Aspects of plant life; with special reference to the British flora

by R. Lloyd Praeger

Another large group of casual abnormalities is concerned with the colour of leaves. The Purple Beech is a case in point. It was not produced by selection, but arose naturally, no doubt as a chance seedling. In this instance the character is usually passed on to the offspring, most seedlings having similar purple leaves, though some individuals are green. The peculiar colour is due in this case to a pigment in the epidermis of the leaf; the green chlorophyll is duly present, though its colour is masked by the purple leaf-skin. To a different category belong the “gold” and “silver” variegations which are so much exploited in shrubberies and borders and greenhouses. These spots or stripes or tintings of pale colour on the leaves are due to the lack of chlorophyll in the chromatophores (chlorophyll corpuscles); sometimes to an absence of the chromatophores themselves; and this omission appears to be caused by an enfeebled condition of the plant. Variegated plants are weaker than normal ones, and hence do not tend to survive in nature. But gardeners have protected and propagated a large number of them. When the variegation arises, as it often does, on a branch of an otherwise normal plant, it usually is not reproducible from seed, and must be perpetuated by cuttings. But where it happens with seedlings, it is often more or less fixed, and may be reproduced generation after generation, as in the Golden Elder, Golder Feather, and the marginal-variegated form of Winter Cress (Barbarea vulgaris).

Flower colour is not so fixed as leaf colour, for obvious reasons, the green colour of leaves being due to chlorophyll, which is an absolutely necessary ingredient of the leaf if plant food is to be manufactured; whereas flower colour is merely for advertisement, and any pigment can be made to serve. In nature most flowers vary in tint, and some in a marked degree—take the little native Milkwort (Polygala), which may be blue or purple or white. Flowers offer great opportunities, therefore, to the gardener, and by selecting on the one hand and hybridizing on the other every known tint has been reproduced in some blossom. Adding to this the variability in size and shape of petals, and the tendency to “doubling,” the flower in the hands of skilful cultivators has been altered almost beyond recognition. Take the Roses, for example, with their infinite variety of form and colour. The bulk of them are derived from a dozen wild species, possessing comparatively small single flowers, white, yellow, or red—Rosa centifolia, damascena, gallica (the source of the older Roses), indica, moschata, odorata, rugosa, Wichuræiana, with our native arvensis and spinosissima. By selecting for colour, shape, and “doubleness,” both from the species themselves and from the offspring produced by hybridizing one of these with another, what a wealth of beauty has been developed! More than any other flowers, the Roses are the crown and glory of the gardener’s art. Well has the Rose been called the Queen of Flowers; but it owes its royal prerogative to man. Nature provided blossoms—elegant, but of no special promise—and a tendency to vary, of priceless value; human skill and industry have done the rest.

CHAPTER VII
PAST AND PRESENT

The dependence of animals upon plants for the food by means of which they continue to inhabit the earth, which was pointed out on a previous page (75), shows that the plant world is older than the animal world; but the immense age of both can be appreciated only by a study of stratigraphical geology. The tens of thousands of feet of sedimentary rocks, laid down in slow succession on the floors of ancient seas and lakes, and still reposing layer upon layer, and no less the great gaps in the series produced when, raised into the air, deposition ceased, and thousands of feet of rock were slowly worn away and washed down again into the sea by the action of frost and wind and water, point to periods incalculably remote as measured by the standards which we apply to human history. A few thousands of years measures the span which separates us from the Neolithic Period; but to the geologist a million years is but a convenient unit for expressing, so far as any expression by our time-standards is possible, the huge periods with which he has to deal. And even when we get back as far as the oldest fossils will take us, we are still a long way from having reached the epoch when life on the earth originated. As we work backward and study the fossils of older and older rocks, the multitudinous assembly of plants and animals which fill the world to-day are replaced by other and more primitive forms, many groups approaching each other and merging in common ancestral types. But still, the very oldest fossil-bearing strata contain the remains of organisms already far up the ladder of evolution. The Lamp Shells (Brachiopods), Pteropods, Trilobites, and Worms of the ancient Cambrian rocks have clearly a long ancestral history. Plants are not so abundantly preserved in the rocks as the skeletons and shells of animals, on account of their softer nature; but in the oldest known plants it is again clear that we are dealing with forms by no means primordial. It is the more interesting, then, to note that many very lowly forms of life have come down to us from times immensely remote, and are still present on the earth in abundance, swarming in every sea and in every pond, or nestling in damp crevices of the land; while higher types of immense antiquity still mingle with the crowd of recent Seed Plants, some of them forming noble forest trees. Of especial interest, taking into account the wide distinction which exists between the higher animals on the one hand and the higher plants on the other, is it to find that there are still in existence organisms which are so much on the border-line between these two great groups of living things that they can be referred to one or other only with hesitation, clearly indicating that animal and vegetable life sprang from a common source. Take the group known as Mycetozoa or Myxomycetes. These names alone show the divergent views which men of science have held regarding them, Myxomycetes signifying “slime-fungi,” while Mycetozoa means “fungus-animals.” These remarkable organisms, of which over 180 species are found in the British Isles, begin life as tiny wind-borne spores. Under suitable conditions of moisture and heat, the spore swells, its wall cracks, and the contents—a tiny globule of protoplasm—creep out, develop a little tail or flagellum, which by lashing about propels the pear-shaped swarm-cell through the drop of water in which it began life. The organism feeds by catching bacteria and other minute particles of organic matter, which are conveyed into the interior of the little mass of protoplasm and digested. The swarm-cells increase in number by division, and ultimately unite in pairs to form a plasmodium, which may, by union with other plasmodia, eventually attain a quite large size. In this naked protoplasmic mass a very remarkable rhythmic movement is set up, the granular protoplasm of the interior streaming rapidly along certain channels for about 1¹/₂ minutes, when the motion is reversed and it streams in the opposite direction. The whole mass now creeps about in moist places, usually in the form of a network of branching veins, feeding as it goes, usually on dead vegetable matter. When fully developed the plasmodium creeps out into some more open spot and transforms itself into masses of spores enclosed in spore-cases, which vary much in different genera as regards size, shape, and colour, and are often borne on delicate stalks. When ripe, the spore-cases, or sporangia, open, and the spores are liberated into the air to be dispersed by wind and eventually to begin growth on their own

