before 1840; Theodore de Saussure observed in 1822 the production of
heat in flowers and its dependence on respiration; ten years later
Goeppert proved the rise of temperature in germinating and vegetating
organs. Dutrochet stimulated enquiry by his researches in various
branches of the science between 1820 and 1840; he was the first to
apply the phenomena of diosmosis to the explanation of the movement
of sap in plants with a lasting influence on the further progress of
physiology. Chemical investigations were less fruitful in results,
though they served to collect a considerable material of single facts,
which could afterwards be turned to theoretical account.
The close of this period, which began with unprofitable doubts, but
in which much was set in a train for further development after 1840,
is marked by the publication of some important compilations, in which
all that had as yet been done in vegetable physiology was presented in
a connected form. In addition to Dutrochet’s collected works (1837)
three comprehensive compendia of vegetable physiology made their
appearance, one by De Candolle, which was translated into German by
Roeper and published with many improvements and additions in 1833
and 1835; this was followed by a work on vegetable physiology by L.
C. Treviranus, 1835-1838, and lastly by Meyen’s ‘Neues System der
Pflanzenphysiologie,’ 1837-1839. These works exhibit the characteristic
features of the period chiefly in this, that physiology finds as yet
no strong support in phytotomy, while the old views of vital force are
brought face to face with more exact physico-chemical explanations of
processes of vegetation.
4. We have already pointed out the wonderful impulse given to the
study of morphology and phytotomy, of embryology and cells about the
year 1840; it was shown also that this was due in a great measure
to discarding the errors of the nature-philosophy and the idea of
vital force, and requiring in the place of such speculations exact
observation and systematic induction, and how Schleiden’s ‘Grundzüge’
soon after 1840 vigorously met the demands of the newer time in
these respects, but without satisfying them by the positive results
obtained. The rapid progress made by phytotomy and the doctrine of
cells in the hands of von Mohl and Nägeli proved specially favourable
to vegetable physiology, by making it possible to follow the processes
of fertilisation in the interior of the ovule. The formation of the
pollen-tube from the pollen-grain had been observed long before
1840, and Schleiden in 1837 had proposed the view that the embryo
of Phanerogams was formed at the end of the pollen-tube by free
cell-formation after it had entered the embryo-sac. But Amici in 1846
and Hofmeister in 1849 showed that this notion was erroneous, and
that the germ-primordium is in existence in the embryo-sac before the
arrival of the pollen-tube and is excited by it to further development,
to the forming the embryo. Similarly Hofmeister’s further observations
on the embryology of Vascular Cryptogams and Mosses left no doubt,
that the spermatozoids of these groups of plants discovered by Unger
and Nägeli serve to fertilise the germ-cell or egg-cell previously
formed in the female organ and to excite it to further development
(1849, 1851). Soon after the sexual act was observed in various
Algae, and these afforded the best opportunity for solving by the
aid of the microscope the questions which experiment had still left
open. Thuret showed in 1854, how the large egg-cells in species of
Fucus are surrounded and fertilised by spermatozoids, and he even
succeeded in producing hybrids by fertilising the egg-cells of one
species with the spermatozoids of another; but it was still uncertain
whether simple contact of the male and female organs was sufficient, or
whether fertilisation is due to the mingling of the substance of the
spermatozoid and the germ-cell; the question was settled by Pringsheim
in 1855; he saw the male organ of fertilisation of a fresh-water alga
penetrate into the substance of the egg-cell and be dissolved in it,
and this proceeding was afterwards observed in higher Cryptogams and is
represented in its simplest form in the sexual act of the Conjugatae,
which De Bary described at length in 1858 and like Vaucher regarded as
a sexual process.
When we consider to what an extent the time and power of work of the
most eminent botanists was devoted after 1840 to long and difficult
observations on the minute anatomy of plants, on cell-formation,
embryology and the history of the development of organs, we cannot
wonder if other parts of vegetable physiology, which require
experiments on vegetation in plants, were cultivated but little and
by the way only; but these studies also gained firmer footing in the
advance of phytotomy, which supplied the physiologist with a more
definite idea of the organism in which the phenonema of vegetative life
are produced.
The chemistry of the food of plants was one of the strictly
physiological subjects, which like the sexual theory was studied
without intermission and with considerable success in the period from
1840 to 1860, but chiefly or entirely by chemists, who connected
their investigations into the processes of nutrition in plants with
Saussure’s results. Agricultural chemists were chiefly engaged till
nearly 1860 with the questions, whether all or certain constituents
of the ash of a plant are indispensable parts of its food, and whence
these constituents are derived, and with cognate considerations on
the exhaustion of the soil by cultivation and its remedy by suitable
manuring. In France Boussingault had undertaken experimental and
analytical investigations on these subjects before 1840, and it was
he who in the course of the next twenty years made the most valuable
physiological discoveries; of these the most important was the fact
that plants do not make use of free atmospheric nitrogen as food, but
take up compounds of nitrogen for the purpose. In Germany the interest
in such questions was increased by the instrumentality of Justus
Liebig, who gathered from the knowledge that had been accumulated up
to 1840 all that was fundamental and of real importance, and drew
attention to the great practical value of the theory of the nutrition
of plants in agriculture and in the management of woods and forests;
considerable state provision was soon made for investigations of the
kind, but these often wandered from the right path for the reason,
that being designed to promote practical interests they lost sight
of the inner connection between all vital phenomena. Still a great
mass of facts was accumulated, which careful sifting might afterwards
render serviceable to pure science. Some of the best agricultural
chemists deserve the credit of vindicating purely scientific as well
as practical points of view, and explained in comprehensive works the
general subject of the nutrition of plants, so far as it was possible
to do so without going deeply into their organisation; among these
were Boussingault and the Germans Emil Wolff and Franz Schulze. But
the questions of the nutrition of plants, which are connected with the
chemical processes of assimilation and metabolism within them, remained
still undecided, though some valuable preliminary work on these points
dates from this time.
In comparison with this important advance in the sexual theory and the
doctrine of the nutrition of plants little was done in the branches
of vegetable physiology which remain to be mentioned, and that little
appeared in an unconnected and fragmentary state; different observers
established the connection between the temperature of plants and
oxygen-respiration; some new single facts were discovered in connection
with the downward curvature of roots, Brücke published in 1848 an
excellent enquiry into the movements of Mimosa-leaves, and Hofmeister
showed in 1857 that the phenomenon, then known as bleeding in the
vine and some other trees, takes place in all woody plants, and not
in spring only but in every period of the year, if the requisite
conditions are present. These and many other isolated observations were
very valuable for the future, but were not used at the time to frame
comprehensive theories, because no one devoted himself exclusively to
questions of the kind with the perseverance, which in these difficult
subjects can alone lead to certain results and to a deeper insight
into the inner connection of the phenomena. Surprisingly small was the
addition to the knowledge of the movement of sap in plants, and still
less was discovered respecting the external conditions of processes of
growth and the movements connected with them. The important question of
the dependence of the phenomena of vegetation on temperature, was it
is true not wholly neglected; but the mistake was made of attempting a
short cut by multiplying the total period of vegetation of a plant by
the mean daily temperature, in the hope of finding in this product an
expression for the total warmth required by a given plant; this mistake
was especially misleading in the geography of plants.
The more valuable knowledge which had been gathered up to 1851 was
brought together by von Mohl in his often-mentioned work on the
vegetable cell with equal perspicuity and conciseness, and current
views were critically examined; vegetable physiology generally was
expounded at greater length but with less critical sifting in
Unger’s text-book of 1855; these were the two books which did most to
disseminate a knowledge of the subject up to 1860, and they performed
their task with credit; that which appears in Schacht’s books after
1852 under the head of vegetable physiology rests on such imperfect
acquaintance with this branch of science, as to diminish rather than
increase its reputation.
* * * * *
Passing from this preliminary survey to a more detailed account of the
subject, it will be found necessary to keep the history of the sexual
theory distinct from other questions in vegetable physiology. This
mode of proceeding is required by the fact, that the establishment and
further elucidation of the decisive points in the sexual theory were
made independently of the rest of physiology, so that the historical
continuity would be interrupted and the account rendered obscure by
any attempt to connect the development of the theory chronologically
with other topics. In like manner the doctrine of the nutrition of
plants and of the movement of the sap was developed uninterruptedly and
in independence of other physiological matters; it will be advisable
therefore to devote a separate chapter to those subjects also. Earlier
discoveries respecting the movements of the parts of plants and the
mechanics of growth will be briefly recounted in a third chapter.
CHAPTER I.
HISTORY OF THE SEXUAL THEORY.
1. FROM ARISTOTLE TO R. J. CAMERARIUS.
It will contribute to a correct appreciation of the discoveries made
towards the end of the 17th century by Rudolph Jacob Camerarius and his
successors in regard to the sexual relations of plants, if we first
make ourselves acquainted with all that was known of the matter up to
that time from Aristotle downwards; we shall learn at the same time
how extremely unfruitful was the superficial observation of the older
philosophy in a question in which inductive research only could lead to
real results.
That Aristotle[90] like many others after him reckoned sexual
fertilisation among processes of nutrition, and thus failed to perceive
the specific and peculiar character of the latter, is shown distinctly
by his assertion, that the nutritive and propagative power of the
soul is one and the same. This hasty generalisation was associated
in Aristotle’s mind with another error arising from very defective
experience, which led him to bring sexuality in organisms into causal
connection with their movement in space. He tells us in his botanical
fragments, that in all animals which have the power of locomotion, the
female is distinct from the male, one creature being female, another
male, but both being of the same species, as in humankind. In plants
on the contrary these powers are combined and the male is not distinct
from the female; each plant therefore reproduces itself and emits
no fertilising material; and he adds, that in animals which do not
move, as those that have shells and those that live attached to some
other substance, male and female are not distinguished, for their
life resembles that of plants; at the same time they are called male
and female by resemblance and analogy, and there is a certain slight
distinction. In like manner some trees produce fruits while others do
not, though they aid fruit-bearing trees in the production of fruit, as
happens in the case of the fig-tree and the caprifig.
In comparison with these views of Aristotle those of his disciple
Theophrastus[91] appear to some extent enlightened, and to rest on a
wider experience, but even his observation supplies nothing of interest
on the subject; for he says that some blossoms of the ‘mali medicae’
produce fruit, and that some do not, and that it should be observed
whether the same thing occurs in other plants, which he might easily
have done for himself in his own garden. He is more concerned with
putting his knowledge into logical order, than with answering the
question whether there is any sexual relation in plants. It is certain,
he says, that among plants of the same species some produce flowers and
some do not; male palms, for instance, bear flowers, the female only
fruit[92]; and he concludes the sentence by the remark, that in this
lies the difference between these plants, and those which produce no
fruit, and that it is obvious that there must be a great difference
in the flowers. In his third book ‘De Causis’ (c. 15, 3) he says,
that terebinths are some male and some female, and that the former are
barren and are therefore called male. That Theophrastus in all these
matters trusted to the relations of others is shown by a passage in
the same book (c. 18, 1), where he says, ‘What men say, that the fruit
of the female date-palm does not perfect itself unless the blossom
of the male with its dust is shaken over it, is indeed wonderful,
but resembles the caprification of the fig, and it might almost be
concluded that the female plant is not by itself sufficient for the
perfecting of the fœtus; but this cannot be the case in one genus or
two, but either in all or in many.’ We observe the grand style in which
the Greek philosopher dismisses this important question, and how far he
is from condescending to make an observation for himself.
It appears that in Pliny’s time the hypothesis of a sexual difference
in plants had grown up and become confirmed in the minds if not of
writers, yet of those who occupied themselves with nature; Pliny in his
‘Historia Mundi,’ describing the relation between the male and female
date-palm, calls the pollen-dust the material of fertilisation, and
says that naturalists tell us that all trees and even herbs have the
two sexes[93].
