in combination with the alkalies, as Kurt

Sprengel informs us. It is by comparison with these and similar notions

that we recognise the full superiority of the utterances of Malpighi

and Mariotte respecting the nutrition of plants, and their sagacity

is still further shown by the fact, that there are some things which

they forebore to say, evidently because they thought that they were not

clearly proved.

The views of Malpighi and Mariotte on the nutrition of plants were

respected and often quoted by their contemporaries and immediate

successors; but as has happened in other cases unfortunately up to

recent times, much that was fundamentally important and significant

in them was neglected from the first for comparatively unimportant

matters, and the views of these clear thinkers were so mixed up with

indistinct ideas and actual misconceptions, that no real advance was

made, though a variety of new facts were from time to time brought to

light. It has been already noticed that Malpighi’s correct idea of

the connection of the leaves with the nutrition of the plant was at

a later time commonly supposed to be equivalent to Major’s theory of

circulation, and since the latter was for various reasons considered to

be incorrect, it was thought that Malpighi’s view was dismissed with

it. Yet even Major’s theory deserved the preference over the views of

those who assumed only an ascent of the sap in the wood, because it

at least attempted to account for certain phenomena of growth. It

found a new supporter in 1680 in the person of Claude Perrault, who

does not however appear[122] to have added anything essentially new

to Malpighi’s conclusive arguments for a returning sap. Nor did his

opponent Magnol in his very weak treatise published in 1709 succeed

in saying anything that will bear examination against the theory of

circulation, which he too ascribed to Malpighi.

Among the phenomena of vegetation in woody plants, there is scarcely

one so striking as the outflow of watery sap from wounded vines and

from some tree-stems in the spring. This phenomenon, like the outflow

of milky juice, gum, resin and the like, could not fail to be regarded

with lively interest by those who occupied themselves with vegetable

physiology in the 17th century. Even supposing the movements of water

in the wood and of the milky and other juices in their passages not

to be necessary accompaniments of the nutrition of plants, yet it was

natural that the physiologists of the 17th century should see in them

striking proofs of that movement of the sap which is connected with

nutrition, and should therefore make them a subject of study. It might

also seem to them that the problem in question was easy to solve,

for it was not till long after that it came to be understood that

these movements are in reality one of the most difficult questions of

vegetable physiology. We discover the interest taken in these matters

from a series of communications in the form of letters from Dr. Tonge,

Francis Willoughby, and especially from Dr. Martin Lister, to be found

in the Philosophical Transactions for 1670[123]. The phenomenon to

which these men chiefly directed their attention was just the one best

calculated to lead to misconceptions respecting the movements of water

in woody plants, namely that which is known as the bleeding of the

wood in winter, and which depends on entirely different causes from

those which produce the weeping of the vine and other woody plants in

spring; but the two things were supposed to be identical, and hence

arose an unfortunate confusion of ideas. Lister indeed showed that it

is possible to force water out of the wood of a portion of a branch

cut from a tree in winter time by warming it artificially, and then

to cause the water to be sucked in again by cooling it; but it was

reserved for a modern physiologist to prove that this phenomenon has

nothing to do with the bleeding of cut stems from root-pressure, and

cannot be used to explain it.

John Ray, who gave a clear and intelligent summary of all that was

known respecting the nutrition of plants in the first volume of his

‘Historia plantarum’ (1693), also communicated some experiments made

by himself on the movements of water in the wood. He follows Grew’s

nomenclature, who called the ascending sap in the wood lymph and the

woody fibres therefore lymph-vessels, and notices particularly that

the lymph especially in spring cannot be distinguished in taste or in

consistence from common water. He agrees with Grew that in spring the

lymph fills the true vascular tubes of the wood and oozes from them

in cross sections, while in summer these are filled with air, and the

lymph at that time, when there is strong transpiration in woody plants,

ascends only in the lymph-vessels, that is in the fibrous elements of

the wood and the bast. By suitable incisions Ray proved that the lymph

can also move laterally in the wood; and by causing water to filter in

opposite directions through pieces of a branch cut off at both ends, he

refuted those who thought that the cavities of the wood and especially

the vessels were furnished with valves to hinder the return of the

lymph. But his knowledge of the mechanical causes of the movement of

water in the wood was not very great.

Some years elapsed before Hales’ labours added materially to the

progress which had been already made in the study of these processes

in vegetation. His important services to vegetable physiology close

our present period, but before we pass on to them, we must first

notice a few less important writers. The pages of Woodward and Beale

on transpiration and the absorption of water are not very valuable

contributions to the theory of nutrition. The fact stated by Woodward,

that a Mentha growing in water took up and discharged by evaporation

through the leaves forty-six times as much water as it retained in

itself, was perhaps the most important of all that he discovered, but

his own conclusions from it were of no value.

