THE SEA CITIZEN THAT INVADED THE LAND

NO more is known of the origin of life than of the origin of the world. The probability that life originated in the sea is discussed elsewhere,1 but the earliest organisms, whether animal or vegetable, cannot have left any traces in the rocks. Indeed, it may be questioned whether the earliest and simplest living beings, which may be envisaged as minute one-celled blobs of protoplasm, could reasonably be classified as either ‘animal ‘or ‘vegetable ‘—the distinctive characters of these two branches may have taken millions of years to evolve.

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Here, at the very beginning of our story, the present gives the only clue to the past. There still exist microscopic aquatic organisms which might equally well be referred to the animal or the vegetable kingdom, and have often been claimed by both zoologists and botanists; these are probably the little-altered descendants of primeval ancestors which, having found their niche in the scheme of things, have managed to escape extinction. There are indeed numerous examples of both animals and plants which have persisted practically unchanged for very long periods.

THE OLDEST PLANTS OF ALL: SEAWEEDS

THE first recognisable fossil plants, and the only ones that have so far been found throughout the Cambrian, Ordo-vician, and most of the Silurian periods, are without exception alga;, or seaweeds. Most of the soft and flimsy seaweeds of our coasts do not seem likely subjects for preservation as fossils—they decay far too quickly—but there are certain groups of alga; which secrete a coral-like skeleton of lime. Forms such as these have frequently been preserved in marine deposits of the past, and were in some cases actually responsible for building up extensive limestone masses which may be called algal reefs. The familiar coral reefs of the present day, as well as of the past, are often indeed partly built up by these calcareous alga;.

The forms found in the early Palaeozoic are very varied; some of them belong to an extinct family of the red alga;, and some to a group of green alga; with whorlcd branches. Lime was deposited by the living plant round these branches, producing a cylindrical, club-shaped, or spherical body. By cutting sections of these bodies it is possible in favourable states of preservation to study the structure, often quite complicated, of the plant itself, which is represented by tubes or hollows, or perhaps merely by lines of discoloration in the hard fossil.

These whorled green alga; are found in rocks of almost every age from the Ordovician onwards and several kinds still exist. Each geological formation has its characteristic types, and a general progression of structure within the group can be traced almost throughout the series. In referring to the pre-Cambrian and early Palaeozoic times as the age of the seaweeds, the existence of numerous fossil seaweeds in later marine sedimentary rocks must not be overlooked, but after

the Silurian period the main interest in fossil botany naturally shifts to the land-plants.

THE SEAWEEDS THAT BUILT ROCKS

IN addition to those alga; which can be related, sometimes distantly, to those of the recent flora, there are others in the far-off Palaeozoic which, though recognisable as belonging to the algal class, can only be dubiously fitted into a classification based on living forms. Like many of the fossils, their preservation is imperfect and their reproductive organs are unknown, but they are often very important geologically as rock-builders or because they show the age of the rocks in which they are found.

There are other bodies again about which controversy still rages; the best known of these are concentrically-banded masses of limestone, for which an algal origin has been claimed, mainly on the ground of their resemblance to certain recent deposits which are supposed to be due to alga;. In these cases there is no question of the deposition of lime in the form of a skeleton round the plant itself; the precipitation of the bands of lime is a by-product of the plant’s physiological activities. But similar banded precipitates may a’so be produced by purely inorganic means, and convincing proof that these large fossil forms are really due to alga; or (as has also been suggested) bacteria is usually lacking.

We may note in passing that minute organisms like bacteria would naturally be singularly difficult to recognise in the rocks and that their presence in the past can as a rule only be inferred from the results of their activities. The same may be said of another group of lowly organisms, the blue-green alga;, which at the present day undoubtedly play an important part in marine sedimentation. Maurice Black has shown that ‘in addition to those forms which actively contribute calcium carbonate to the sediment there are other species which function primarily as sediment-binders, without necessarily precipitating any lime themselves.’ The latter are usually responsible for the characteristic form of the sediments, and in the Bahamas ‘where such deposits are now accumulating over large areas, structures are being produced which are reminiscent of those formed in some of the great limestone formations of the Lower Palaeozoic and Upper Pre-Cambrian.’ It is therefore quite possible that the rock-forming processes which are going on in West Indian seas at the present day are identical

with those of six hundred million years ago, and are clue to similar organisms.

PLANTS TAKE TO LAND AND LEARN TO BREATHE IN THE AIR WE do not know when marine (or at any rate aquatic) plants first invaded the dry land nor how long they took to develop those adaptations which fitted them for life in an aerial instead of a watery medium. We may speculate on the possibility that certain seaweeds of the shore, left dry at every low tide, and already differentiated into an attached or rooted portion and a more or less erect stem, developed a stronger skin, with pores (stomata) for breathing, and a central strand of thick-walled elements which served to strengthen and give rigidity to the stem as well as to conduct water within the plant. Finally, they must have produced air-borne instead of water-borne spores.

The earliest known land-plants fit in extraordinarily well with this simple hypothetical description. A black rock found at Rhynie in Aberdeenshire, which is practically a petrified peat of Middle Devonian age, contains beautifully preserved examples of the oldest-known land-flora, whose minute anatomy can be studied in every detail. In this region of Aberdeenshire there was in Devonian times a peat-bog which underwent periodic inundation, resulting in the formation of peat beds alternating with thin sandy layers. The period was one of declining volcanic activity, and the final petrifaction of the peat was probably due to an influx of water highly charged with silica in solution, from hot springs or geysers. The inundation was probably quite sudden, since in the upper layers the plants are embedded, almost in their position of growth, in a practically pure flinty rock.

The plants have a simple generalised structure : one of them, called Rhynia, was rootless and leafless, and consisted of tufts of forked erect stems continued into a creeping underground portion which bore root-hairs. The simple spore-sacs, containing only one kind of spore, were borne at the tips of the branches. There was a solid central conducting strand, which was somewhat more elaborate in another genus, Asteroxylon (so called from the star-shaped appearance of the woody strand in transverse section). The stem of Asteroxylon was clothed with small veinless leaves, and the spore-sacs were borne on special branches; both genera had breathing-pores (stomata) similar to those of modern plants.

A DISCOVERY THAT IS EAGERLY ANTICIPATED THE simple form and primitive anatomy of these plants suggest on the one hand that they are derived from seaweed-like ancestors, and on the other that they may themselves be members of a group from which various higher types of plants have been evolved. It would be unwise, however, to assume either that these particular early Devonian forms were the direct ancestors of later plants, or that land-plants originated in Devonian times. One of the discoveries to which botanists and geologists eagerly look forward is of pre-Devonian land or freshwater deposits in which plant relics have been preserved. Whether or not because the conditions under which the sedimentary rocks were deposited were unfavourable to the preservation of plants, such fossils are at present unknown, except perhaps in the Uppermost Silurian beds which may here be classified with the Devonian.

The Rhynie plants may have had a long history as land-dwellers, and the fortunate accident of their preservation in the Aberdeenshire chert may have taken place in the old age of their race, almost at the moment of extinction. The group to which they belong is unrepresented in fossil floras of later date than the Middle Devonian, and there are no living plants which can definitely be claimed as closely allied to them. In addition to those found in the petrified peat of Rhynie, plants of similar type (so far as can be judged from less well-preserved material) are found in early Devonian rocks of every continent.

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