If this theme supplied little material for reflection to philosophers,
it did not fail to excite the fancy of the poets. De Candolle cites
the verses of Ovid and Claudian on the subject, and passing over
the intervening centuries for a very sufficient reason notices the
lively poetic description of two date-palms in Brindisi and Otranto
by Jovianus Pontanus in 1505. But nothing was gained in this way for
natural science.
Treviranus in his ‘Physiologie der Gewächse[93]’ (1838), II. p. 371,
has well described the state of knowledge on this subject among the
botanists of Germany and the Netherlands in the 16th century. ‘The
idea of a male sex in such plants as Abrotanum, Asphodelus, Filix,
Polygonum mas et femina, was founded only on difference of habit, and
not on the parts which are essential to it. But it should be observed
that it is the less learned among the older botanists, Fuchs, Mattioli,
Tabernaemontan, who make most frequent use of this mode of designating
plants; the more learned, as Conrad Gesner, de l’Écluse, J. Bauhin
employ it only in the case of a plant already known. De l’Écluse it
is true in describing the plants which he found often notes the form,
colour, and even the number of the stamens; in Carica Papaya he calls
the individual with stamens the male, and the one with carpels the
female, since he holds them to belong to different sexes, though of the
same species; but he is satisfied with saying, that it is affirmed that
the two are so far connected, that the female produces no fruit if the
male is separated from it by any great distance (‘Curae posteriores,’
42).
The case of the botanists above-mentioned is simply one of ignorance;
in the botanical philosopher Cesalpino on the contrary we see a
consequence of the Aristotelian system, which leads him distinctly to
reject the hypothesis of separate sexual organs in plants as opposed to
their nature. It is difficult to understand how De Candolle, at page 48
of his ‘Physiologie végétale,’ can say that Cesalpino recognised the
presence of sexes in plants. His conception of vegetable seed-grains
as analogous to the male seed in animals must have made it impossible
for him to understand sexuality in plants. So too his notion that the
seed is derived from the pith as the principle of life in plants,
in connection with which he says at page 11 of the first of his
sixteen books; ‘Non fuit autem necesse in plantis genituram aliquam
distinctam a materia secerni, ut in animalibus, quae mari et femina
distinguuntur.’ He regarded the parts of the flower which surround the
ovary, or are separate from it, together with the stamens as simply
envelopes of the foetus; and though he knew, as has been already shown,
that in some plants, the hazel, chestnut, Ricinus, Taxus, Mercurialis,
Urtica, Cannabis, Mais, the flowers are separate from the fruit, and
even mentions that the barren individuals are called male, and the
fruit-bearing female, he understood this only as a popular expression,
without really admitting a sexual relation. Respecting the words male
and female he says at page 15: ‘Quod ideo fieri videtur quia feminae
materia temperatior sit, maris autem calidior; quod enim in fructum
transire debuisset, ob superfluam caliditatem evanuit in flores,
in eo tamen genere feminas melius provenire et fecundiores fieri
aiunt, si juxta mares serantur, ut in palma est animadversum, quasi
halitus quidam ex mari efflans debilem feminae calorem expleat ad
fructificandum.’
There is no mention of the pollen here, still less any attempt to
extend what had been observed in dioecious plants to the ordinary
cases, in which flowers and pistil, as Cesalpino would say, are united
in the same individual. His view of the relation between the seed and
the shoot, cited above on page 47, shows that he conceived of the
formation of seeds as only a nobler form of propagation than that by
buds, but not essentially distinct from it. The idea of sexuality in
plants was not in fact consonant with Cesalpino’s interpretation of
Aristotelian teaching.
Prosper Alpino’s account (1592) of the pollination of the date-palm
contains nothing new, except that he had seen it in Egypt himself[94].
The Bohemian botanist Adam Zaluziansky[95] made no observations of his
own, but attempted in 1592 to reduce the traditional knowledge on the
subject to some kind of theory. The foetus, he says, is a part of the
nature of plants, which they produce out of themselves, and is thus
distinguished from the shoot which grows from the plant, as a part
from the whole, but the other as a whole from a whole. He quotes Pliny
almost word for word where he says, that observers of nature maintain
that all plants are of both sexes, but in some the sexes are conjoined,
in others they are separate; in many plants the male and female are
united, and these have the power of propagation in themselves, like
many androgynous animals; and he explains this, more explicitly than
Aristotle, from defect of locomotion in plants. This is the case, he
says, with the majority of plants. In some, as the palm, the male
and female are separated, and the female without the male produces
no fruit, and where the dust from the male does not reach the female
plant by natural means, man can assist. Zaluziansky like other writers
is anxious that plants of different sexes should not be taken for
different species. He refers also to the popular distinction of many
plants into male and female according to certain external peculiarities.
Jung again must certainly have known the facts and views that were
current in his time; but there is nothing in his botanical writings to
show that he entertained the idea of a real sexuality in plants, of the
necessity of the co-operation of two sexes in the work of propagation.
It might almost be believed that the most learned and serious men, such
as Cesalpino and Jung, were just those, who regarded the hypothesis of
sexuality in plants as an absurdity, and shrunk from its consideration.
This impression is conveyed too by Malpighi’s ‘Anatomie des Plantes.’
It was Malpighi who gave the first careful account of the development
of the seed, and studied the earlier stages in the growth of the embryo
in the embryo-sac; and yet even he says nothing of the co-operation
of the dust contained in the anthers in the formation of the embryo,
and does not once mention the views of former writers. Malpighi,
like Cesalpino, regarded the formation of seeds as only another kind
of ordinary bud-formation, and propagation as only another kind of
nutrition. He mentions (p. 52) incidentally that plants with unfruitful
flowers are designated as male, but treats this as a popular expression
merely, and ultimately propounds the theory that the stamens and the
floral envelopes remove a portion of the sap from the flower, in order
to purify the sap for the production of the seeds (p. 56).
In all accounts of the theory of sexuality in plants, a botanist
otherwise unknown in history, Sir Thomas Millington, is named as the
person who first claimed for the stamens the character of male organs
of generation. The only record of the fact, however, is contained in
the following words of Grew in his ‘Anatomy of Plants’ (1682), ch. 5,
sect. 3, p. 171: ‘In conversation on this matter (namely the connection
of the stamens, called by Grew the attire[96], with the formation of
seeds) with our learned Savilian Professor Sir Thomas Millington, he
told me he was of opinion that the attire served as the male organ in
the production of the seed. I replied at once, that I was of the same
opinion, and gave him some reasons for it, answering at the same time
some objections that might be brought against it.’ Grew gives on p.
172 the following summary of his ideas on the subject[97]; it would
appear, he says, that the attire serves to remove some superfluous
parts of the sap, as a preparatory process to the production of seed.
As the floral envelopes (foliature) serve to remove the volatile and
saline sulphur-parts, so the attire serves to lessen and adjust the
gaseous, in order that the seed may become more oily and its principles
be better fixed. Here we find ourselves on the ground of the chemistry
of the day, in which sulphur, salt, and oil play the chief parts.
Consequently, continues Grew, the flower has usually a stronger smell
than the attire, because the saline sulphur is stronger than the
gaseous, which is too subtle to affect the sense. Closely adhering to
Malpighi’s view he goes on to compare these processes in the flower
with processes in the ovary of animals, inasmuch as they qualify
the sap in the ovary for the approaching formation of seed, and he
says that as the young and early attire before it opens contains the
superfluous part of the female organ, so after it is opened it probably
performs the office of the male. But how confused his ideas still were
on this point may be further seen by examination of the passage which
follows in his book (page 172, section 7), where, speaking of the
single flowers in the head of the Compositae, he regards the blade,
that is the style and stigma, of the floral attire as a portion of a
male organ, and the globulets (pollen-grains) and other small particles
upon the blade and in the thecae (anthers) of the seed-like attire as
a vegetable sperm, which subsequently when the parts are duly matured
falls down upon the seed-case and so touches it with a prolific virtue.
He meets the objection, that the same plant must consequently be both
male and female, with the fact, that snails and other animals are
similarly constituted. That the pollen-grains communicate a prolific
virtue to the ovary (uterus) or to its juices by simply falling upon
it, he thinks is rendered probable by comparing this with the process
of fertilisation in many animals, and here Grew has some curious
remarks. The section closes with the observation that to expect
complete similarity in this matter between plants and animals, is to
require that the plant should not only resemble an animal, but should
actually be one.
If now we ask ourselves, what it really was that was gained from
Millington and Grew, we find that it was simply the conjecture, that
the anthers produce the male element in fertilisation, and that this
view was closely connected in their minds with the strangest chemical
theories and analogies from animal life. It is remarkable by what
indirect ways science sometimes advances. If Grew had only been
prepared to assume some kind of sexuality in plants, he need only have
taken up Theophrastus’ statement, that the anther-dust of the male
palm is shaken over the female to produce fertilisation; and since
both Grew and Malpighi observed the pollen in the anthers, they might
at once and in reliance on this experiment of a thousand years before
have come to the conclusion that the stamens are the male organs. But
Grew never mentions the ancient views and experiences. Like other
writers before Camerarius, he made no attempt to answer the question
by experiment. It was a step in advance, when Ray in his ‘Historia
Plantarum’ (1693), I. cap. 10, p. 17; II. p. 1250, threw some light on
the very obscure train of thought in Grew’s mind, and did something to
put it on the right track, by referring to the case of dioecious plants
and to the old experience of the date-palm, but he too made no attempt
to settle the question by experiment. The true discoverer of sexuality
in plants, Camerarius, was however engaged in the experimental solution
of the problem two years before the appearance of Ray’s ‘Historia
Plantarum.’ Ray’s remarks on the subject in the preface to his ‘Sylloge
Stirpium’ (1694) are only assertion founded on no experiments. But
if any are prepared to attribute greater value to the utterances of
Grew and Ray, the comparison of them with the way in which Camerarius
addressed himself to the question will show at once, that it was he
who so far advanced the theory of the subject as to make it accessible
to experimental treatment, as he undoubtedly was the first who not
only undertook experiments on the subject but carried them out with
the skill which will appear in the following section. Linnaeus was
right when he says in his ‘Amoenitates’ (1749), I. p. 62, that it was
Camerarius who first clearly demonstrated (perspicue demonstravit) the
sexuality of plants and the mode of their propagation.
2. ESTABLISHMENT OF THE DOCTRINE OF SEXUALITY IN PLANTS BY RUDOLPH
JACOB CAMERARIUS. 1691-1694.
We have seen that all that was known with regard to sexuality in
plants up to 1691 was comprised in the facts related by Theophrastus
concerning the date-palm, the terebinth, and the ‘malus medica,’ and in
the conjectures of Millington, Grew, and Ray, while Malpighi’s views in
opposition to these later authors were considered to be equally well
founded. The sexuality of plants could only be raised to the rank of
a scientific fact in one way, that namely of experiment; it had to be
shown that no seed capable of germination could be formed without the
co-operation of the pollen. All historic records concur in proving,
that Camerarius was the first who attempted to solve the question
in this way, and that he followed up this attempt by many other
experiments. It is quite another question how the fertilising matter
reaches the germ which is capable of being fertilised, and this could
not be entertained till experiment had established the fact, that the
pollen is absolutely indispensable to fertilisation.
To Johann Christian Mikan, Professor of Botany in Prague, is due
the merit of having collected the scattered and therefore almost
forgotten writings of RUDOLPH JACOB CAMERARIUS[98], and published
them, together with some similar works of Koelreuter, at Prague in 1797
under the title, ‘R. J. Camerarii Opuscula Botanici Argumenti.’ This
book, apparently little known, will be my principal authority for the
following remarks. The short preliminary communications are printed
without alteration from the ninth and tenth year of the second, and
from the fifth and sixth year of the third decury of the Ephemerides
of the Leopoldina; the letter to Valentin, which will be noticed again
further on, together with an abstract of the same and an answer of
Valentin, are given according to Gmelin’s edition of 1749.