None of Malpighi’s doctrines had from the first excited so much

attention as the one which makes the air which is necessary for the

respiration of the plant circulate in the spiral vessels of the wood,

as it does in the tracheae in insects; while Grew and Ray after him

agreed with Malpighi in the main, his countryman Sbaraglia in 1704

ventured even to deny the existence of such vessels, and before long

phytotomy was fallen into such a state of decadence that the question,

whether there were any vessels, or as they were then called spiral

vessels, at all, was repeatedly affirmed and as often denied again,

and ultimately it was thought better in the interest of physiological

questions to take counsel of experiment rather than of the microscope.

Thus in 1715 Nieuwentyt endeavoured with the help of the air-pump to

make the air contained in the vessels issue in a visible form under

a fluid. Here we again encounter the philosopher Christian Wolff as

a zealous representative of vegetable physiology in Germany; in the

third part of his work, ‘Allerhand nützliche Versuche,’ 1721, among

other experiments he mentions some which confirmed the presence of air

in plants; the question was more interesting, in the state in which

physics and chemistry then were, than that of the anatomical character

of the air-conducting organs. Wolff submitted leaves lying in water

containing no air to the vacuum of the air-pump, and saw air-bubbles

issue, especially on the under side; but when he allowed the

atmospheric pressure to come into play again the leaves became filled

with water, and a piece of fir-wood treated in a similar manner sank

after the infiltration. In similar experiments with apricots air issued

from the rind and especially from the stalk. Wolff’s pupil Thümmig

described similar experiments in his ‘Gründliche Erläuterung der

merkwürdigsten Begebenheiten in der Natur,’ 1723, and both continued in

this question, as in all their physiological and phytotomical views,

faithful adherents of Malpighi, as it was wisest then to be. We must

linger a moment longer over Christian Wolff, because he published a

few years later a general view of the nutrition of plants in a popular

form. Wolff’s services in the dissemination of natural science in

Germany seem not to have been as highly appreciated up to the present

time as they deserve to be; his various works on natural science,

some of which took a wide range and were partly founded on his own

observations, were full of matter and for his time very instructive;