Fig. 26.—A Myxomycete (Comatricha typhoides) in Fruit.

a, Natural size; b, enlarged.

account. This story partakes about equally of incidents characteristic of the life-history of the lower animals and of the lower plants. The fruiting stage and the wind dispersal of the spores recall the arrangements familiar in the Fungi, and are not matched in any section of the animal kingdom; while the creeping plasmodium, devouring food as it goes, is entirely suggestive of animal life, and is not paralleled anywhere in the vegetable kingdom. There is no reason to look on the Mycetozoa as a group of animals which have taken on certain plant-like characters, any more than as a group of plants which have evolved certain animal characteristics: we appear to see in them a very ancient group which has come down to us from a time when plants and animals, as we know them, had not yet become differentiated.

Among plants, as distinguished generally from animals by the production and abundant use of chlorophyll and of cellulose, we have still existing on the Earth a range of forms extending from almost the most primitive organism that we can imagine up to the splendid Seed Plant, specialized in a hundred ways. Every pool, every soil, swarms with bacteria, the lowliest form of life—organisms exceedingly minute, exceedingly simple, and capable of existing under highly diverse conditions both physical and chemical. Thence we can trace an irregular ascending scale through the Fungi, the Algæ, Mosses, Horsetails, Ferns, and Club-mosses, to the Conifers, and on to the highest of the Seed Plants, which exceed in their beauty of structure and complicated life anything that has gone before them. In fact, as Theophrastus says, your plant is a thing various and manifold. And this existing vegetation with its thousand forms is but the present manifestation of the vital activity which has populated the earth during tens of millions of years. The oldest rocks which have been preserved to us in such a condition as to yield remains of plants and animals in a recognizable form are those known as Cambrian, the deposition of which occurred at a period which geologists have variously calculated as from, say, 20 to 100 or more millions of years ago. Yet even at that immensely remote period, life, both vegetable and animal, was already abundant and diverse, as well as highly organized. As Darwin long ago pointed out, the geological record does not go back nearly far enough to allow us any insight into the evolution of the earlier forms of life. Below the Cambrian rocks, as represented in these islands and in Europe generally, with their well-developed fauna, are tens of thousands of feet of strata which once, no doubt, were sediments at the bottom of the sea, and later on hardened into slates and sandstones in which were embedded remains of more primitive organisms; but these rocks have been so altered during the immense period of their existence by heat and pressure and the other vicissitudes to which the restless crust of the earth is subject that they now present a mass of granite-like material in which all trace of organic life has been destroyed. In America the rocks of corresponding age are better preserved, and have yielded a limited fauna displaying an already advanced stage of evolution. To account for the strange paucity of animal remains it has been suggested that the creatures of these earliest times were soft-bodied, so that after death they left no trace behind. It may be noted that the pre-Cambrian rocks contain beds of limestone and of carbon (in the form of graphite); such beds, in later rocks, are composed of organic materials, the limestones being formed of the skeletons of minute marine creatures, particularly Foraminifera, and the carbon deposits of the remains of plants.