Camerarius had observed, that a female mulberry-tree once bore fruit,
though no male tree (amentaceis floribus) was in its neighbourhood,
but that the berries contained only abortive and empty seeds, which he
compared to the addled eggs of a bird. His attention was roused, and
he made his first experiment on another dioecious plant, Mercurialis
annua; he took in the end of May two female specimens of the wild plant
(they were usually called male, but he knew them to be the female) and
set them in pots apart from others. The plants throve, the fruit was
abundant and filled out, but when half ripe they began to dry up, and
not one produced perfect seeds; his communication on this subject is
dated December 28, 1691. In the third decury of the Ephemerides, year
5, he relates that in a sowing of spinach he had found monoecious as
well as dioecious plants, as Ray had observed in Urtica romana, and he
himself again in three other species. The disregard of this fact was
afterwards the cause of erroneous interpretation of the experiments and
of doubt about sexuality.
But Camerarius’ chief composition on the subject of sexuality in
plants is his letter ‘De sexu Plantarum,’ which is often mentioned but
apparently little read, and which he addressed to Valentin, Professor
in Giessen, on Aug. 25, 1694. It is the most elaborate treatise on the
subject which had as yet been written, or indeed which appeared before
the middle of the 18th century, and contains more profound observations
than were made by any other botanist before Koelreuter. The style
contrasts favourably with the style of the writers of the time, and
is thoroughly that of modern natural science; it combines perfect
knowledge with careful criticism of the literature of the subject;
the construction of the flower is explained more clearly than it had
ever been before, or was again for a long time after, and expressly
for the purpose of making the meaning of his experiments on sexuality
intelligible. The whole tone of the letter shows that Camerarius was
deeply impressed with the extraordinary importance of the question, and
that he was concerned to establish the existence of sexuality by every
possible means.
After detailed examination of the parts of the flower, the anthers and
pollen, the behaviour of the ovules before and after fertilisation,
the phenomena of double flowers and similar matters, from all which he
cautiously deduces the meaning of the anthers (apices), he proceeds
to bring forward direct proofs. He says, ‘In the second division of
plants, in which the male flowers are separated from the female on the
same plant, I have learnt by two examples the bad effect produced by
removing the anthers. When I removed the male flowers (globulos) of
Ricinus before the anthers had expanded, and prevented the growth of
the younger ones but preserved the ovaries that were already formed,
I never obtained perfect seeds, but observed empty vessels, which
fell finally to the ground exhausted and dried up. In like manner I
carefully cut off the stigmas of Mais that were already dependent, in
consequence of which the two ears remained entirely without seeds,
though the number of abortive husks (vesicularum) was very great.’
He then refers to his former communications to the Ephemerides on
dioecious plants, and says that the case of the spinach confirmed these
results. After alluding to similar relations in animals he continues,
‘In the vegetable kingdom no production of seeds, the most perfect
gift of nature, the general means for the maintenance of the species,
takes place, unless the anthers have prepared beforehand the young
plant contained in the seed (nisi praecedanei florum apices prius ipsam
plantam debite praeparaverint). It appears, therefore, justifiable to
give these apices a nobler name and to ascribe to them the significance
of male sexual organs, since they are the receptacles in which the
seed itself, that is that powder which is the most subtle part of the
plant, is secreted and collected, to be afterwards supplied from them.
It is equally evident, that the ovary with its style (seminale vasculum
cum sua plumula sive stilo) represents the female sexual organ in the
plant.’ Further on he assents to Aristotle’s theory of the mixture of
sexes in plants, and adduces Swammerdam’s discovery of hermaphroditism
in snails, which he says is the exception in animals but the rule in
plants. One erroneous notion which was only seen to be erroneous a
hundred years later by Konrad Sprengel, and not finally refuted till
within the last few years, was his belief that hermaphrodite flowers
fertilise themselves, and this by comparison with the snails he thinks
is strange, though most botanists till down to our own times, in spite
of Koelreuter and Sprengel, did not find it strange. That sexuality
in plants was admitted by botanists, Ray excepted, at the close of
the 17th century at most in a figurative sense, but that Camerarius
conceived of it as in the animal kingdom, and sought to make this
conception prevail, is apparent from the strong expressions, which he
uses to show that in dioecious plants the distinction between male and
female plants is not to be understood figuratively. He says that the
new foetus, the young plant contained in the seed, is formed inside
the coat of the seed after the plant has flowered, exactly as the new
foetus is formed in animals. The authority of the ancients was still
great at that time, for Camerarius thinks it necessary to insist that
the views of Aristotle, Empedocles, and Theophrastus are not opposed
to his sexual theory. Camerarius appears as the true investigator
of nature, endowed with the true discerning spirit in disregarding
the question which had already been raised with respect to animals,
whether the ovum or the spermatozoid (vermis) produces the foetus,
because the first thing to be done was to establish the fact of a
sexual difference, not the mode of generation; he thinks it certainly
desirable to examine and see what the pollen-grains contain, how far
they penetrate into the female parts, whether they advance uninjured
as far as the seed which receives them, or what they discharge if they
burst before reaching it. He does full justice to Grew’s services in
connection with the knowledge of the pollen and its function.
It does all honour to the scientific spirit in Camerarius, that he
raises a number of objections to his own theory; one was, that Lycopods
and Equisetaceae produce, as he thinks, no young plants from their
pollen; he suspected therefore that they have no seed. It should be
remembered that the germination of Equisetaceae and Lycopods was not
observed till the 19th century. An objection, more important at the
time, was that a third ear of a castrated maize plant contained eleven
fertile seeds, of whose origin he could give no account. He was even
more disturbed by finding that three plants of hemp taken from the
field and cultivated in the garden produced fertile seeds, and he tries
to explain it by supposing various ways in which pollination might have
taken place unobserved. This led him to make a fresh experiment; next
year he placed a pot containing seedlings of hemp in a closed room;
three male and three female plants grew up; the three male were cut off
(not by himself) before their flowers opened; the female produced a
great number of abortive seeds, but also a good many fruitful ones. His
opponents and those who sought to appropriate his honours fastened,
as is usual, on these failures, without being able to account for the
experiments which had been successful. The statement of his failures is
our best proof of the exactness of his observations, for we now know
the cause of failure, which Camerarius himself observed, but did not
apply in explanation. We may assume that he would have cleared up this
point in his splendid investigations in a quieter time, for at the end
of his letter he laments the unjust war then raging; it was the time
of the predatory campaign of Louis XIV. To his letter is appended a
Latin ode of twenty-six stanzas by an unknown poet, probably a pupil
of his own; it is an epitome of the ‘Epistola de sexu Plantarum,’ as
Goethe’s well-known poem contains the chief points of his doctrine
of metamorphosis, but it resembles Goethe’s composition in no other
respect; it begins
Novi canamus regna cupidinis,
Novos amores, gaudia non prius
Audita plantarum, latentes
Igniculos, veneremque miram.
3. DISSEMINATION OF THE NEW DOCTRINE; ITS ADHERENTS AND OPPONENTS.
1700-1760.
No part of botany has so often engaged the pen of the historian, as the
doctrine of sexuality in plants; but the majority of writers have not
gone to the original sources for their information, and the consequence
has been that the merits of the real founders and promoters of the
doctrine have often been thrown into the shade for the benefit of
others; even German botanists have ascribed the services of Camerarius
to Frenchmen and Englishmen, because they were unacquainted with his
writings, or were unable to judge of the question and its solution.
We shall here endeavour to show from the records of the 18th century
how far anyone before Koelreuter really contributed anything of value
to the establishment of the sexual theory. As is usually the case
in great revolutions in science, some simply denied the new theory,
many adopted it without understanding the question, others formed a
perverse and distorted conception of it under the influence of reigning
prejudices, while others again sought to appropriate to themselves
the merit of the real discoverer; there were but few who with a right
understanding of the question advanced it by new investigations.
The botanists who endeavoured to aid in determining the matter by their
own observations may be distinguished into those, to whom the important
point was the enquiry whether the pollen is absolutely necessary to
the formation of seed, such as Bradley, Logan, Miller, and Gleditsch,
and those who like Geoffroy and Morland assumed that sexuality was no
longer an open question, and who were bent on observing in what way
the pollen effects fertilisation in the ovule. But there was another
class of writers altogether, who, believing that they could deal with
the subject without making observations and experiments of their own,
either like Leibnitz, Burckhard, and Vaillant, simply accepted the
results of the observations of others on general grounds, or like
Linnaeus and his disciples, endeavoured to draw fresh proofs from
philosophical principles, or like Tournefort and Pontedera, simply
rejected the idea of sexuality in plants. Lastly, we might mention
Patrick Blair who did nothing himself, but merely appropriated the
general results of Camerarius’ observations, and has had his reward
in being quoted even by German writers as one of the founders of the
sexual theory[99].
We have now to see what was really brought to light by further
experiment and observation. BRADLEY appears to have been the first who
experimented on hermaphrodite flowers with a view to establish the
sexuality of plants (‘New improvements in Gardening’ (1717), I. p.
20). He planted twelve tulips by themselves in a secluded part of his
garden, and as soon as they began to flower removed the anthers; the
result was, that not one of them produced seeds, while four hundred
tulips in another part of the same garden produced seeds in abundance.
Twenty years pass by before another experiment is made. JAMES
LOGAN[100], Governor of Pennsylvania, an Irishman by birth, set some
plants of maize in each corner of a plot of ground, which was forty
feet broad, and about eighty long, and experimented on them in various
ways. In October he noted the following results:—the cobs of the
plants, from which he had removed the male panicles when the stigmas
were already dependent, presented a good appearance; but closer
examination showed that they were unfertilised, with the exception of
one which was turned in the direction from which the wind might have
conveyed pollen from other plants. On the cobs, from which he had
removed some of the stigmas, he found exactly as many grains as he had
left stigmas. One cob, which had been wrapped in muslin before the
appearance of the stigmas, produced only empty husks.
Miller’s experiments in 1751, which Koelreuter has extracted from the
‘Gardener’s Dictionary,’ part II[101], are specially interesting,
because the aid of insects in pollination was then observed for the
first time. Miller planted twelve tulips, six or seven ells apart, and
carefully removed the stamens as soon as the flowers began to open;
he imagined that he should thus entirely prevent fertilisation; some
days after he saw some bees load themselves with pollen in an ordinary
tulip-bed and fly over to his imperfect flowers. After they were gone,
he observed that they had left on the stigmas a quantity of pollen
sufficient for fertilisation, and these tulips did in fact produce
seed. Miller also kept some female plants of spinach apart from the
male, and found that they bore large seeds without embryos.
Professor GLEDITSCH, Director of the Botanic Garden in Berlin,
described in the same year (‘Histoire de l’Académie royale des sciences
et des lettres’ for the year 1749, published in 1751 at Berlin), an
experiment on the artificial fertilisation of Palma dactylifera folio
flabelliformi, which was no doubt our Chamaerops humilis, since he says
himself in page 105 that it was Linnaeus’ Chamaerops, and Koelreuter
speaks of the plant in his report by that name. This treatise, in point
of scientific tone and learned handling of the question, is the best
that appeared between the time of Camerarius and that of Koelreuter.
We learn from the introduction, that in the year 1749 there were few
who doubted the existence of sexuality in plants. The author says
that he has endeavoured to attain to perfect conviction on the point
by many years’ experiments with plants of the most various kinds. Of
late years he had chiefly selected dioecious plants for investigation,
Ceratonia, Terebinthus, Lentiscus, and the species of date-palm
which is commonly called Chamaerops. After relating the formation of
fertile seeds in Terebinth and the mastic-tree produced by artificial
pollination, he turns to Chamaerops, of which species Prince Eugene
had repeatedly caused specimens of considerable size to be brought
over from Africa; a specimen cost as much as a hundred pistoles; but
they died without flowering. ‘Our palm in Berlin,’ he continues, ‘is
a female, and may be eighty years old; the gardener asserts that it
has never borne fruit, and I have myself never seen fertile seeds on
it during fifteen years.’ As there was no male tree of the kind in
Berlin, Gleditsch procured some pollen from the garden of Caspar Bose
in Leipsic. In the course of the nine-days’ journey the greater part
of the pollen escaped from the anthers, and Gleditsch feared that it
was spoilt; but he was reassured by the Leipsic botanist Ludwig, who
had had experience in Algiers and Tunis, and who informed him that the
Africans usually employ dry pollen that has been kept for some time for
the purpose of fertilisation. Though the flowering of the female tree
was nearly over, he strewed the loose pollen on its flowers, and tied
the withered inflorescence of the male plant to a late-blowing shoot of
the female. The result was that fruit ripened in the following winter,
and germinated in the spring of 1750. A second attempt conducted in a
similar manner produced an equally favourable result[102].
