experiments had been set in a window, and inattention to this point
caused some of his attempts to repeat the experiment to fail. He was
further perplexed by using vessels which had become coated with a film
of "green matter," probably Euglæna. Such vessels restored vitiated
air, though no leaves were present, and when placed in the sun, gave
off considerable quantities of a gas, Priestley's "dephlogisticated
air" (oxygen). Hardly any oxygen was given off when the green matter
was screened by brown paper. Water impregnated with carbonic acid
was found to favour the production of the green matter. To us,
who have been taught at school something about the properties of
green plant-tissues, it seems obvious that Priestley ought to have
ascertained by microscopic examination whether his "green matter"
was not a living plant. But he had always avoided the use of the
microscope, his eyes being weak, and after some imperfect attempts
in this way he made up his mind that the green matter was neither
animal nor vegetable, but a thing _sui generis_. Neglecting his
most instructive experiments, and not waiting till he could devise new
ones, or even disentangle his thoughts, he sent to the press a confused
explanation, which seemed to teach that vitiated air may be restored by
sunlight alone.
A Dutch physician, named John Ingenhousz, who was then living in
England, read Priestley's narrative and began to investigate on his own
account. Without detailing his numerous experiments, we may give his
own clear summary (condensed). "I observed," Ingenhousz says, "that
plants have a faculty to correct bad air in a few hours; that this
wonderful operation is due to the light of the sun; that it is more
or less brisk according to the brightness of the light; that only the
green parts of the plant can effect the change; that leaves pour out
the greatest quantity of oxygen from their under surfaces; that the sun
by itself has no power to change the composition of air." It will be
seen that Priestley started the inquiry, devised and executed the most
necessary experiments, and got excellent results. Then he lost his way,
and bewildered by conflicting observations, which he was too impatient
to reconcile, published a barren and misleading conclusion. Nothing was
left for him but to acknowledge that Ingenhousz had cleared up all his
perplexities.
Nicholas Theodore de Saussure, son of the Alpine explorer, showed in
1804 that when carbon is separated from the carbonic acid of the air
by green plants, the elements of water are also assimilated, a result
which owes its importance to the fact that starch is a combination of
carbon with the elements of water. Saussure also proved that salts
derived from the soil are essential ingredients of plant-food, and that
green plants are unable to fix the free nitrogen of the air; all the
nitrogen which they require is obtained from the ground.
We are unable to follow the history further. Though the main facts
were established as early as the beginning of the nineteenth century,
experimental results of scientific and practical interest have never
ceased to accumulate down to the present time.
The Metamorphoses of Plants.
Speculations concerning the nature of the flower roused at one time
an interest far beyond that felt in most botanical questions. The
literary eminence of Goethe, who took a leading part in the discussion,
heightened the excitement, and to this day often prompts the inquiry:
What does modern science think of the Metamorphoses of Plants?
Let us first briefly notice some anticipations of Goethe's famous
essay. In the last years of the sixteenth century Cesalpini, taking
a hint from Aristotle, tried to establish a relation between certain
parts of the flower and the component layers of the stem. Linnæus
worked out the same notion more elaborately, and with a confidence
which sought little aid from evidence. His wonderful theory of
Prolepsis (Anticipation) need not be described, far less discussed,
here. He also borrowed and adapted an analogy which had been thrown
out by Swammerdam. The bark of a tree, which according to the theory
of Prolepsis gives rise to the calyx of the flower, he compared to the
skin of a caterpillar, the expansion of the calyx to the casting of
the skin, and the act of flowering to the metamorphosis by which the
caterpillar is converted into a moth or butterfly. More rational than
the speculations just cited, and more suggestive to the morphologists
of the future, are his words: "Principium florum et foliorum idem
est" (Flower and leaf have a common origin)—which was not, however,
a very novel remark in the eighteenth century. Long before Linnæus
early botanists had remarked the resemblance of sepals, petals, and
seed-leaves to foliage-leaves; Cesalpini has a common name for all
(folium).
At the very time when Linnæus was occupied with his fanciful analogies,
a young student of medicine named Caspar Friedrich Wolff, who was
destined to become a biologist of great note, published a thesis
which he called _Theoria Generationis_ (Halle, 1759). This thesis
marks an epoch in the history of animal embryology, but what concerns
us here is that Wolff examined the growing shoot, and there studied
the development of leaf and flower. He found that in early stages
foliage-leaves and floral-leaves may be much alike, and thought that he
could trace both to a soft or even fluid substance, which is afterwards
converted into a mass of cells. It seemed to him possible to resolve
the flowering shoot into stem and leaves only. Wolff's thesis, or at
least that part of it which dealt with the plant, was little read and
soon forgotten; his studies of the development of animals were carried
further and became famous.