they contributed moreover to introduce more liberal habits of thought

at a time when gross superstitions, such as that of palingenesia,

reigned even among men who published scientific treatises in the German

Academy of Sciences (the ‘Acta of the Leopoldina).’ If Wolff’s own

scientific researches show more good will than skill, yet he had an

advantage over many others in a really philosophical training, a habit

of abstract thought which enabled him to fix with certainty on what

was fundamentally important in the observations of others, and thus

to expound the scientific knowledge of his day from higher points of

view. For this reason his work which appeared in 1723, ‘Vernünftige

Gedanken von den Wirkungen der Natur,’ deserves recognition. It is a

work of the kind which would now be called a ‘Kosmos,’ and treats of

the physical qualities of bodies generally, of the heavenly bodies

and specially of our own planet, of meteorology, physical geography,

and lastly of minerals, plants, animals and men. In accordance with

his chief object, general instruction, it is written in German and

in a good homely style, and contains the best information that was

at that time to be obtained on scientific subjects; among these he

gives an account of the processes of nutrition in plants, in which he

made careful and intelligent use of all that had been written on the

subject, bringing together all the serviceable material which he could

gather from Malpighi, Grew, Leeuwenhoek, Van Helmont, Mariotte and

others into a connected system, and occasionally introducing pertinent

critical remarks. If we consider the state of scientific literature in

Germany in the first years of the 18th century, we shall be inclined

to assign as great merit to comprehensive text-books of this popular

character as to new investigations and minor discoveries. Wolff’s

chapter on nutrition has however a special interest for us, because it

contains several observations of value which were lost sight of after

his time. These refer chiefly to the chemistry of nutrition and touch

many problems which were not solved before our time; for instance,

the statement that it is a well-known fact that the earth loses its

fruitfulness, if much is grown on it; that it requires much to feed

it, and must be manured with dung or ashes; in these few words we have

the questions of the exhaustion of the soil, and the restitution of

the substances taken from it by the crop, brought into notice by Wolff

at this early period. ‘It should be particularly noted,’ continues

Wolff, ‘how fruitful nitre makes the soil; Vallemont has praised

the usefulness of nitre, and has mentioned other things which have

a like operation by reason of their saline and oily particles, such

as horn from the horns and hoofs of animals; dung likewise contains

saline and oily particles, which are present in the ash also, and we

see therefore that such particles should not be wanting, if a plant

is to be fed from water. The seed also, which supplies the first food

of the plant, shows the same thing, for there are none which do not

contain oil and salt, and there are many from which the oil may be

squeezed out; and oil and salt are found in all plants if they are

examined chemically.’ He insists on the correctness of the view taken

by Malpighi and Mariotte, that the constituents of the food must be

chemically altered in the plant. Since every plant, he says, has its

own particular salt and its own particular oil, we must readily allow

that these are produced in the plant and not introduced into it. But at

the same time since plants cannot grow where the soil does not supply

them with saline and especially with nitrous particles, it is from

these that the salts and oils in the plant must be produced, and the

water also changed into a nutritious juice. Further on he alludes to

the saline, nitrous and oily particles which float in the air, and says

that daily experience shows that most of the substance of putrefying

bodies passes into the air, and that if we admit light through a

narrow opening into a dark place, we can see a great number of little

particles of dust floating about; water also readily takes up salt and

earth, and mineral springs show that metallic particles are mixed with

it. Therefore there is no reason to doubt that rain-water also contains

a variety of matters which it conveys to the plant. Alluding once more

to the chemical changes in the constituents of the food which must be

supposed to take place in the plant, he connects the subject with some

remarks on the organs of plants, in which he closely follows Malpighi;

he says that these changes cannot take place in tubes, because the

sap merely rises or falls in them; we can only therefore suppose that

it is in the spongy substance (the cellular tissue) that the nutrient

sap is elaborated, and accordingly the vesicles or utriculi are a kind

of stomach; but the change in the water can only be this, that the

particles of various substances which are in rain-water are separated

from it and united together in some special manner, and this cannot be

effected without special movements. But his ideas on these movements

in the sap are somewhat obscure. He employs the expansion of the air

and the capillarity of the woody tubes as his moving forces. He agrees

decidedly with those who postulated a returning sap as well as an

ascending crude sap, but he appeals in this matter to Major, Perrault,

and Mariotte, and not to Malpighi; yet like Malpighi he notices the

growth of trees set upside down as a proof that the juices can move

in opposite directions in the conducting organs, and with Mariotte he

ascribes the enlargement of growing organs to the expanding power of

the juices which force their way into them.

But these well-meant efforts on the part of Christian Wolff, and indeed

all that was done from Malpighi and Mariotte to Ingen-Houss to advance

the knowledge of the nutrition of plants, was thrown into the shade by

the brilliant investigations of STEPHEN HALES[124] in whom we see once

more the genius of discovery and the sound original reasoning powers of

the great explorers of nature in Newton’s age. His ‘Statical Essays,’

first published in 1727, reappeared in two new editions in English, and

afterwards in French, Italian and German translations; in the last with

a preface by Christian Wolff. This was the first work devoted to a more

complete account of the nutrition of plants and of the movements of the

sap in them, and while it noticed what had been already written on the

subject, it was chiefly composed of the author’s own investigations.

An abundance of new experiments and observations, measurements and

calculations combine to form a living picture of the whole subject.

Malpighi endeavoured to discover the physiological functions of organs

by the aid of analogies and a reference to their structure; Mariotte

discerned the main features of the connection between plants and

their environment by combining together physical and chemical facts;

Hales may be said to have made his plants themselves speak; by means

of cleverly contrived and skilfully managed experiments he compelled

them to disclose the forces that were at work in them by effects made

apparent to the eye, and thus to show that forces of a very peculiar

kind are in constant activity in the quiet and apparently passive

organs of vegetation. Penetrated with the spirit of Newton’s age,

which notwithstanding its strictly teleological and even theological

conception of nature did endeavour to explain all the phenomena of life

mechanically by the attraction and repulsion of material particles,

Hales was not content with giving a clear idea of the phenomena of

vegetation, but sought to trace them back to mechanico-physical laws as

then understood. He infused life into the empirical materials which he

collected by means of ingenious reflections, which brought individual

facts into connection with more general considerations. Such a book

necessarily attracted great attention, and for us it is a source of

much valuable instruction on matters of detail, though we now gather up

the phenomena of vegetation into a somewhat differently connected whole.

His investigations into transpiration and the movement of water in

the wood were greeted with the warmest approbation. He measured the

quantity of water sucked in by the roots and given off by the leaves,

compared this with the supply of moisture contained in the earth,

and endeavoured to calculate the rapidity with which the water rises

in the stem, and to compare it with the rapidity of its entrance

into the roots and its exit by the leaves. The experiments, by which

he showed the force of suction in wood and roots, and that of the

root-pressure in the case of the bleeding vine, were particularly

striking and instructive. His measurements and the figures, on which

he founded his calculations, were not so exact as they were often at a

later time supposed to be, but he was himself satisfied with obtaining

round, approximative numbers; these under given circumstances supplied

a sufficient basis for propositions which were new and afforded a

certain amount of insight into the economy of the plant. This mode of

proceeding showed his understanding; for the case of living bodies

is different from that of metals and gases; in these we seek for

constants which can then be inserted in general formulae, and to which

therefore the nicest accuracy is applied; but in plants we have to

deal with individual cases, and it is from a right interpretation of

the measurements taken from them that we can arrive at general laws of

vegetation.