Koelreuter, who repeats this account in his ‘Historie der Versuche,’
a record of the experiments made between the years 1691 and 1752 on
the sexes of plants, ends his narrative with these words: ‘These
are, as far as I know, all the attempts which have been made and
described since the year 1691 to prove the existence of sexes in
plants.’ Koelreuter’s book was written to show that experiment only can
determine the question of sexuality in the vegetable kingdom, and that
no one beside Camerarius, Bradley, Logan, Miller, and Gleditsch had
pursued this method up to 1752.
While these botanists occupied themselves with the question whether
there was a distinction of sexes in the vegetable kingdom, we meet with
two writers at the beginning of the 18th century who regard sexuality
as proved, and who take up the question of the mode in which the
pollen brings about the formation of the embryo. Both were adherents
of the theory of evolution, bad observers, and not familiar with
the literature of the subject. The first is SAMUEL MORLAND. In the
‘Philosophical Transactions’ of 1702 and 1703, p. 1474, he names Grew
as the man who had observed that the pollen answers to the male semen,
but he makes no allusion to Camerarius’ experiments, the only ones
which had as yet been made. He himself suggests that the young seeds
may be compared to unfertilised ova, while the pollen-dust (farina)
contains embryo plants, one of which must find its way into every
ovule (ovum) in order to fertilise it. If so, the style must be a tube
through which the embryos pass into the ova. He supposes the pollen in
Fritillaria imperialis to be washed by wind and rain from the stigma
through the style into the ovary, without reflecting that the movement
must be an upward one in the hanging flower. If I could prove, he says,
that embryos are never found in unfertilised seeds, this would be a
demonstration; but I have never been so fortunate as to settle this
point. He does not mention that Camerarius had shown this ten years
before; he can only give as the main argument for his conjecture,
that in beans the embryo lies near the orifice of the seed-coat (the
micropyle), which shows that he was not aware that the two large
bodies in the seed of the bean (the cotyledons) belong to the embryo,
a fact which his countrymen Grew and Ray had already pointed out. It
appears therefore, that Morland supplied no answer to the question how
fertilisation takes place; his treatise contains nothing more than the
assertion that the embryo is already contained in the pollen-grain, and
that it reaches the seed through a hollow style and is there developed,
an entirely erroneous and not even an original idea, for it was the
offspring of the theory of evolution which was at that time in vogue.
GEOFFROY’S communications (‘Histoire de l’Académie royale des
sciences,’ Paris, 1714, p. 210) contain a few more facts. He mentions
neither Grew, Camerarius, nor even Morland, but connects his own
observations of 1711 on the structure and purpose of the more important
parts of the flower with those of Tournefort, who was a decided
opponent of the doctrine of sexuality in plants. The parts of the
flower are hastily described, figures are given of some forms of
pollen-grains, and the notion that the style is a tube receives some
apparent confirmation from the experiment of drawing water through
the style of a lily. The view that the pollen is not an excrement, as
Tournefort and Malpighi had maintained, is defended partly by arguments
which prove nothing, for instance, by the erroneous assertion that
the anthers are always so disposed that the extremity of the pistil
must necessarily receive their dust. The only proof offered for the
fact that seeds are infertile if deprived of the co-operation of the
pollen, is a very hasty account of some experiments with maize and
Mercurialis. The result of these experiments, as well as some other
remarks of Geoffroy, remind us of the text of Camerarius’ letter to an
extent which mere accident will hardly account for. If Geoffroy really
made these experiments, which is open to some doubt, yet they were
made fifteen years later than those of Camerarius, who did make the
same experiments among others and has described them better. Geoffroy
next endeavours to show how the pollen effects the fertilisation,
and offers two views on the subject; first, that the dust contains
much sulphur and is decomposed on the pistil, the more subtle parts
forcing their way into the ovary, where they set up a fermentation
and cause the formation of the embryo; the second view is, that the
pollen-grains already contain the embryos, which find their way into
the seeds and are there hatched. This is Morland’s notion, who however
is not mentioned. Geoffroy considers the latter to be the more probable
hypothesis, chiefly because no embryo is found in the ovule before
fertilisation, and also because the seed of the bean has an orifice
(the micropyle); it does not occur to him that these facts speak as
much for the first as for the second view.
Enough has been produced to show that Morland and Geoffroy contributed
nothing either to the establishment of the fact of sexuality in
plants, or to the decision of the question how the pollen effects
fertilisation in the ovule. Nevertheless I have mentioned these two
men immediately after those who really developed the sexual theory,
because they at least took their stand on experience, and endeavoured,
though unsuccessfully, to demonstrate conditions of organisation
which should explain the process of fertilisation. We come now to the
names of men—Leibnitz, Burckhard, Vaillant, Linnaeus—who are usually
supposed to have aided in establishing the sexual theory, but who
may be proved to have contributed nothing whatever to the scientific
demonstration of that doctrine. First as regards the philosopher
Leibnitz; he says in a letter of 1701, from which Jessen has quoted
the most important parts in his ‘Botanik der Gegenwart und Vorzeit,’
1864, p. 287: ‘Flowers are closely connected with the propagation
of plants, and to discover distinctions in the mode of propagation
(principiis generationis) is very useful,’ etc.; again, ‘A new and
extremely important point of comparison will be hereafter supplied
by the new investigations into the double sex in plants,’ alluding,
according to Jessen, to those of Camerarius and Burckhard. We shall
not expect to find that Leibnitz made experiments himself, and the
words quoted merely indicate that he wished to see the parts of the
flower employed for purposes of classification, because according to
the observations of others they are the instruments of propagation.
The remark applies in a still higher degree to Burckhard, who in his
letter to Leibnitz of 1702, quoted above on p. 83, further developed
the idea intimated by Leibnitz, for he too accepted the sexuality of
plants as an established and self-evident truth. The address with which
Sebastian Vaillant opened his lectures at the Royal Gardens in Paris in
1717 has often been noticed by the historians of botany. De Candolle,
who assigns to him an important share in developing the sexual theory,
says[103], that in this address he propounded the sexuality of plants
most expressly and as an acknowledged fact, and that he described very
graphically the way in which the anthers fertilise the pistil, into
which description little that was correct probably found its way, since
it required Koelreuter, Sprengel, and the botanists of quite modern
times to clear up this point. Vaillant therefore can only have the
credit of an eloquent description of what was then accepted. However,
De Candolle goes on to say what Vaillant’s discoveries were, and on
the following page we read that Linnaeus confirmed these discoveries
in the year 1736 in his ‘Fundamenta Botanica,’ and made skilful use of
them in the year 1735 in laying the foundations of his sexual system.
We have already in the second chapter of the first book explained the
confusion of ideas which lies at the bottom of these and many similar
statements, and in the same chapter have sufficiently indicated
our opinion respecting Linnaeus’ share in the establishment of the
doctrine of sexuality. It was the character of Linnaeus’ mind to
attach slight value to the experimental proof of a fact, even when,
like that of sexuality, it could only be proved by experiment; from
the point of view of his scholastic philosophy it was more important
with him to deduce the existence of this fact, in what seemed to
him the philosophic way, from the idea of the plant or from reason,
and in doing so to drag in a variety of analogies from the animal
kingdom; hence he acknowledged the services rendered by Camerarius,
but troubled himself little about his experiments which alone could
decide the question, while he undertakes himself to prove the existence
of sexes in plants on grounds of reason and the like in his peculiar
fashion. How he did this in the ‘Fundamenta’ and in the ‘Philosophia
Botanica’ has been already shown. Here we will briefly notice the
often-quoted dissertation, ‘Sponsalia Plantarum,’ in the first volume
of the ‘Amoenitates Academicae’ (1749). He first gives the views of
Millington, Grew, Camerarius and others; then on p. 63 he accepts the
statement of Gustav Wahlboom, that he, Linnaeus, had devoted infinite
labour to this question in 1735 in the ‘Fundamenta Botanica,’ and had
there (§§ 132-150) proved the sexes of plants with so great certainty
that no one would hesitate to found on it a detailed classification
of plants. Here then we have once more the construction of Linnaeus’
so-called sexual system introduced into the question of sexuality,
as if it had anything whatever to do with the establishing the
existence of sexes in plants, and as to the infinite labour (infinito
labore) which Linnaeus is supposed to have given to the question,
the paragraphs cited from the ‘Fundamenta’ contain the scholastic
subtleties quoted in Book I. chap. 2, but not one single really new
proof. The arguments in the dissertation we are considering are of
exactly the same kind, and it is itself only a lengthy paraphrase of
Linnaeus’ propositions in the ‘Fundamenta Botanica,’ illustrated by
experiments made by others, and with the addition of a few unimportant
observations, some of which are misinterpreted. We read, for instance,
p. 101, ‘Nectar is found in almost all flowers, and Pontedera thinks
that it is absorbed by the seeds that they may be the longer preserved;
it might seem that bees must be hurtful to flowers, since they
carry away the nectar and the pollen;’ but Linnaeus, differing from
Pontedera, remarks that ‘bees do more good than harm, because they
scatter the pollen on the pistil, though it is not yet ascertained what
is the importance of the nectar in the physiology of the flower.’ This
fact of the assistance rendered by insects, which was soon afterwards
better described by Miller, is not further examined in this place, for
Linnaeus goes on to speak of gourds, that they do not perfect their
fruit under glass, because the wind is prevented from effecting the
pollination.
One experiment only is mentioned, but not the person by whom it
was made. We read at p. 99 that in the year 1723 in the garden of
Stenbrohuld, the male flowers of a gourd in bloom were daily removed,
and that no fruit was formed. Soon after allusion is made to the
artifices used by gardeners to obtain hybrid varieties of tulips and
cabbage, but the matter is treated rather as agreeable trifling. In the
third volume of the Amoenitates of the year 1764, in which Koelreuter’s
first enquiries into hybridisation had been already published, we find
a dissertation on hybrids by Haartman, which was certainly written
as early as 1751. In this treatise the necessary existence of hybrid
forms is concluded from philosophic principles, as Linnaeus had deduced
sexuality from similar principles; no experiments are made, but
certain forms are arbitrarily assumed to be hybrids; a Veronica spuria
gathered in the garden at Upsala in 1750 is asserted to be the product
of Veronica maritima as the mother and of Veronica officinalis as the
father, but the only reason for assigning the paternity to the latter
plant is that it grew close by. We find also a Delphinium hybridum
stated on similar grounds to be the offspring of Delphinium elatum
fertilised by Aconitum napellus, and a Saponaria hybrida to have arisen
from the pollination of Saponaria officinalis by a Gentiana; and we are
told among other things that Actaea spicata alba is the offspring of
Actaea spicata nigra fertilised by Rhus toxicodendron. It is obvious
that in all this there was no observation of decisive facts, but simple
conclusions from arbitrary premises.
We conclude therefore that neither Linnaeus nor his disciples in the
interval that elapsed between the labours of Camerarius and Koelreuter
contributed a single new or valid proof to the establishment of the
fact, that there is a sexual difference in plants and that hybrids are
formed between different species; and if many later botanists talked
of the great services rendered by Linnaeus to the sexual theory, and
even regarded him as its most eminent founder, this arose partly from
the fact that they were unable to distinguish between his scholastic
deductions and scientific proof, and partly from that confusion of the
idea of sexuality with a classification of plants founded on the sexual
organs, to which we have before called attention. Such a confusion
of ideas gave rise to the claims which Renzi asserted on behalf of
Patrizi, but which Ernst Meyer, in his ‘Geschichte der Botanik,’ iv.
p. 420, has refuted on this very ground. Even in our own century De
Candolle has been blamed by Johann Jacob Roemer for not giving Linnaeus
the credit of being the actual founder of the sexual theory.