Goethe in 1790 revived Wolff's theory of the flower, without suspicion
that he had been anticipated. It is only our ignorance, he said, when
the fact came to his knowledge, that ever deludes us into believing
that we have put forth an original view. As soon as he realised the
true state of the case, he spared no pains to do Wolff full justice.
The aim of Goethe's _Metamorphoses of Plants_ was to determine the
Idea or theoretical conception of the plant, and also to trace the
modifications which the Idea undergoes in nature. These two inquiries
constituted what he called the Morphology of the plant, a useful,
nay, indispensable term, which is still in daily use. He thought
that he could discover in the endless variety of the organs of
the flowering plant one structure repeated again and again, which
gradually attained, as by the steps of a ladder, what he called the
crowning purpose of nature—viz., the sexual propagation of the race.
This fundamental structure was the leaf. The proposition that all the
parts of the flower are modifications of the leaf he defended by three
main arguments—viz., (1) the structural similarity of seed-leaves,
foliage-leaves, bracts, and floral organs; (2) the existence of
transitions between leaves of different kinds; and (3) the occasional
_retrogression_, as he called it, of specially modified parts to a
more primitive condition. These lines of argument were illustrated by
many well-chosen examples, the result of long and patient observation.
Goethe did not, however, fortify his position by the likeness of
developing floral organs to developing foliage-leaves, which had been
Wolff's starting-point. He arrived independently at Wolff's opinion
that the conversion of foliage-leaves into floral organs is due to
diminished nutrition.
Linnæus's exposition of the nature of the flower had been read
attentively by Goethe, who must have remarked that the conversion of
organs to new uses was there described as a _metamorphosis_. That
word had been, long before the time of Linnæus, appropriated to a
particular kind of change—viz., an apparently sudden change occurring
in the life-history of one and the same animal. It was therefore unlucky
that Goethe should have been led by the example of Linnæus to employ the
word in the general sense of adaptation to new purposes. He did not,
however, expressly compare flower-production with the transformation of
an insect, as Linnæus had done.
The reception of Goethe's _Metamorphosen der Pflanzen_ was at first
cold, but the doctrine which it enforced gradually won the attention of
botanists, and by 1830 he was able to show that it had been accepted by
many good judges.
Then came the discoveries of Hofmeister, followed by Darwin's _Origin
of Species_. Naturalists soon ceased to put the old questions, and
the old answers did not satisfy them. Wolff and Goethe had generalised
the flowering plant until it became a series of leaf-bearing nodes
alternating with internodes, but no such abstract conception could
throw light upon the common ancestor of all the flowering plants,
nor upon the stages by which the flowering plant has been evolved,
and it was these which were now sought. Hofmeister brought to light
a fundamental identity of structure in the reproductive organs of
the flowering plants and the higher cryptogams. There has since
been no doubt in what group of plants we must seek the ancestor of
the flowering plant. It must have been a cryptogam, not far removed
from the ferns, and furnished with _sporophylls_—_i.e._,
leaf-like scales, on which probably two kinds of sporangia, lodging male
and female spores respectively, were borne. The careful investigation of
the fossil plants of the coal measures has brought us still nearer
to the actual progenitor. Oliver and Scott[31] have pointed out that
the carboniferous Lyginodendron, though showing unmistakable affinity
with the ferns, bore true seeds, as a pine or a cycad does. Many other
plants of the coal measures are known to have combined characteristics
of ferns with those of cycads, while some of them, like Lyginodendron,
crossed the frontier, and became, though not yet flowering plants, at
least seed-bearers.
The discovery of a fossil plant which makes so near an approach to
the cryptogamic ancestor of all the flowering plants may remind us
how little likely it was that the ideal plant of Wolff and Goethe,
consisting of leaves, stem, and other vegetative organs, but without
true reproductive organs, should fully represent the type from which
the flowering plants sprang. No plant so complex as a fern could
maintain itself indefinitely without provision for the fertilisation of
the ovum; the only known asexual plants are of low grade, and, it may
be, insufficiently understood.
What substratum of plain truth underlies the doctrine of the
metamorphoses of plants? Botanists would agree that all sporophylls,
however modified, are homologous or answerable parts. Carpels and
stamens are no doubt modified sporophylls. Petals are sometimes,
perhaps always, modified stamens, and therefore modified sporophylls
also. We must not call a sporophyll a _leaf_, for it contains a
sporangium of independent origin, and the sporangium is the more
essential of the two. The common origin of foliage-leaf, bract,
perianth-leaf, sporophyll (apart from the sporangium), and seed-leaf
is unshaken. We may picture to ourselves a plant clothed with nearly
similar leaves, some of which either bear sporangia or else lodge
sporangia in their axils. Part of such a primitive flowering plant
might retain its vegetative function and become a leafy shoot,
while another part, bearing crowded sporophylls, might yield male,
female, or mixed cones. From an ancestor thus organised any flowering
plant might be derived. But the chief wonder of the theory of
Metamorphoses—viz., the derivation of stamen and pistil from mere
foliage-leaves—disappears. Anther and ovule take their real origin
from the sporangium, whose supporting leaf is only an accessory.