To show that the forces of suction and pressure which operate in

plants are not something sui generis, but prevail also in dead matter,

in other words that they are an example of the general attraction of

matter, a subject of particular interest at that time, Hales observed

the absorption of water by substances with fine pores; and measured

the force employed. These processes he compared with the force which

swelling peas exert on the obstacles which they encounter, and thus

obtained a more correct idea of the forces concerned in the movement of

water in the plant than that given by the capillarity of glass-tubes,

which Mariotte and Ray had employed to illustrate them.

Hales failed to appreciate the value of Malpighi’s observations on

the function of leaves, and was induced by the copiousness of the

evaporation of water from their surfaces to overrate the physiological

importance of that process; hence he saw in leaves chiefly organs of

transpiration, which raise the sap by suction from the roots through

the stem. In accordance with this view he denied the existence of a

descending sap in the bark, and only admitted that the ascending sap

in the wood might possibly sink in the night in consequence of the

lowering of the temperature, like the quicksilver in a thermometer, and

that so far there might be a return-movement. This was the weak point

in Hales’ system.

One of his most important discoveries has generally been overlooked

even in modern times, probably because it was entirely neglected by

his successors in the 18th century; he was the first who proved,

that air co-operates in the building up the body of the plant, in

the formation of its solid substance, and that gaseous constituents

contribute largely to the nourishment of the plant; consequently

that neither water, nor the substances which it carries with it from

the earth, alone supply the material of which plants are composed,

as had been generally imagined. He showed also with the aid of the

air-pump, and better than Nieuwentyt and Wolff, that air enters the

plant not only through the leaves but also through apertures in the

rind, and circulates in the cavities of the wood. He then connected

this with the fact which he had confirmed by numerous experiments,

that large quantities of ‘air’ are obtained from vegetable substance

by fermentation and dry distillation; the air thus set free by

fermentation and heat must in his opinion be condensed and changed to

a solid condition during the period of vegetation. He says in chap. 7,

that we find by chemical analysis (dry distillation) of vegetables,

that their substance is composed of sulphur, volatile salt, water

and earth; these principles are all endowed with mutual power of

attraction (of their parts). But air also enters into the composition

of the plant, and this in its solid state is powerfully attractive,

but in an elastic condition has the highest powers of repulsion. It is

on infinitely various combinations, actions, and reactions of these

principles that all activity in animal and vegetable bodies depends. In

nutrition the sum of the forces of attraction is greater than that of

the forces of repulsion, and thus the viscid ductile parts are first

produced, and then by evaporation of the water the harder parts.

But if the latter again absorb water, and the forces of repulsion

consequently gain the preponderance, then the consistence of the

vegetable parts is dissolved, and this decomposition restores to them

the power of forming new vegetable products; therefore the stock of

nutritive substance in nature can never be exhausted; this stock is the

same in animals and plants, and is fitted by a small change of texture

to feed the one or the other.

He goes on to say, that it results from his experiments, that leaves

are very useful for the nourishing of the plant, inasmuch as they

draw up the food from the earth; but they seem also to be adapted to

other noble and important services; they remove the superfluous water

by evaporation, retaining the parts of it that are nutritious, while

they also absorb salt, nitre, and the like substances, and dew, and

rain; and since, like Newton, he regarded light as a substance, he

concludes by asking: ‘may not light, which makes its way into the outer

surfaces of leaves and flowers, contribute much to the refining of the

substances in the plant?’

It might be gathered from these expressions that Hales attributed

importance for purposes of nutrition only to the substances suspended

in the air; but this was not the case; for we read in the 6th

chapter, that he had proved by experiment that a quantity of true

permanently elastic air is obtained from vegetable and animal bodies by

fermentation and dissolution (dry distillation); the air is to a great

extent immediately and firmly incorporated with the substance of these

bodies, and it follows therefore that a large quantity of elastic air

must be constantly used in forming them.

But Hales not only regards the air as a nourishing substance, but

he sees also in its elasticity, which counteracts the attraction of

other substances, the origin of the force which maintains the internal

movements in the plant. He says that if all matter were endowed only

with forces of attraction, all nature would at once contract into an

inactive mass; it was therefore absolutely necessary in order to set

in movement and animate this huge mass of attracting matter, that a

sufficient quantity of strongly repellent and elastic matter should be

mixed with it; and since a large portion of these elastic particles

are constantly changing to a solid condition through the attraction of

the other parts, they must be endowed with the power of again assuming

their elastic condition, when they are set free from the attracting

mass. Thus the formation and dissolution of animal and vegetable bodies

go on in constant succession. Air is therefore very important to the

production and growth of animals and plants in two ways; it invigorates

their juices while it is in the elastic state, and contributes much to

the firm union of the constituent parts, when it has become fixed.