A few words in conclusion on those writers, who after Camerarius’
investigations still denied sexuality in plants, because they knew
nothing of what had been written on the subject or were incapable of
appreciating scientific proof. Tournefort must first be mentioned
on account of the great authority which he enjoyed with botanists
during the first half of the 18th century. In his ‘Institutiones rei
herbariae’ of the year 1700 (Book I. p. 69), with which we have already
made acquaintance, he treats of the physiological significance of the
parts of the flower, apparently in entire ignorance of Camerarius’
researches, and at any rate with a leaning to Malpighi’s views. He
makes the petals take up nourishment from the flower-stalks, which
they further digest and supply to the growing fruit, while the
unappropriated parts of the sap pass through the filaments into the
anthers and collect in the loculaments, to be afterwards discharged as
excreta. Tournefort even doubted the necessity of the pollination of
the female date-palm. The truth is that he was not well acquainted with
the facts, and was led astray by his preconceptions. The same was the
case with the Italian botanist Pontedera; in his ‘Anthologia’ of 1720
he reproduces Malpighi’s unlucky notion, and at the same time makes the
ovary absorb the nectar for the perfecting of the seed; he regarded
the male flower in dioecious plants as a useless appendage.
Valentin, to whom Camerarius addressed his famous letter ‘De sexu
plantarum’ in 1694, did his correspondent a disservice in publishing a
short abstract of it, which contained some gross misapprehensions of
the facts[104]. Alston in 1756 relying on these incorrect statements
disputed the conclusions of Camerarius, and doubted the sexual
importance of the stamens on very insufficient grounds. More reasonable
doubts were suggested by a German botanist, Möller, who observed that
female plants of spinach and hemp produced seeds even after the removal
of the male plants, and appealed to the apparently asexual propagation
of Cryptogams; these objections were answered by Kästner of Göttingen,
who pointed to the fact that dioecious plants, the willow for instance,
sometimes bear hermaphrodite flowers. The botanists in question would
never have entertained these doubts, if they had read and understood
the writings of Camerarius, or had been acquainted with the literature
of the subject.
4. THE THEORY OF EVOLUTION AND EPIGENESIS.
We have already observed the influence of the theory of evolution on
the doctrine of the fertilisation of plants in the case of Morland
and Geoffroy. We learn more about it in the work, already quoted,
of the philosopher Christian Wolff, ‘Vernünftige Gedanken von den
Wirkungen der Natur,’ Magdeburg, 1723; it will be well to give his
own words, for they will serve to show at the same time the amount of
knowledge possessed by a cultivated and well-read man in the country
of Camerarius and thirty years after the appearance of his treatise
on the sexuality of plants. In the second chapter of the fourth part,
which treats of the life, death, and generation of plants, Wolff
says: ‘Ordinarily plants are produced from seeds, for the seed not
only contains the plant in embryo but also its first food.’ He says
that propagation by means of buds is equally natural, for each bud
contains a branch in little. ‘We find inside in the flower a number of
stalks disposed in a circle, and something at the top of each, which
is full of dust and lets the dust fall on the upper part of that which
holds the seed; this organ is compared by some to the genitals of the
animal, and the dust to the male seed; they think also that the seed
is made fruitful by the dust, and that therefore the embryo must be
conveyed by the dust into the seed-case and there be formed into seeds.
I have proposed to examine into the matter, but I have always let it
escape me.’ ... ‘Since all that has been hitherto adduced is found
also in flowers which spring from bulbs, and it is also certain that
the leaves of bulbs have consequently embryos in them ... it is easy
to see that the embryos must come from the leaves of the bulbs. And
since they could as easily be conveyed from there into the seed-grains
with the sap, as into the dust which is produced in the upper part of
the flower, I am inclined to think that this is the true account of
the matter and that it will be confirmed by experience. But now comes
the main question, whence come the embryos into the sap; since they
have not an external figure only but an internal structure also, it is
not plain how they can be formed either by the mere inner movement of
the sap, or by separation of certain parts.... And this is certainly
more credible, that the embryos already exist in little in the sap and
the plant, before they are brought by some change into the condition
in which they are met with in the seed and in buds. But there is the
further question where they were previously. They must either lie one
in another in a minute form, as Malebranche especially maintains, or
they are brought from the air and the earth with the nourishing sap
into the plant, an idea which Honoratus Fabri advanced and Perrault and
Sturm developed after him. According to the first opinion the first
seed-grain must have contained everything in itself, which has grown
from it to this hour.’ But this demand goes beyond even Wolff’s powers
of belief; for, says he, it is too great a tax on the imagination to
conceive of this inclosing of germs one in another like box within box.
It is well known that such notions as these were very prevalent in
the 18th century, and that the spermatozoids of animals were thought
to lend considerable support to them; even Albert Haller after 1760
was an adherent of the theory of evolution. However confused Wolff’s
general train of thought may be, we should notice his perception of
the fact, that the theory of evolution does away with the sexual
significance of the anthers. We shall see by-and-bye, that Koelreuter
was able to form a very different idea of sexual propagation. His great
importance in the history of the sexual theory will be best learnt
from a consideration of the speculative views of his predecessors
and contemporaries. It will not be amiss therefore to disregard
chronology for a while, and to notice here the views of the Baron von
Gleichen-Russworm, and the feeble arguments of Kaspar Friedrich Wolff
against the theory of evolution. The first-named writer in his work
‘Das Neueste aus dem Reich der Pflanzen,’ 1764, relying principally on
microscopic observation of the contents of pollen-grains, supported the
view that the granules in them answer to spermatozoids in animals, and
that they find their way into the ovule and are there developed into
embryos. Yet Gleichen was at the same time a zealous supporter of the
sexual theory, and endeavoured to meet well-known objections to it by
pointing to the occurrence of female flowers on male plants of spinach;
he also made some experiments on maize and hemp in the interests of
the theory. He did not perceive that hybrids supply convincing proof
against the theory of evolution, but he rightly appealed to them as
affording strong arguments in favour of sexuality. His real knowledge
of hybrids is partly drawn from the statements of Linnaeus, with
which we have already made acquaintance; he even describes a hybrid
between a goat and a cow, and other similar ones, and he is angry
with Koelreuter for fixing such narrow limits to the occurrence of
hybrids; thus the first person who produced hybrids systematically in
the vegetable kingdom must submit to be scolded for refusing to accept
the imaginary hybrids of his contemporaries. Gleichen’s book and the
selection from his microscopic discoveries, which appeared in 1777,
abound in good detached observations; he was the first who saw and
figured the pollen-tubes of Asclepias, without of course suspecting
their real nature and importance.
Kaspar Friedrich Wolff is usually said to be the writer who refuted the
theory of evolution. It is certainly true that in his dissertation for
his doctor’s degree in 1759, the well-known ‘Theoria generationis,’
he appeared as the decided opponent of evolution; but the weight of
his arguments was not great, and the hybridisation in plants which was
discovered at about the same time by Koelreuter supplied much more
convincing proof against every form of evolution. Wolff conceived of
the act of fertilisation as simply another form of nutrition. Relying
on the observation, which is only partly true, that starved plants are
the first to bloom, he regarded the formation of flowers generally as
the expression of feeble nutrition (vegetatio languescens). On the
other hand the formation of fruit in the flower was due to the fact,
that the pistil found more perfect nourishment in the pollen. In this
Wolff was going back to an idea which had received some support from
Aristotle, and is the most barren that can be imagined, for it appears
to be utterly incapable of giving any explanation of the phenomena
connected with sexuality, and especially of accounting for the results
of hybridisation. Wolff may have rejected the theory of evolution on
such grounds as these, but he failed to perceive what it is which is
essential and peculiar in the sexual act.
5. FURTHER DEVELOPMENT OF THE SEXUAL THEORY BY JOSEPH GOTTLIEB
KOELREUTER, AND KONRAD SPRENGEL. 1761-1793.
Camerarius had shown by experiment that the co-operation of the pollen
is indispensable to the production in plants of seeds containing an
embryo, and later observers had confirmed the fact of sexuality by
further and varied experiments. The next step in the strict scientific
investigation of the matter was to determine by the same method of
experiment the share of each principle, the male and the female, in the
formation of the new plant which resulted from the sexual act. When
pollen and ovule belong to the same individual plant, the offspring
assumes the same form and the question remains undecided. It was
necessary to bring together the pollen and ovule of different plants;
this must show whether some characters are derived to the offspring
from the pollen, and others from the ovule, and what the characters are
which are thus distinguished, supposing of course that such a union of
different forms is possible. The answer to these questions could only
be obtained by experiment, that is by artificial hybridisation; for
until hybrid forms had actually been produced in this manner, it must
be quite unsafe to assume that certain wild plants owed their origin to
cross-fertilisation.
Camerarius had already raised the question in his letter, whether
cross-fertilisation in plants is possible, and had added another,
whether the progeny varies from its parents (an et quam mutatus
inde prodeat foetus). Bradley is our authority for the statement
that a gardener in London had obtained a hybrid between Dianthus
caryophyllus and Dianthus barbatus by artificial means as early as
1719; but KOELREUTER[105] was the first who investigated the question
scientifically and thoroughly. He was the first moreover who recognised
all its importance, and he applied himself to it with such admirable
and unexampled perseverance and judgment, that the results which he
obtained are still the best and most instructive, though a thousand
similar experiments have been made since his time. He also made the
first careful study of the different arrangements inside the flower in
their connection with the sexual relation, discovered the purpose of
the nectar and the co-operation of insects in pollination, and proposed
that view of the sexual act, which with some considerable modification
we must still in the main consider to be the true one, namely, that it
is a mingling together of two different substances.
If we compare Koelreuter’s writings, which are full of matter in a
small compass, with all that was produced after Camerarius, we are
astonished not only at the abundance of new thoughts, but still more
at their wonderful clearness and perspicuity, and the sureness of the
foundation laid for them in observation and experiment. In reading the
observations of Linnaeus, Gleichen, and Wolff on the sexual theory
we step into a world of thought which has long been strange and is
scarcely intelligible to us, and which in the present day possesses
only a historical interest. Koelreuter’s works on the contrary seem
to belong to our own time; they contain the best knowledge which we
possess on the question of sexuality, and have not become antiquated
after the lapse of more than a hundred years. We see by his example
that one really gifted thinker with the requisite perseverance will
effect more in a few years, than many less gifted observers in the
course of many years. But the same thing happened now, which happens
often in similar cases and which happened to Camerarius; a much longer
time elapsed before others learnt to understand the meaning and
importance of Koelreuter’s labours, than he had found necessary for
making his discoveries.
Koelreuter’s most important and best-known work appeared in four
portions in 1761, 1763, 1764 and 1766 under the title, ‘Vorläufige
Nachricht von einigen das Geschlecht der Pflanzen betreffenden
Versuchen und Beobachtungen’; we shall endeavour to give a brief
summary of the more important results.
At different places in this work occur remarks and experiments on
arrangements for pollination, which up to that time had been seldom and
only hastily observed. As the pollen-tube had not yet been discovered,
and Koelreuter himself set out with the view, that a fluid finds its
way from the pollen-grains as they lie on the stigma to the ovules, it
was important first of all to determine the quantity of pollen which is
required for the complete fertilisation of an ovary; with this object
in view Koelreuter counted the pollen-grains formed in a particular
flower and compared them with the number required to be applied to
the stigma in order to effect complete fertilisation, and he found
that the latter number was much the smaller. For instance, he counted
four thousand eight hundred and sixty-three pollen-grains in a flower
of Hibiscus venetianus, while from fifty to sixty were sufficient to
produce more than thirty fertile seeds in the ovary; in Mirabilis
jalapa and Mirabilis longiflora he counted about three hundred grains
of pollen in the anthers, while from two to three or even one sufficed
for fertilisation in the one-ovuled ovary. He also tried, whether in
flowers with divided and even deeply-cleft styles fertilisation could
be effected in all compartments of the ovary through one of them only,
and he found that it could.