The chief steps by which the morphology of the flowering plant has
been attained are these:—Cesalpini (1583), followed by several other
early botanists, recognized the fundamental identity of foliage-leaf,
perianth-leaf, and seed-leaf. Linnæus (1759) added stamen and carpel
to the list, identifications of greater interest, but only partially
defensible. Wolff (1759) justified by similarity of development the
recognition of floral organs as leaves. Goethe (1790) traced structural
similarity, transitions, and retrogression in leaves of diverse
function. Hofmeister (1849-57) showed a relationship between the
flowering plant and the higher cryptogams. Oliver and Scott (1904),
inheriting the results of Williamson's work, discovered a carboniferous
seed-bearing plant, one of a large group intermediate between ferns and
cycads. It is now possible to explain the resemblance of the various
leaf-like appendages of the flowering plant by derivation either from
the leaves or the sporophylls (the latter not being wholly leaves) of
some extinct cryptogam, which was either a fern or a near ally of the
ferns.
Early Notions about the Lower Plants.
The fathers of botany neglected everything else in order to concentrate
their attention upon the flowering plants, from which very nearly
all useful vegetable products were derived. The lack of adequate
microscopes rendered it almost impossible to investigate the structure
and life-history of ferns, mosses, fungi, and algæ until the nineteenth
century. As late as the time of Linnæus it was possible to maintain
that they developed spontaneously, though the great naturalist himself
called them Cryptogamia, thereby expressing his conviction that they
reproduce their kind like other plants, but in a way so far not
understood. Gaertner, a contemporary of Linnæus, pointed out one
important respect in which the spores of cryptogams differ from the
seeds of flowering plants, viz. that they contain no embryo.
_Ferns._—Even before the age of Linnæus it was known that little
ferns spring up around the old ones, and that a fine dust can be shaken
from the brown patches on the back of ripe fern-leaves. The dust was
reputed to be the seed of the fern, and in an age which believed in
magic the invisibility of fern-seed made it easy to suppose that the
possessor of fern-seed would become invisible also. When the microscope
began to be applied to minute natural objects, the brown patches of the
fern-leaf were closely examined. William Cole of Bristol (1669),
Malpighi, Grew, Swammerdam, Leeuwenhoek, and others, found the stalked
capsules (sporangia), their elastic ring and the minute bodies (spores)
lodged within them; it seemed obvious to call the capsules ovaries and
the spores seeds. Some time in the latter part of the seventeenth
century Robert Morison, professor of botany at Oxford, who died in 1683,
sowed spores of the harts-tongue fern, and next year got an abundant
crop of prothalli, which he took to be the cotyledons. A little later,
when it had been proved that flowering plants possess male and female
organs, diligent search was made for the stamens and pistils of ferns
and mosses, which of course could not be found, though identifications,
sometimes based upon a real analogy, were continually announced. Late
in the eighteenth century one John Lindsay, a surgeon in Jamaica, who
was blest with leisure and a good microscope, repeated the experiment
of Morison, which seems to have been almost forgotten. Having remarked
that after the rains young ferns sprang up in shady places where the
earth had been disturbed. it occurred to him to mix the fine brown
dust from the back of a fern-leaf with mould, sow the mixture in a
flower-pot, and watch daily to see what might come up. About the
twelfth day small green protrusions were observed, which enlarged,
sent down roots, and formed bilobed scales, out of which young ferns
ultimately grew. In 1789 Sir Joseph Banks, who was reputed to be the
best English botanist of the day, asked Lindsay's help in sending West
Indian ferns to Europe. Lindsay replied that it would be easier to send
the seed, and that the seed would grow if properly planted. This was
new to Banks, who demanded further information. Lindsay then prepared a
short illustrated paper, which Banks communicated to the newly formed
Linnean Society. It will be seen that Lindsay was able to add nothing
of much importance to what Morison had ascertained a century before.
The spores were still identified with seeds, the prothallus was still
a cotyledon, and for years to come botanists continued to seek anthers
on fern-leaves. At this point we suspend for a time the history of the
discovery (see below, p. 108).
_Mosses._—Linnæus observed that the large moorland hair-moss
(Polytrichum) is of two forms, only one of which bears capsules, and
further that in dry weather the capsules emit masses of fine dust.