We see what good use Hales could make of the small stock of ideas in

physics and chemistry at his disposal, and that he succeeded with their

help in rising to a point of view, from which he was able to form some

idea of the phenomena of vegetation in their most important relations

to the rest of nature, and in their inner course and connection. But

his successors did not comprehend the fundamental importance of these

considerations, and made no use of the pregnant idea, that a much

larger part of the substance of plants comes from the air and not from

the water or the soil; they were for ever wondering that so little is

furnished by the soil to the plant, as Van Helmont had shown, though

they did not confess to supposing that the water was changed into the

substance of the plant, as he had imagined. Thus physiologists lost

sight of the principle, which might long before the time of Ingen-Houss

have sufficiently explained the most important of all the relations of

the plant to the outer world, namely that it derives its food from the

constituents of the atmosphere, and so neglected further experimental

enquiry into the matter; they quoted and repeated Hales’ experiments

and observations again and again, but forgot that which in his mind

bound all the separate facts together.

Hales is the last of the great naturalists who laid the foundations of

vegetable physiology. Strange as some of their ideas may seem to us,

yet these observers were the first who gained any deep insight into the

hidden machinery of vegetable life, and handed down to us a knowledge

both of individual facts and of their most important relations. If we

compare what was known before Malpighi’s time with the contents of

Hales’ book, we shall be astonished at the rapid advance made in less

than sixty years, while scarcely anything had been contributed to the

subject in the period between Aristotle and Malpighi.

3. FRUITLESS ATTEMPTS TO EXPLAIN THE MOVEMENT OF THE SAP IN PLANTS.

1730-1780.

If those, who studied the nutrition of plants and especially the

movement of their sap in the period between Hales and Ingen-Houss, had

kept a firm hold on Malpighi’s view, that the nutritive substances are

elaborated in the leaves, and had combined it with Hales’ idea, that

plants derive a large portion of their substance from the air, they

would have had a principle to guide them in their investigations into

the movement of the sap; and by experimenting on living plants they

might have succeeded in giving a more definite expression to these

ideas, even though chemistry and physics supplied during that time no

new aids. We have said already that such was not the course of events;

physiologists confined their attention to the obvious phenomena of

vegetation, and trusted in so doing to gain a firmer footing, but in

this they never got beyond a commonplace and unreflecting empiricism,

because their observation was without an object, and their conclusions

without a principle. They wandered from the right direction, as always

happens when observation is not guided by a well-considered hypothesis;

and their conceptions were rendered more obscure by their imperfect

acquaintance with one of the most important aids to understanding

the movement of the sap, namely the structure of the more delicate

parts of the plant, the knowledge of which had not advanced since the

days of Malpighi and Grew. Since most of them made no phytotomical

investigations of their own, and only partially understood the

descriptions of those writers, they had to be content with misty and

often quite inaccurate ideas of the inner structure of wood and bark,

and yet expected to obtain an insight into the movement of the sap

in them. In reading the writings of Malpighi, Grew, Mariotte, Hales

and even Wolff, notwithstanding many mistakes in details we find a

pleasure in the connected reasoning, and in the sagacity which knew how

to distinguish between what was important and what was not; whereas

the observers, whom we have now to mention, give us only isolated

statements, nor have we the satisfaction of feeling that we are

conversing with men of superior understanding.

We may pass over the unimportant writings of Friedrich Walther (1740),

Anton Wilhelm Platz (1751) and Rudolph Böhmer (1753), as merely barren

exercises; but some notice should be taken of those of De la Baisse

and Reichel, since these authors at least endeavoured to bring to

light something new. But the method which they employed of making

living plants suck up coloured fluids was calculated to give rise to

serious errors both at the time and afterwards. Magnol had mentioned

experiments of the kind in 1709, and the Jesuit father Sarrabat, known

by the name of DE LA BAISSE, occupied himself with them and described

them in a treatise, ‘Sur la circulation de la sêve des plantes,’ 1733,

which received a prize from the academy of Bordeaux[125]. He set the

roots of different plants in the red juice of the fruit of Phytolacca,

and found that in two or three days the whole of the bark of the roots

and especially the tips of the root-fibres were coloured red inside.

