Part 9

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 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).

Reden, Vol. I., pp. 109, 154.

Traité d'Insectologie, première partie. Two vols.12 mo. Paris, 1745.

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."

"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.)

For an account of other early hypotheses of the same kind the reader may refer to Edward Clodd's Pioneers of Evolution.

Life and Letters, Vol. II., p. 212.

Origin of Species, ed. i., p. 484.

White uses anecdote in the old sense, meaning by it a piece of unpublished information.

Réaumur, Hist. des Insectes, Vol. V., Mém. viii.

Darwin, Origin of Species, chap. vii.

Vol. III., Mém. iv.

Hist. Nat., Vol. IV.

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.

Hist. Animalium, VIII., i.

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.

Lloyd Morgan, Habit and Instinct, Introduction.