No further progress was made until 1782, when Hedwig, in a memoir of
real merit, described the antheridium and archegonium of the moss,
and traced the capsule to the archegonium. Interpreting the organs of
the moss by those of the flowering plant, he called the antheridia
anthers, the capsule was a seed-vessel, the spores were seeds, and
the green filament emitted by the germinating spore a cotyledon. Such
misinterpretations were then inevitable.
_Fungi._—Micheli in 1729 found the spores of several fungi,
germinated them, and figured the product. The figures show the
much-branched filament (mycelium) which burrows in the soil and
constitutes the vegetative part of the fungus, and also here and there
a pileus (mushroom, toadstool, &c.), which is the fructification
springing out of the mycelium. His account comprises the best part of
what is known down to the present time of the reproduction of that group
of fungi to which the mushroom belongs.
_Algæ._—Some early observers (Réaumur among the rest) studied the
enlarged and fleshy branches of brown seaweeds, and discovered the
seed-like spores.
This scanty knowledge of the life-history of cryptogams sufficed
until the nineteenth century, when the study was resumed with better
microscopes and in a far more connected way, with results of the
highest interest and importance (see below, p. 108).
[15] Reden, Vol. I., pp. 109, 154.
[16] _Traité d'Insectologie_, première partie. Two vols.12 mo. Paris,
1745.
[17] In circles untouched by general European thought such beliefs
lasted much later. Sir Francis Galton (_Memories of My Life_, p. 67)
says: "The horizon of the antiquarians was so narrow at about the
date (1840) of my Cambridge days that the whole history of the early
world was literally believed, by many of the best-informed men, to
be contained in the Pentateuch. It was also practically supposed
that nothing more of importance could be learnt of the origin of
civilisation during classical times than was to be found definitely
stated in classical authors."
[18] "If anything could disentitle Montesquieu's _Esprit des Lois_ to
the proud motto, _Prolem sine matre creatam_, it would be its close
relationship to the Politics." (A. W. Benn's _Greek Philosophers_, Vol.
II., p. 429.)
[19] For an account of other early hypotheses of the same kind the
reader may refer to Edward Clodd's _Pioneers of Evolution_.
[20] _Life and Letters_, Vol. II., p. 212.
[21] _Origin of Species_, ed. i., p. 484.
[22] White uses _anecdote_ in the old sense, meaning by it a piece of
unpublished information.
[23] Réaumur, _Hist. des Insectes_, Vol. V., Mém. viii.
[24] Darwin, _Origin of Species_, chap. vii.
[25] Vol. III., Mém. iv.
[26] _Hist. Nat._, Vol. IV.
[27] The first edition of the _Nouvelles Observations sur les Abeilles_
(1792) was the work of François Huber alone; the second (1814) was
prepared by Pierre with the co-operation of his father, and is here
credited to the son.
[28] _Hist. Animalium_, VIII., i.
[29] Huxley's _Hume_, chap. v. Some few naturalists, who are entitled
to respectful attention, such as Father Wasmann, author of _The
Psychology of Ants_, do not even now receive the conclusions of Hume.
[30] Lloyd Morgan, _Habit and Instinct_, Introduction.
[31] _Phil. Trans._, 1904.
PERIOD IV.
1790-1858
Characteristics of the Period.
The first French republic and the first French empire were associated
with a great outburst of scientific energy. French mathematics,
astronomy, and physics were pre-eminent. England suffered from
isolation during the continental war, but Davy, Young, the Herschels,
Watt (now past his prime), Dalton, and William Smith supported the
scientific reputation of their country. In Germany this was the age
of Goethe and Schiller; Alexander von Humboldt was prominent among
the scientific men of Prussia. The forty years' peace, during which
reaction prevailed in many parts of Europe, was in England and America
a time of steady growth and progress.
Sprengel and the Fertilisation of Flowers.
Conrad Sprengel, an unsuccessful schoolmaster who lived in a Berlin
attic and got his bread by teaching languages or whatever else his
pupils wished to learn, wrote a book which marks an epoch in the study
of adaptations. This was his _Secret of Nature Discovered_, which
appeared in 1793. Half a century passed before its merit was recognised
by any influential naturalist; even then the recognition was private,
and never reached the author, who had died long before. There was no
striking of medals, no jubilee-celebration, nothing more than this,
that Robert Brown recommended the book to Charles Darwin, who found in
it, as he says, "an immense body of truth."
In 1787 Sprengel had remarked that the bases of the petals of Geranium
silvaticum are beset with long hairs. Persuaded that no natural
structure can be devoid of meaning, Sprengel asked what purpose
these hairs might serve. A honey-gland in their midst suggested that
they might protect the honey by keeping off the rain, which easily
enters this shallow flower. Other honey-secreting flowers were found
to possess mechanisms adapted to the same end. His first question
suggested a second: Why should flowers secrete honey?