It was a natural conclusion at that time, that it was these parts which

chiefly absorbed the red colouring matter, and in fact this opinion was

maintained till quite recent times, and it was on such results that

Pyrame de Candolle founded his theory of the spongioles of the root,

which is still accepted in France. At present it is known, that the

bark and especially the youngest tips of the fibres of the root are not

coloured under these circumstances, until they have been first poisoned

and killed by the colouring matter; these experiments therefore,

which have been frequently repeated since De la Baisse’s time, prove

nothing respecting the action of living roots, but they were from the

first the cause of a pernicious error in vegetable physiology, which

as we shall see gave rise to others also. One result however of De la

Baisse’s experiments was less misleading; he placed the cut ends of

branches of woody plants in the coloured fluid, and found that not only

the general body of the wood, but the woody bundles which pass from

it into the leaves and parts of the flowers, were coloured red, while

the succulent tissue of the bark and leaves remained uncoloured. It

appeared therefore that the red juice passed only through the wood,

and a somewhat bold analogy might lead to the further conclusion that

this is true also of the nutrient substances dissolved in the watery

sap; but the view so stated is not at present considered to be correct,

and that the sap which ascends from the roots to the leaves, the

water especially, is conveyed through the wood only, and not through

the rind, had been already sufficiently proved by the experiments of

Hales and others. The uncritical treatment of experiments of this kind

by GEORG CHRISTIAN REICHEL[126] afterwards led to new errors, though

his dissertation, ‘De vasis plantarum spiralibus,’ shows to advantage

by the side of similar productions of the day owing to its careful

notices of the literature, and the author’s original researches in

phytotomy. Reichel was not satisfied with the arguments of Malpighi,

Nieuwentyt, Wolff, Thümmig and Hales for the view that the vessels of

the wood contain air. He observed quite correctly, that if branches

are cut off from woody and herbaceous plants and the cut surfaces

are placed in red decoction of brazil-wood, the red colouring matter

spreads through all the vascular bundles, even those of the flowers

and fruit; but on examination with the microscope he found the red

fluid to some extent in the cavities of the vessels, and hastily

concluded that they too in the natural condition convey sap and not

air. His description and his drawing show however, that only some

vessels had received any of the red fluid and that none of these were

filled with it. Reichel and the many who repeated his statements

forgot to ask whether the vessels had contained air or fluid before

the experiment, or whether the result would have been the same, if

plants with uninjured and living roots had absorbed the coloured fluid,

and no divided vessels had therefore come in contact with it. There

was no reason why observers of that day should not have been alive to

the simple consideration, that the vessels of a branch parted from

the stem and placed in a fluid must necessarily show the capillary

action of narrow glass tubes if they are filled with air in their

natural condition, and that in the experiment the transpiration of the

leaves must favour the ascent of the red juice in the cavities of the

vessels, as was to be gathered from other and better experiments made

by Hales. But these obvious reflections were not made; the supposed

results of the experiment were heedlessly accepted, and the unfounded

notion, that vessels are natural sap-conducting organs, was set up

in opposition to the trustworthy decision of Malpighi and Grew, that

they convey air. Thus on the strength of badly interpreted experiments

one of the most important of physiological discoveries was called in

question, and a hundred years later there were persons, who, relying

on the same experiments as Reichel, supposed that the vessels of

the wood convey the ascending sap, a view which made it impossible

from the first to arrive at any real understanding of the movement

of the sap in plants provided with organs of transpiration. But even

the other great discovery which we owe to Malpighi, that leaves are

organs for elaborating the food, was denied by Bonnet, who substituted

for it the utterly false view, that they chiefly serve to absorb

rain-water and dew. BONNET[127], who had previously done good service

to insect-biology, and had discovered the asexual propagation of

aphides, having injured his eyes in these studies, found an agreeable

pastime in a variety of experiments on plants. Much that he did was

unimportant, yet he obtained some results, which could afterwards be

turned to account by more competent persons, for the weakness of his

own judgment is shown even in his more serviceable observations, such

as those on the curvature of growing plants. We notice the same defect

in his observations on the part played by leaves in the nutrition of

the plant. It shows the character of the time that a book like Bonnet’s

‘Recherches sur l’usage des feuilles des plantes,’ a mere accumulation

of undigested facts, should have been generally considered an important

production. He tells us, that his attention was called by Calandrini

to the fact, that the structure of the under side of leaves seems to

show that they were intended to absorb ‘the dew that rises from the

ground’ and introduce it into the plant. Starting from this sensible

suggestion, as he calls it, he proceeded to make a variety of senseless

experiments with leaves, which were cut off from their plants, and

having been smeared over with oil or other hurtful substances were laid

on water, some on their upper some on their under side, the object

being to note the time which they took to perish. It is impossible to

imagine worse-devised experiments on vegetation; for if Bonnet wished

to test Calandrini’s ‘sensible’ conjecture, he ought certainly to have