Koelreuter directed special attention to the arrangements, by which in
the natural course of things the pollen finds its way from the anthers
to the stigmas. He ascribed perhaps too much to the agency of the wind
and the oscillations of the flower from any cause; at the same time he
was the first who recognised the great importance of the insect-world
to pollination in flowers. ‘In flowers,’ he says, ‘in which pollination
is not produced by immediate contact in the ordinary way, insects are
as a rule the agents employed to effect it,’ (later observation has
shown that they are generally so employed even in cases where actual
contact is possible), ‘and consequently to bring about fertilisation
also; and it is probable that they render this important service if
not to the majority of plants at least to a very large part of them,
for all the flowers of which we are speaking have something in them
which is agreeable to insects, and it is not easy to find one such
flower, which has not a number of these creatures busy about it.’ He
noticed the dichogamous construction in Epilobium, but did not further
pursue his observation. He next examined the substance in flowers which
is agreeable to insects; he collected the nectar of many flowers in
considerable quantities, and found that it gave after evaporation of
the water a kind of sweet-tasted honey; this honey was unpalatable only
in Fritillaria imperialis, which is avoided by the humble-bees. He had
no doubt therefore, that bees procure their honey from the nectar of
flowers. How greatly he was interested in the relations between the
existence of certain plants and that of certain animals, relations
which were neglected till Darwin once more brought them into notice in
quite recent times, is shown by his investigation into the propagation
of the mistletoe (1763); he calls special attention to the fact, that
not only must the pollination of this plant be effected by insects, but
that the dissemination of its seeds is also exclusively the work of
birds, and that the existence of the plant therefore is dependent on
two different classes of living creatures.
Again we find observations on the movements of anthers and stigmas,
especially those caused by sensitiveness. Count Giambattista dal Covolo
had made the first observations in 1764 on the sensitiveness of the
anthers of thistle-like plants, and had endeavoured to explain their
mechanism. Koelreuter did not trouble himself about this point, so much
as about the connection between the irritability of the anthers and the
pollination of the stigmas. He took into consideration the sensitive
stamens of Opuntia, Berberis and Cistus, which Du Hamel had already
noticed, and discovered for himself the irritability of the lobes of
the stigma in Martynia proboscidea and Bignonia radicans. He noticed
that the lobes when touched close, but soon open again; but if pollen
is placed upon them, they remain closed till fertilisation is secured.
How perfectly insects effect the pollination of flowers he showed by a
comparative trial, in which he applied pollen himself to three hundred
and ten flowers with a brush, while he left the same number to the
operation of insects; the number of seeds formed in the latter case was
very little less than in the former, though the insects had to contend
with unfavourable weather.
He endeavoured also to ascertain the time required for the quantity
of ‘seminal matter’ sufficient for fertilisation to reach the ovary
after pollination; he also showed that pollination is followed by
fertilisation without the aid of light; later botanists incorrectly
maintained the contrary.
Koelreuter was less successful in his observations on the structure of
pollen-grains; here the microscope was indispensable and microscopes
were still very imperfect. Nevertheless he discovered that the
outer covering of the pollen-grain consists of two distinct coats,
and noticed the spines and sculpturings on the outer coat and its
elasticity; he observed the lids of the orifices in the exine of
Passiflora coerulea, and went so far as to see the inner coat in
moistened pollen-grains protrude in the form of conical projections,
which then however burst and allowed the contents to escape. But he
explained the pollen-tube, which he had thus seen, incorrectly by
supposing that these projections were intended to prevent the bursting
of moistened grains. It was not till sixty or seventy years later that
the matter was fully understood. Koelreuter supposed the contents of
the pollen-grain to be a ‘cellular tissue,’ and the true fertilising
substance to be the oil which adheres to the outside of the grains,
but is formed inside them and finds its way out through fine passages
in the coat. The bursting of the pollen-grains, which his opponent
Gleichen thought must take place to allow of the escape of his supposed
spermatozoids, seemed to him an unnatural proceeding.
Starting from the hypothesis, that the oil which clings to the
pollen-grains is the fertilising substance, Koelreuter propounds his
view of the process of fertilisation in accordance with the chemical
notions of the day; he first rejects the idea that the pollen-grains
themselves can reach the ovary, and then says: ‘Both the male seed and
the female moisture on the stigmas are of an oily nature, and therefore
when they come together enter into a most intimate union with one
another, and form a substance which, if fertilisation is to ensue,
must be absorbed by the stigma and conveyed through the style to the
so-called ovules or unfertilised germs.’ Koelreuter therefore made the
fertilisation really take place on the stigma, the mingled male and
female substance making its way into the ovary and there producing the
embryos in the seed. He had expressed this view before in 1761; he
repeated it in 1763 with the idea that the male and female moistures
unite together, as an acid and an alkali unite to form a neutral
salt; a new living organism is the result at once or later of this
union. In an investigation which he made in 1775 into the conditions
of pollination in Asclepiadeae he reverted to this idea, and insisted
that the act of fertilisation in the whole vegetable and animal kingdom
is a mingling of two fluids. But at a later period he seems to have
no longer considered the moisture of the stigma to be the female
principle, for experiment had taught him, that if a stigma exchanges
the moisture from another stigma for its own, and is then dusted with
its own pollen, no hybrid form is produced[106]. In any case Koelreuter
had a more correct idea of the nature of sexual fertilisation than
any of his predecessors, and it was one specially adapted to enable
his contemporaries to understand the results of experiments in
hybridisation, while the hybrids themselves supplied most convincing
arguments against the prevailing theory of evolution.
We have arrived at Koelreuter’s most important performance, the
production of hybrids. Here was a case for skilful experimentation,
not for microscopic observation, and here he obtained results in which
nothing afterwards required to be changed, but which when combined
with later observations have been used for the discovery of general
laws in hybridisation. The first hybrid which he obtained by placing
the pollen of Nicotiana paniculata on the stigmas of N. rustica,
produced pollen that was impotent; but he soon after obtained hybrids
from the two species which produced seeds capable of germination, and
in 1763 he described a considerable number of hybrids in the genera
Nicotiana, Kedmia, Dianthus, Matthiola, Hyoscyamus, and others. In the
last portion of his great work (1766) he speaks of eighteen attempts
to obtain hybrids with five native species of Verbascum, and submits
Linnaeus’ views on hybrid plants, which we have already described,
to a withering criticism. He shows at the same time from experiment,
that if the stigma of a plant receives its own pollen and pollen from
another plant at the same time, the former only is effectual, and that
this is one reason why hybrids which can be raised artificially are
not found in nature. We must not attempt to give a detailed account of
his famous hybrids of the third, fourth, and fifth degrees, nor of his
experiments on other points, such as the reverting of hybrids to the
original form by the repeated employment of its pollen; the value of
these experiments for theoretical purposes was afterwards fully brought
out by Nägeli.
It is impossible to rate too highly the general speculative value of
Koelreuter’s artificial hybridisation. The mingling of the characters
of the two parents was the best refutation of the theory of evolution,
and supplied at the same time profound views of the true nature of the
sexual union. It was shown by his numerous experiments that only nearly
allied plants and not always these are capable of sexual union, which
at once disposed of Linnaeus’ vague ideas in the judgment of every
capable person, though it was long before science candidly accepted
Koelreuter’s results. The plant-collectors of the Linnaean school as
well as the true systematists at the end of the 18th century had little
understanding for such labours as Koelreuter’s, and incorrect ideas on
hybrids and their power of maintaining themselves prevailed in spite
of them in botanical literature. Hybrids were necessarily inconvenient
to the believers in the constancy of species; they, disturbed the
compactness of their system and would not fit in with the notion that
every species represented an ‘idea.’
Koelreuter’s doctrines however did not always fall on unfruitful soil;
two botanists at least were found in Germany who adopted them, Joseph
Gärtner the author of the famous Carpology and father of Carl Friedrich
Gärtner who at a later time spent twenty-five years in experimenting
on fertilisation and hybridisation, and Konrad Sprengel who took up
Koelreuter’s discovery of the services rendered by insects and arrived
at some new and very remarkable results.
JOSEPH GÄRTNER made no fresh observations on sexuality himself, but
in the Introduction to his ‘De fructibus et seminibus plantarum’
(1788) he made use of Koelreuter’s results for the purpose of
distinguishing more clearly between different kinds of propagation,
and strengthening his own attack on the theory of evolution. The
germ-grains or spores of cryptogamic plants were at that time often
regarded on insufficient grounds as true seeds; Gärtner distinguished
them from seeds, because they are formed without fertilisation and
yet are capable of germination, whereas ovules become seeds capable
of germination only under the influence of the pollen. He distinctly
denied the sexuality of the Cryptogams; it was not till fifty years
later that strict scientific proof was substituted in this department
of botany for vague conjecture, and it was more in the interest of
true science in Gärtner’s day to deny sexuality in the Cryptogams
altogether, than to take the stomata in Ferns with Gleichen, or the
indusium with Koelreuter, or the volva in Mushrooms for the male organs
of fertilisation. Gärtner rightly appealed to Koelreuter’s hybrids
against the defenders of the theory of evolution; and to those who saw
in the seed only another form of vegetative bud, he said, that the bud
can produce a new plant without fertilisation but that the seed cannot.
We have already given an account in the chapters on Systematic Botany
of the services rendered by Gärtner to the knowledge of the seed in
its immature and in its mature condition; as regards the process of
fertilisation he adopted in the main Koelreuter’s view, that it is
the result of the union of a male and female fluid, from which the
germ-corpuscle in the ovule is developed by a kind of crystallisation.
Konrad Sprengel also fully committed himself to this view, and was
thereby prevented from understanding the process of fertilisation in
Asclepiadeae.
In KONRAD SPRENGEL[107] we encounter once more an observer of genius,
like Camerarius and Koelreuter, who however surpassed them both in
boldness of conception and was therefore even less understood by his
contemporaries and successors, than they had been by theirs. The
conclusions, to which his investigations led him, were so surprising,
they suited so little with the dry systematism of the Linnaean school
and with later views on the nature of plants, that they had become
quite forgotten when Darwin brought them again before the world and
showed their important bearing on the theory of descent. As Camerarius
first proved that plants possess sexuality, and Koelreuter showed that
plants of different species can unite sexually and produce fruitful
hybrids, so now Sprengel showed that a certain form of hybridisation
is common in the vegetable kingdom, namely the crossing of different
flowers or different individuals of the same species. In his work,
‘Das neu entdeckte Geheimniss der Natur in Bau und Befruchtung der
Blumen,’ Berlin, 1793, he says at page 43: ‘Since very many flowers
are dioecious, and probably at least as many hermaphrodite flowers
are dichogamous, nature appears not to have intended that any flower
should be fertilised by its own pollen.’ This was however only one of
his surprising conclusions; still more important perhaps was the view,
that the construction and all the peculiar characters of a flower can
only be understood from their relation to the insects that visit them
and effect their pollination; here was the first attempt to explain the
origin of organic forms from definite relations to their environment.
Since Darwin breathed new life into these ideas by the theory of
selection, Sprengel has been recognised as one of its chief supports.
It is highly interesting to read, how this speculative mind by the
study of structural relations in flowers, which were apparently trivial
and open to the eyes of all men, first arrived at ideas which in the
course of a few years were to lead to such far-reaching results. He
says: ‘In the summer of 1787 I was attentively examining the flowers
of Geranium sylvaticum, and observed that the lower part of the petals
was provided with slender rough hairs on the inside and on both edges.
