Fig. 6.—Diagram illustrating Succession of Vegetation in Lakes.
a, Marsh zone; b, reed zone; c, zone of floating vegetation; d, zone of submerged vegetation.
superabundant. In such places we are faced with a vegetation exhibiting a great number of species and a marked variety of form, and by no means so easy to correlate with its environment as those which we have been considering. In a wide sense, the nature of the vegetation is largely dependent on the degree of aeration of the water and the amount of dissolved mineral salts which it contains, an increase of either (within limits) resulting in a richer flora. But in any one area it is clear that depth of water is the controlling factor: the plants are arranged in zones, one succeeding another as the bottom shelves. Two main zones are conspicuous: (1) A zone of tall reed-like plants near the margins, which farther out is succeeded by (2) a zone of lax floating plants which either have leaves resting on the surface or grow entirely submerged. Above the former a belt of marsh plants links the reed zone with the vegetation of the soils of normal moisture; below the latter, should the water increase in depth, we reach an aquatic desert region, where the reduction of light renders plant growth difficult, and eventually inhibits it. Let us consider the conditions prevailing in the reed zone. Here the plants are essentially aerial, and though they have their feet in water, the stems and leaves rise far above it. Water-level is variable in lakes and rivers; the plants are usually tall, so that even in case of flood the leaves and flowers will not be drowned. Wave action on lake-shores is somewhat violent, and in flooded rivers a strong current may sweep through the vegetation; we see the advantage of the slender elastic stems and narrow leaves that characterize the plants: compare Reed (Phragmites), Reed-mace (Typha), Flag (Iris), Bur-reed (Spargarium), Bulrush (Scirpus); and these characters also fit them for the windy nature of their habitat. The denuding effect of wave or current action is countered by the network of creeping stems and abundant roots which the plants possess, forming a tough felt which floats, and by its growth and decay helps materially to form fresh land. Another effect of the creeping and branching stem-systems is the production of extensive and dense groves of many of the species.
When we pass beyond the reed zone, a completely different type of vegetation prevails. Here the plants are essentially aquatic. They make no effort to raise their stems and leaves above the water surface; but almost all of them raise their flowers into the air, though the seed is often ripened below the surface by a downward curving of the stem. These plants, surrounded by water, use their roots chiefly as anchors, and absorb through their stems and leaves the water from which they obtain the necessary mineral salts. As regards the supply of oxygen and carbon dioxide which the air supplies to them, those with floating leaves absorb it from the atmosphere, while those whose leaves are submerged have to subsist on the small quantity of these gases which is dissolved in the water—no wonder that such plants are rare in stagnant waters where aeration is poor. To assist respiration and transpiration, abundant and often comparatively gigantic air-spaces are provided in roots or stems or leaves, giving them a cellular appearance, and making them singularly light and spongy in texture. The leaf system of those plants which possess floating leaves—such as Water Lily (Castalia and Nymphæa) or Common Pondweed (Potamogeton natans), are well worth study. They are tough, to withstand battering by waves; the stomata are situated, not on the lower side of the leaf, as in land plants, but on the upper side, where they are in contact with the atmosphere; and the upper surface is waxy or oily, so that it is not wetted and the stomata are not blocked. Changes of water-level are met by means of long flexible stems, rising not vertically from the root, but at an angle, so that the leaves can rise with a rise of water-level. But not all the plants are anchored to the bottom. Some, which favour especially ditches and quiet waters, float freely with roots hanging down in the water—the Frog-bit (Hydrocharis) and Duckweeds (Lemna) are familiar examples. In the Duckweeds true leaves are absent, but the tiny stems are flattened and green and serve the same purpose, the minute flowers being borne on their edges. A few plants, such as the smallest of the Duckweeds (Wolffia arrhiza) and the Bladderworts (Utricularia), have gone farther still, and have dispensed with roots altogether. In Wolffia, indeed, the degeneracy of structure which results from the simplification of life problems in plants which live thus floating freely in water, is carried to its extreme limit. Leafless, rootless, and almost flowerless, it maintains itself by the budding of its tiny green fronds, a life-history as primitive as that of the lowly Algæ among which it lives. In the Bladderworts, the long flaccid stems, clothed with much-divided leaves converted in part into ingenious insect-traps (see p. 188), hang limply in the water, sending up boldly into the air their flowering shoots with yellow Snapdragon-like blossoms. In most of such free-floating plants, compact buds are formed at the tips of the shoots in autumn, and while the rest of the stem dies away these sink to the bottom and remain there safe from frost and storm until the spring, when they rise to the surface and produce a new crop of plants.