left the leaves on the living plants and have observed the effect of

the supposed absorption of dew on the vegetation. It is to be observed,

that by rising dew he evidently meant aqueous vapour, for the real dew

descends chiefly on the upper side of the leaf; and what could he have

expected to learn by laying cut leaves on water? how could this prove

that leaves absorb dew? Nevertheless Bonnet came to the conclusion that

the most important function of leaves was to absorb dew, and in order

to make this result agree with Hales’ investigations on transpiration,

he propounded the theory[128], that the sap which rises by day from

the roots into the stem is carried by the woody fibres assisted by

the air-tubes into the under side of the leaves, where there are many

stomata to facilitate its exit (evaporation). At the approach of night,

when the leaves and the air in the air-tubes are no longer under the

influence of heat, the sap returns to the roots; then the under side

of the leaves commences its other function; the dew slowly rising

from the earth strikes against it, condenses upon it, and is detained

there by the fine hairs and by other contrivances (this really takes

place to a much greater extent on the upper side). The fine tubes of

the leaves absorb it at once, (this is evidently not so, since the dew

increases in quantity till sunrise), and conduct it to the branches,

whence it passes into the stem. He thought so highly of this strange

theory, that he believed he found in it a teleological explanation of

the heliotropic and geotropic curvature of leaves and stems, two things

which he did not distinguish, and of the position of leaves on the

stem. Bonnet’s view of the functions of leaves, foolish as it is, is

historically important and therefore required to be noticed, because it

was really accepted during many years in preference to the older and

better ideas, and because it shows how the power of judging of such

matters had fallen off since Malpighi’s time. It appears to have been

the praise lavished on Bonnet by his contemporaries that made later

physiologists, who might have known better, take him for an authority

on the nutrition of plants. His experiments on the growth of plants

in another material than earth are if possible more worthless than

those with cut leaves. Here too the idea was not his own; for hearing

that land-plants had been grown in Berlin in moss instead of earth,

he made numerous experiments of the kind, and found that many plants

grow vigorously in this way, and bloom and bear seed. But the theory

of nutrition gained nothing by these experiments, which were only a

childish amusement. The few pages which Malpighi wrote on the nutrition

of plants are worth more than all Bonnet’s book on the use of leaves;

the former by the help of some simple considerations and conclusions

from analogy really discovered the use of leaves; Bonnet on the faith

of many unmeaning experiments ascribed to them another function than

the true one.

We are unable to pass a much more favourable judgment on the views

respecting the nutrition of plants of another writer, who otherwise

did good service to vegetable physiology, and to whom we shall return

in our last chapter. It is true that DU HAMEL[129], of whom we speak,

was not an investigator of nature, as were Malpighi, Mariotte or

Hales; compared with those great thinkers he was only a compiler, and

a somewhat uncritical one. But he was not a dilettante in science,

like Bonnet; he made the vegetable world the subject of serious and

diligent study, and he endeavoured to turn the results of that study

to practical account. Long familiarity with plants gave him a kind

of instinct for the truth in dealing with them, as is shown in his

observations and experiments, many of which are still instructive;

but he had neither that faculty of combination which can alone bring

a meaning out of experiments and observations in physiological

investigations, nor the power to distinguish between matters of

fundamental and secondary importance. So thinks also his biographer Du

Petit-Thonars.

The merits and the faults here mentioned are combined in an especial

degree in Du Hamel’s most famous work, ‘Physique des arbres,’ which

appeared in two volumes in 1758 and is a text-book of vegetable anatomy

and physiology with numerous plates. His remarks on the nutrition of

plants and the movement of the sap are a lengthy compilation chiefly

from Malpighi, Mariotte and Hales, though he has not succeeded in

appropriating exactly that which is theoretically important or adopting

the most commanding points of view. He introduces the results of his

own experiments into his account, and these are often instructive in

themselves, but are never made use of to establish a definite view with

respect to the connection between the processes of nutrition. He hits

upon the right view only when he is dealing with plain and obvious

matters; for instance, he restores the vessels of the wood to their old

rights, and concludes from experiments, as had been already done in the

17th century, that an elaborated sap moves in the reverse direction in

the rind; so too he perceives that if bulbs, tubers, and roots, with

or without the help of water which they have absorbed, produce shoots

and even flowers, this must be done at the expense of material laid

up in reserve, but he does not turn this fact to any further account.

But he utterly spoilt the best part of his subject; he made the leaves

nothing but pumps that suck up the sap from the roots; he quotes

Malpighi’s better view as a curiosity, and never mentions it again; but

he accepts Bonnet’s unfortunate theory, though he himself adduces many

facts, which make for Malpighi’s interpretation of the leaves. He is

almost more unsuccessful with chemical points in nutrition; he repeats

Mariotte’s statements with regard to the necessity of a chemical change

in the nutrient substances in the plant, and even supplies further

proof of it; but he cannot shake off the Aristotelian dogma, that

the earth like an animal stomach elaborates the food of plants, and

that the roots absorb the elaborated matter like chyle-vessels (II.