Convinced that the wise framer of nature has not produced a single hair
without a definite purpose, I considered what end these hairs might be
intended to serve. And it soon occurred to me, that on the supposition
that the five drops of juice which are secreted by the same number of
glands are intended for the food of certain insects, it is not unlikely
that there is some provision for protecting this juice from being
spoiled by rain, and that the hairs might have been placed where they
are for this purpose. Since the flower is upright, and tolerably large,
drops of rain must fall into it when it rains. But no drop of rain can
reach one of the drops of juice and mix with it, because it is stopped
by the hairs, which are over the juice-drops, just as a drop of sweat
falling down a man’s brow is stopped by the eye-brow and eye-lash, and
hindered from running into the eye. An insect is not hindered by these
hairs from getting at the drops of juice. I examined other flowers
and found that several of them had something in their structure,
which seemed exactly to serve this end. The longer I continued this
investigation, the more I saw that flowers which contain this kind of
juice are so contrived, that insects can easily reach it, but that
the rain cannot spoil it; but I gathered from this that it is for the
sake of the insects that these flowers secrete the juice, and that it
is secured against rain that they may be able to enjoy it pure and
unspoilt.’ Next year, following out an idea suggested by the flowers of
Myosotis palustris, he found that the position of spots of different
colours on the corolla have some connection with the place where the
juice is secreted, and with the same ready reasoning as before he came
to the further conclusion: ‘If the corolla has a particular colour in
particular spots on account of the insects, it is for the sake of the
insects that it is so coloured; and if the particular colour of a part
of the corolla serves to show an insect which has lighted on the flower
the direct path to the juice, the general colour of the corolla has
been given to it, in order that insects flying about in search of their
food may see the flowers that are provided with such a corolla from a
long distance, and know them for receptacles of juice.’
He afterwards discovered that the stigmas of a species of Iris were
absolutely unable to be fertilised in any other way than by insects,
and further observation convinced him more and more, ‘that many,
perhaps all flowers, which have this juice, are fertilised by the
insects which feed on it, and that consequently this feeding of insects
is in respect of themselves an end, but in respect of the flowers only
a means, but at the same time the sole means to a definite end, namely,
their fertilisation; and that the whole structure of such flowers can
be explained, if in examining them we keep in sight the following
points, first, that flowers were intended to be fertilised by the
agency of one or another kind of insects, or by several; secondly, that
insects in seeking the juice of flowers, and for this purpose either
alighting upon them in an indefinite manner, or in a definite manner
either creeping into them or moving round upon them, were intended to
sweep off the dust from the anthers with their usually hairy bodies or
with some part of them, and convey it to the stigma, which is provided
either with short and delicate hairs, or with a viscid moisture, that
it may retain the pollen.’
In the summer of 1790 he detected dichogamy, which he first observed
in Epilobium angustifolium. He found, ‘that these hermaphrodite
flowers are fertilised by the humble-bee and by other bees, and that
the individual flower is not fertilised by its own pollen, but the
older flowers by the pollen which the insects convey to them from
the younger.’ Having observed the same thing in Nigella arvensis,
he afterwards found exactly the opposite arrangement in a species of
Euphorbia, in which the stigmas can receive the pollen by the aid of
insects only from older flowers.
He goes on to say that he grounds his theory of flowers on these
his six chief discoveries made in the course of five years; he then
gives his theory at length, first of all explaining the nature of
juice-secreting glands (nectaries), and organs for receiving or
covering the nectar, and the contrivances for enabling insects to find
their way readily to the juice. He calls attention to Koelreuter’s
excellent observations on the fertilisation of nectar-bearing flowers
by insects, and notices that no one has hitherto shown that the whole
structure of such flowers has this object in view, and can be fully
explained by it. He finds the chief proof of this important proposition
in dichogamy.
‘After the flower,’ he says, ‘has opened in dichogamous plants, the
filaments have or assume either all at once or one after another a
definite position, in which the anthers open and offer their pollen for
fertilisation. But at this time the stigma is at some distance from
the anthers and is still small and closed. Hence the pollen cannot be
conveyed to the stigma either by mechanical means or by insects, for
there is as yet properly no stigma. This condition of things continues
a certain time. When that time is elapsed, the anthers have no longer
any pollen, and changes take place in the filaments the result of which
is that the anthers no longer occupy their former position. Meanwhile
the pistil has so changed that the stigma is now exactly in the place
where the anthers were before, and as it now opens, or expands the
parts of which it is composed, it often fills about the same space as
the anthers filled before. Now the spot, which was at first occupied by
the ripe anthers and is now occupied by the ripe stigma, is so chosen
in each flower, that the insect for which the flower is intended cannot
reach the juice without touching with a portion of its body the anthers
in a young flower, and the stigma in an older; it thus brushes the
pollen from the anthers and conveys it to the stigma, and so the pollen
of the younger flower fertilises the older.’ It has been already said,
that Sprengel was also acquainted with the opposite form of dichogamy;
and the result of his explanation of both kinds is the conclusion,
that some flowers can only be fertilised by the aid of insects, and
he adds that some cases are to be found, in which the arrangements
in the flower are of such a nature as to involve the injury and even
the death of the insect that gives its services. Further on he tells
us, that all flowers, ‘which are without a proper corolla and have no
calyx of any importance in its place, are destitute of nectar, and
are not fertilised by insects but by some mechanical means, as by the
wind, which either blows the pollen from the anthers on to the stigmas,
or shakes the plant or the flower and makes the pollen fall from the
anthers on to the stigmas.’ He observes, that such flowers always
produce a light pollen and in large quantities, whereas the pollen
of nectar-bearing plants is heavy. Then he shows how his principles
explain all the physiological characters of flowers, position, size,
colour, smell, form, time of flowering and the like.
Sprengel set out with the idea, that the nectar and certain
arrangements in flowers are expressly intended for the service of
insects; but his investigations led him ultimately to the conclusion,
that insects themselves serve not only to effect the fertilisation of
plants generally, but also in all ordinary cases to bring about the
crossing of different flowers of the same plant or of different plants
of the same species. There remained a question, which from Sprengel’s
strictly teleological point of view especially required an answer,
what was the object of this crossing of flowers or individual plants?
Sprengel was content, as we have seen, with simply stating the fact,
and with saying, that nature apparently did not choose that any flower
should be fertilised by its own pollen. Who would make it a reproach
to the discoverer of such remarkable and widely-prevalent phenomena
in nature, that he did not answer this question and give the final
touches to the body of doctrine which he created, and which could
only be developed by many experiments and the labour of long years?
Neither his worldly circumstances nor the reception accorded to his
work with all its genius were such as to encourage him to undertake
the solution of this last and most difficult problem, even if he had
been inclined to do so. Botanists were just at that time and for some
time after preoccupied with views, which allowed such biological and
physiological facts in vegetable life to lie neglected, nor were
Sprengel’s results at all favourable to the doctrine of the constancy
of species; from that point of view the wonderful relations between the
organisation of flowers and that of insects must have seemed absurd and
repulsive. In such cases it is the character of less-gifted natures,
rather to deny the facts or to disregard them, than to sacrifice their
own favourite views to them; this is one explanation of the neglect
which Sprengel’s book met with everywhere. Then notwithstanding the
labours of a Camerarius and a Koelreuter there were many even at the
beginning of our own century who still doubted the sexuality of plants.
Even after Knight and William Herbert, with a right understanding of
the question left open by Sprengel, had obtained experimental results
which helped to answer it, the new doctrine did not make its way. The
earlier simple-minded but consistent teleology had been succeeded
by a rejection of all teleological explanations in the treatment of
physiological questions, and this spirit conduced to make Sprengel’s
results seem inconvenient in proportion as they appeared to admit only
of such explanation. With regard to phenomena of this kind botanists
before 1860 were in a position, in which they were without the means
of forming a judgment; they shrank from the teleological point of
view and from believing with Konrad Sprengel, that every, even the
least-obvious, arrangement in an organism was the direct work of a
Creator; but they had nothing better to put in the place of this idea,
and hence Sprengel’s discoveries not being understood were neglected
till Darwin recognised all their importance, and by opposing the theory
of descent and selection to the principle of design was in a position
not only to show that they had a scientific meaning, but also to employ
them as powerful supports of the theory of selection. Then, too, it
became possible rightly to appreciate the contributions of Knight,
Herbert, and K. F. Gärtner to the further completion of Sprengel’s
doctrine, for their discoveries also were for a while neglected. A
few years after the appearance of Sprengel’s book, Andrew Knight[108]
relying on the results of experiments made for the purpose of comparing
self-fertilisation and crossing in the genus Pisum, laid down the
principle, that no plant fertilises itself through an unlimited number
of generations; in 1837 Herbert summed up the results of his numerous
experiments in fertilisation in the statement, that he was inclined
to believe that he attained a better result, when he fertilised the
flowers from which he wished to obtain seeds with pollen from another
individual of the same variety or at least from another flower, than
when he fertilised it with its own pollen; K. F. Gärtner came to the
same conclusion after experiments in fertilising Passiflora, Lobelia,
and species of Fuchsia in 1844. In these observations lay the first
germ of the answer to the question left undecided by Sprengel, why
most flowers are so constructed that fertilisation can only be fully
effected by the crossing of different flowers or of different plants of
the same species; the artificial crossings of this kind, which Knight,
Herbert, and Gärtner compared with the self-fertilisation of single
flowers, showed that crossing procures a more complete and vigorous
impregnation than self-fertilisation. It was but a short step from this
fact to the idea, that the arrangements in the flower discovered by
Sprengel together with the aid of insects serve to secure the strongest
and most numerous progeny possible. Darwin was the first who fixed his
eye distinctly on this idea also, in order to employ it in his theory
of selection, and sought support for it in a number of experiments made
after 1857.
6. NEW OPPONENTS OF SEXUALITY AND THEIR REFUTATION BY EXPERIMENTS.
1785-1849.
Those who have read the writings of Camerarius and Koelreuter
carefully find it difficult to believe, that after their time doubts
were still entertained not about the manner in which the processes
of fertilisation are accomplished but about the actual existence
of difference of sex in plants. And yet such doubts were expressed
repeatedly during the succeeding sixty years in various quarters and
with the greatest confidence, and this not in consequence of increased
accuracy in experimental research or of contradictions that could
be proved in the views of the founders of the sexual theory, but
because a number of observers made unskilful experiments and obtained
contradictory results, or made inaccurate observations on the plants on
which they experimented, and generally had not the requisite experience
and circumspection. Such were Spallanzani and later Bernhardi, Giron
de Bouzareingue and Ramisch. Schelver, his pupil Henschel, and their
adherents erred still more grossly and from a different cause; they
thought themselves justified by preconceived opinions and conclusions
from the nature-philosophy in denying facts established by experiment.
The destructive effects of the nature-philosophy on the powers of
the understanding at the beginning of the 19th century was shown
in the case of many botanists, who were no longer able to estimate
the result of simple experiments, and to trace back the phenomena
of nature to the scheme of causes and effects. As Linnaeus once
imagined that he could prove sexuality in plants on philosophical
grounds and paid comparatively slight attention to their behaviour
as shown by experiment, so we have in Schelver a nature-philosopher
who conversely endeavoured to prove the impossibility of sexuality in
plants on philosophical grounds. As Linnaeus deduced sexuality from
the nature or idea of the plant, Schelver denied it from the same
nature or idea; as a matter of logic one was as much in the right as
the other, but the question could not be decided in this way but only
by experiment. However our nature-philosophers thought it advisable
to get some empirical support for their theories, and they found it
in SPALLANZANI[109]. He published his enquiries into fertilisation
in animals and plants under the title ‘Expériences pour servir à
l’histoire de la génération des animaux et des plantes,’ Geneva,
1786; his account of those relating to plants, with which only we
are concerned, betrays a very defective acquaintance with botanical
literature, for he reckons Cesalpino among those who had admitted
sexuality in plants. His experiments themselves testify to very slight
knowledge of the biological considerations by which the cultivation of
plants for experiment must be guided, and generally little botanical
acumen, as is often the case with amateurs who without sufficient
preparation suddenly turn their attention to questions of vegetable
physiology; his treatment of his topics is superficial, his criticism
of others is dogmatic and bitter without exciting confidence in the
author’s own skill and judgment. His experiments were often undertaken
in haste and with little consideration, and some of them were made on
plants the least suitable for such investigations, as for instance
on Genista, beans, peas, radishes, Basilicum, Delphinium. It is no
matter of surprise therefore that in the case of some plants, as
Mercurialis and Basilicum, he arrived at the conclusion that the pollen
is necessary to the production of fertile seeds, while he makes others,
as the gourd, the water-melon, hemp, and spinach produce such seeds
without fertilisation. His countryman Volta, a greater man, repeated
his experiments and impugned the results which he had obtained from
them.