pp. 189, 230). He concludes from his own attempts to grow land-plants

without earth and in ordinary water that the latter supplies the plant

with very little matter in solution, but he makes no use of Hales’

statements with regard to the co-operation of the air in the building

up of the plant, and ends by saying (II. p. 204) that he only wished to

prove that the purest and simplest water can supply plants with their

food, which his experiments do not prove. Thus almost all that Du Hamel

says on the nutrition of plants is a mixture of right observations

in detail with wrong conclusions, and reflections which never rise

above the individual facts and give no account of the connection of

the whole. These faults appear in a still higher degree in a later

and almost more comprehensive work, the ‘Traité théorique et pratique

de la végétation’ of Mustel (1781). The further the distance from the

founders of vegetable physiology, the larger were the books that were

written on the subject; but the thread that held the single facts

together became thinner and thinner, till at last it broke. The theory

of nutrition, like a forced plant, needed light that it might recover

strength. This light came with the discoveries of Ingen-Houss, and

with the mighty strides made by chemistry after 1760 in the hands of

Lavoisier.

4. THE MODERN THEORY OF NUTRITION FOUNDED BY INGEN-HOUSS AND THEODORE

DE SAUSSURE. 1779-1804.

The two cardinal points in the doctrine of the nutrition of plants,

namely that the leaves are the organs which elaborate the food, and

that a large part of the substance of the plant is derived from the

atmosphere, were established, as we have seen, by Malpighi and Hales,

and employed by them in framing their theory; it remained to supply a

direct and tangible proof of the fact that the green leaves take up a

constituent of the atmosphere and apply it to purposes of nutrition. It

was evidently the want of such direct proof which caused the successors

of the first great physiologists to overlook the importance of the

propositions thus obtained by deduction, and so to grope their way in

the dark with no principle to guide them.

The discoveries of Priestley, Ingen-Houss and Senebier, and the

quantitative determinations of de Saussure in the years between 1774

and 1804, supplied the proof that the green parts of plants, and the

leaves therefore especially, take up and decompose a constituent of

the air, while they at the same time assimilate the constituents of

water and increase in weight in a corresponding degree; but that this

process only goes on copiously and in the normal way, when small

quantities of mineral matter are introduced at the same time into

the plant through the roots. The discoveries and facts, from which

this doctrine proceeded, were those which overthrew the theory of

the phlogiston, and from which Lavoisier deduced the principles of

modern chemistry; the new theory of the nutrition of plants was indeed

directly due to Lavoisier’s doctrines, and it is necessary therefore

to take at least a hasty glance at the revolution which was effected

in chemistry between 1770 and 1790. It is a well-known fact[130] that

this revolution dates from the discovery of oxygen-gas by Priestley in

1774. Priestley himself was and continued to be a stubborn adherent of

the phlogiston; but his discovery was made by Lavoisier the basis of an

entirely new view of chemical processes. By the combustion of charcoal

and the diamond, Lavoisier proved as early as 1776 that ‘fixed air’

was a compound of carbon and ‘vital air.’ In like manner phosphoric

acid, sulphuric acid and, after a preliminary discovery by Cavendish,

nitric acid also were found to be compounds of phosphorus, sulphur

and nitrogen with vital air; in 1777 Lavoisier showed that fixed air

and water are produced by the combustion of organic substances, and

after establishing within certain limits the quantitative composition

of fixed air, he named it carbonic acid, and the gas which had up

to that time been known as vital air he called oxygen. Cavendish

in 1783 obtained water by the combustion of hydrogen-gas, and then

Lavoisier proved that water is a compound of hydrogen and oxygen. These

discoveries not only did away step by step with the old theory of the

phlogiston, and supplied the principles of modern chemistry, but they

also affected exactly those substances which play the most important

part in the nutrition of plants; every one of these discoveries in

chemistry could at once be turned to account in physiology. In 1779

PRIESTLEY discovered that the green parts of plants occasionally

exhale oxygen, and in the same year Ingen-Houss described some fuller

investigations, which showed that this only takes place under the

influence of light, and that the green parts of plants give off carbon

dioxide in the dark, as those parts which are not green do both in

the light and the dark. A correct interpretation of these facts was

not however possible in 1779; it was not till 1785 that Lavoisier

succeeded in setting himself quite free from the old notions, and

developed his antiphlogistic system into a connected whole. It should

be mentioned that he had discovered in 1777 that the respiration of

animals is a process of oxidation which produces their internal heat,

heat being the product of every form of combustion. This fact was

equally important for vegetable physiology, but it was some time before

it was used to explain the life of plants.

The establishment of the fact, that parts of plants give off oxygen

under certain circumstances, did little or nothing to further the

theory of their nutrition[131]; and that was all that vegetable

physiology owes to Priestley. Ingen-Houss on the other hand determined

the conditions under which oxygen is given off, and further showed that

all parts of plants are constantly giving rise to carbon dioxide; on

these facts rests the modern theory of the nutrition and respiration of

plants, and we must therefore consider that Ingen-Houss was the founder

of that theory. But since we are dealing here with a discovery of more

than ordinary importance, it seems necessary to go more closely into

the details.