Such was the character of the experiments to which Franz Joseph
Schelver, Professor of Medicine in Heidelberg appealed in his ‘Kritik
der Lehre von dem Geschlecht der Pflanzen,’ 1812. It is unnecessary
to give a detailed account of this strange production of a mind
misled, even though a considerable number of German botanists as late
as 1820 took its nonsense for profound wisdom. Schelver dismissed
the experiments of Camerarius in four lines, and while he treated
Koelreuter with contempt, he praised Spallanzani as the weightiest
author on the subject. The statements of Camerarius and Koelreuter
are true, he said, but they do not prove the fertilisation. He is
more concerned to decide the question from the nature of vegetative
life, and from this nature constructed by himself he concludes that
the organs of plants are of no use at all, that they cannot even tend
to be of use to one another and to propagate life together, because
this one end of their action can be a living one only where all the
parts are present at the same time, which of course disposes of the
fertilising effect of the pollen; accordingly he does not refer the
effect of a male plant on a neighbouring female plant, which results
in the formation of seeds, to pollination by the former, but it is the
proximity itself which has the fertilising effect. But these are very
insufficient specimens of his reasoning.
The writings of his pupil Henschel[110] are even worse than those
of his master, and the worst of these is his large work ‘Von der
Sexualität der Pflanzen’ of 1820. He thought himself obliged to prove
the doctrines of the nature-philosophy by countless experiments; but
the way in which these are devised, managed and described displays
the extreme of dulness and incapacity to form a sound judgment. The
doubt which must occasionally rise in the mind of the reader as to the
accuracy of his reports, and the remarks which have been made on this
point by Treviranus and Gärtner, are not needed to disgust him with the
scientific efforts of this writer.
It would be superfluous to give an account of the contents of
Henschel’s book, which is interesting from the pathological rather
than from the historical point of view. To what an extent better men
than Henschel even later than 1820 lost under the influence of the
nature-philosophy their capacity for judging such questions as we are
discussing, how even investigators of merit thought it worth while
to treat the productions of Schelver and Henschel with a certain
respect, is shown among other works, by a collection of letters, which
were published by Nees von Esenbeck as a second supplement to the
‘Regensberg Flora’ of 1821, and by the later remarks of Goethe on the
metamorphosis of plants, to be found in Cotta’s edition of his works
in forty volumes (vol. xxxvi. p. 134) under the title ‘Verstäubung,
Verdunstung, Vertropfung.’ But there were some who set themselves
distinctly against these pernicious ideas, such as Paula Schrank
(‘Flora,’ 1822, p. 49) and C. L. Treviranus, who published in 1822
a full refutation of Henschel in his ‘Lehre von dem Geschlecht der
Pflanzen in Bezug auf die neuesten Angriffe erwogen.’ A few stray
supporters of the dying nature-philosophy were still to be found at a
later time; among them Wilbrand, Professor in Giessen, who (‘Flora,’
1830, p. 585) adopted the very subtle distinction that there is in
plants something analogous to sexuality in animals, but no real
sexuality. We see in the whole literature of the nature-philosophy
an incapability of judging of experiments simply with the sound human
understanding; an imaginary something was constantly introduced into
the results of experiments which had not the remotest connection with
their conditions and results.
The doubts expressed by Bernhardi in 1811, by Girou in 1828-30, and by
Ramisch in 1837 were of a different kind: these men made experiments
and judged of them in a scientific manner; but they were insufficiently
acquainted with what had been done before them, and their experiments
were not devised with the requisite knowledge of the conditions of the
problem, or carried out with sufficient precautions. Camerarius and
Ray had noticed in the previous century the occasional occurrence of
male flowers on female plants of spinach, hemp and mercury; and yet
the observers above mentioned chose these plants for their experiments
without being on their guard against the possible appearance of these
exceptional circumstances, or of other means of pollination.
We see then that doubts were entertained till as late as after 1830
with regard either to sexuality in plants altogether, or to its general
prevalence in Phanerogams; the Cryptogams were not mentioned, for they
were assumed to be devoid of sex in spite of many valuable observations
of earlier times. The great majority of botanists however admitted the
sexual significance of the organs of the flower; most of them rested in
entire faith on Linnaeus’ authority, while some were able to appreciate
the experimental proofs of Camerarius, Bradley, Logan, Gleditsch and
Koelreuter. But all who took up the subject in earnest between 1820
and 1840 were naturally led to desire that the question should once
more be thoroughly examined. The Berlin Academy of science had offered
in 1819 at Link’s suggestion a prize for an essay on the question,
whether there is such a thing as hybrid fertilisation in the vegetable
kingdom, in the hope of stimulating botanists to new investigations
into the decisive points in the sexual theory. The only reply to
this offer, an essay by Wiegmann which was not sent in till 1828, did
not come up to the requirements of the Academy, and was rewarded with
only half the prize. The Dutch Academy at Haarlem was more successful
when induced by Reinwardt in 1830 to propose the question in a
somewhat altered form and in connection with practical horticulture.
This prize was contended for by KARL FRIEDRICH GÄRTNER[111], whose
essay delayed by circumstances till 1837 received the prize of honour
and an extraordinary reward. But the whole body of his results,
derived from the experimental researches of five-and-twenty years,
were not published till 1849 and then in a large volume, ‘Versuche
und Beobachtungen über die Bastardzeugung,’ Stuttgart, 1849, having
been preceded by an introductory work of equal extent, ‘Versuche und
Beobachtungen über die Befruchtungsorgane der vollkommeneren Gewächse
und über die natürliche und künstliche Befruchtung durch den eigenen
Pollen.’ The two works together are the most thorough and complete
account of experimental investigation into sexual relations in plants
which had yet been written. They are a brilliant termination of the
period of doubt with respect to sexuality in plants which succeeded
to the age of Koelreuter—a termination which coincides in time with
the lively discussion which was being maintained on the strength of
microscopical investigations by Schleiden and Schacht on the one
side and by Hofmeister on the other respecting the processes in the
formation of the embryo.
Gärtner’s writings derive their importance not so much from new and
surprising discoveries or brilliant ideas and unexpected combinations,
as from their very searching examination into all the circumstances and
relations which can come under consideration in the sexual propagation
of Phanerogams. His experiments in hybridisation, of which he kept
most exact accounts, exceeded the number of nine thousand; in these
and in normal cases of pollination he studied all the sources of error
which could in any way affect his experiments, and took into careful
consideration all the conditions of fertilisation connected with the
development of the plant itself and with its external circumstances; at
the same time he examined critically all that had been written on the
subject, and submitted every experiment reported by former observers to
the test of his own wide experience. The volume on self-fertilisation
is a complete account of the biology and physiology of flowers. The
phenomena connected with the unfolding and fertilisation of the flower
are described from the writer’s own observations, some of which are
quite new; it specially investigates the relations between the calyx,
the corolla, the secretion of nectar and the opening of the anthers,
also the temperature of flowers and the physiological processes in
the ovary, the style and the stigma; all that was then known of
irritability and the phenomena of movement in the flower and in the
organs of fructification was collected together and elucidated by fresh
observations, and thus a picture was drawn complete to the smallest
detail of the life of a flower, such as we do not yet possess of any
other organ. It would be idle to think of giving in a small compass
a clear idea of the wealth of these observations. But all this was
only preliminary to the main point, the proof that Camerarius was
right, that notwithstanding the objections of a hundred years the
co-operation of the pollen is indispensable to the formation of the
embryo in the growing seed, and that plants therefore have sexuality
exactly as animals have it. Gärtner did not content himself with simply
making new experiments in fertilisation; he refuted the objections of
Spallanzani, Schelver, Henschel, Girou and others in detail from fresh
experiments and from other sources of information, paying particular
regard to all the circumstances which could come under consideration
in each case; he exposed the inaccuracy of the observations of the
opponents of sexuality point by point, and finally called attention to
a number of remarkable phenomena observable in the ovary even before
fertilisation, and to the circumstances under which the pollen may find
its way to it in cases where ordinary pollination has been apparently
prevented. These observations once more confirmed the existence of
sexuality in plants, and in such a manner that it could never be
again disputed. When facts were observed in 1860, which led to the
presumption that under certain circumstances in certain individuals of
some species of plants the female organs might produce embryos capable
of development without the help of the male, there was no thought of
using these cases of parthenogenesis to disprove the existence of
sexuality as the general rule; men were concerned only to verify first
of all the occurrence of the phenomena, and then to see how they were
to be reasonably understood side by side with the existing sexuality,
as had to be done also in the corresponding cases in the animal kingdom.
Gärtner’s work on hybridisation had been preceded by other enquiries
into the same subject, those namely of Knight mentioned above at the
beginning of the century, and Herbert’s more ample investigations
published in his work on Amaryllideae in 1837. Gärtner did not neglect
to compare his observations at all points with the results of his
predecessors, especially those of Koelreuter, and he deduced from
the astonishing mass of material a number of general propositions
respecting the conditions under which the production of hybrids is
possible, the results of crossing, and the causes of failure. A
special interest attaches to his mixed and compound hybrids, to his
experiments on the various degrees of influence which foreign pollen
exercises on the behaviour of the female organ, and the connection of
this point with the formation of varieties. It is impossible to give
a more distinct account of Gärtner’s results without entering into
discussions which would exceed the limits of a historical survey. It is
the less necessary to do so, since Nägeli undertook in 1865 to give a
summary view of all the important results to be found in the wealth of
material supplied by Koelreuter, Herbert and Gärtner[112]. Gärtner’s
experiments in hybridisation were conducted at Calw in Würtemberg,
the place where Koelreuter had made his in 1762 and 1763. And thus
it was in two small cities of Würtemberg that the foundations of the
sexual theory were laid and the theory itself perfected, as far as
it could be by experiment only, by three of the most eminent among
observers. Camerarius in Tübingen, Koelreuter and K. F. Gärtner in Calw
contributed so largely to the empirical establishment of the theory,
that all that was done by others would seem of small importance, if
artificial pollination only were in question. But Koelreuter was
imperfectly acquainted with the methods by which pollination is
usually effected in nature; Sprengel was the first who saw into all
their more important relations, and the fact must not be concealed,
that Gärtner in regarding Konrad Sprengel’s observations as unworthy
of serious consideration, neglected the most fruitful source of new
and magnificent results. His careful study of the secreting of nectar
and of the sensitiveness of the organs of fertilisation, and his many
observations on other biological relations in flowers, would have
found their natural termination, if he had connected them at all
points with Sprengel’s general conclusions respecting the relation
of the structure of the flower to the insect world. This Gärtner
entirely failed to do, and hence in this case also it was reserved
for Darwin’s wonderful talent for combination to sum up the product
of the investigations of a hundred years, and to blend Koelreuter’s,
Knight’s, Herbert’s, and Gärtner’s results with Sprengel’s theory
of flowers into a living whole in such a manner, that now all the
physiological arrangements in the flower have become intelligible both
in their relations to fertilisation, and in their dependence on the
natural conditions under which pollination takes place without the aid
of man. Here, as in morphology and systematic botany, Darwin found
the premisses given and drew the conclusion from them; here too the
certainty of his theory rests on the results of the best observers, on
investigations which find in that theory their necessary logical and
historical consummation.
7. MICROSCOPIC INVESTIGATION INTO THE PROCESSES OF FERTILISATION IN
THE PHANEROGAMS; POLLEN-TUBE AND EGG-CELLS[113]. 1830-1850.