We have now glanced at the most distinctive of the plant formations which we meet with in our own country, and find that they accompany extreme conditions relating to water and soil: it remains to return to the consideration of the vegetation which develops under conditions of a more normal character—on ordinary soils, in fact, which are neither very wet nor very dry. Such conditions are precisely those which are required for agricultural purposes; and over the wide areas where they prevail, we find, as pointed out already, mere fragments of the native associations remaining in an undisturbed condition. This renders their study more difficult, and the difficulty is heightened by the fact that while the physical conditions show no contrasts so marked as those which we have been considering, the formations which can be distinguished are several, and each contains several associations—often a woodland, a scrub, and a grassland type. Thus, the formation which occupies calcareous soils exhibits characteristic woodlands—woods of Ash (Fraxinus excelsior), for instance, and on the downs peculiar woods or scrub of Box (Buxus sempervirens), Juniper (Juniperus communis), Yew (Taxus baccata), or Hazel, as on Farleton Fell. It also bears some very marked types of grassland, as on the chalk downs; and the limestone pavement of Farleton Fell is a special variant of this. Similarly, clays and loams, sands, and siliceous soils possess similar characteristic types of vegetation. But the consideration of these would occupy more space and lead us into more technical detail than the scope of this book warrants. For an account of these associations, written by botanists who have made a special study of them, the reader is referred to Tansley’s “Types of British Vegetation.”
CHAPTER III
PLANT MIGRATION
All organisms, animal as well as vegetable, are at some period of their existence provided with an opportunity of migration. In the animal world, most land creatures have legs or wings, which allow them to roam about freely—a freedom which is of special importance as enabling them to obtain nourishment and to avoid disadvantageous conditions. Aquatic animals are likewise to a great extent possessed of powers of locomotion, but such powers are not so essential to them as to terrestrial creatures, since the water itself is full of small organisms, both animal and vegetable, on which they can feed; hence a large variety of water creatures are content to remain during much of their lives fixed to one spot, extracting from the water as it passes by both the supply of organic food and the inorganic substances, such as oxygen or carbonate of lime, which they require for their life processes. These sedentary creatures, of which barnacles, sea-anemones, and zoophytes will serve as examples, once attached, do not move from the spot where they have settled down; but it is important to note that not only are their eggs or young mostly liberated into the water, and by it transported to new homes, but in their juvenile stages they often swim vigorously, and thus achieve a wide dispersal. In the plant world, the higher forms, with very few exceptions, spend their lives attached to one spot, like sea-anemones, deriving their food-supply from the air and from the soil; but they similarly are given the opportunity, after birth, of migrating. In our familiar wild flowers, for instance, the young plant, at an early stage of its existence, while it is still minute, becomes covered with a coat often of very resistant qualities, and is then cast loose by the parent in the form of seed, mostly in great numbers, to achieve what travels it can before it takes root and settles down, like its parent before it, to a humdrum existence. In the Cryptogams, or so-called Flowerless Plants, this temporary compression of the organism into very narrow limits suitable for easy dispersal takes place at a different period in the life cycle, but for mechanical purposes the results are similar. Minute bodies, or spores (much smaller than the seeds of the Seed Plants), are cast loose by the parent often in vast numbers, and eventually settle down and reproduce the species. In many of the lower aquatic plants these spores are provided with means of locomotion in the form of a tail-like appendage, which by its movement propels the germs through the water, giving them the same advantage which is possessed by the young of many of the sedentary animals.
The opportunity for migration thus offered to sedentary plants once at least in each cycle is of very great importance. A plant, living on one spot and drawing, from that portion of the soil which its roots can reach, certain mineral salts essential for its continued growth, tends to exhaust the available supply of these materials, and the succeeding generation needs to reach fresh ground if it in turn is to attain healthy development. And it is undoubtedly of advantage to plants, if they are to continue to exist on the Earth, to be able to jump barriers and to colonize fresh suitable habitats which may arise in the course of natural changes, which sooner or later may render old habitats untenable. Thus the very existence of plants upon the Earth depends on the adequacy of seed-dispersal. This being so, the imaginative mind, viewing the marvellous and infinitely varied contrivances of Nature, will possibly be struck more by the want of special provision for dispersal shown by the majority of the higher plants—their helplessness in this respect—than by the beautiful devices exhibited by the few. In the first place, seeds are inert, devoid of any power of locomotion—though in some instances the last act of the parent is to discharge them with an explosive action into the air. They are dependent on the movements of external media—air, or water, or wandering animals—for transportation of any magnitude, and while many possess very beautiful devices for enabling them to take advantage of opportunities in this regard, the majority are devoid of any special structures. They are as inert as pebbles or grains of sand: but they possess two attributes which form important assets—namely, numbers and vitality. The amount of seed produced annually is hundreds, or more usually thousands, sometimes hundreds of thousands, for each parent. What matter if myriads perish? If one in so many thousands takes root and grows, the species will not diminish in numbers. Vitality also largely affects the problem. The seed can endure extremes of heat and cold which would be fatal to the parent; it can be drowned, or scorched, or dashed about, or in many cases eaten by animals without injury; it can lie buried in the soil for a long period of years, yet if turned up again and placed within reach of the requisite amount of air and heat, will spring up vigorously.
As a matter of fact, investigation soon shows that absence of special devices for dispersal provides no measure of the breadth of a plant’s distribution, nor is profuse seed-production necessarily related to abundance of offspring. Many factors come into play, and conclusions of this obvious kind will generally only lead us astray. But that does not render the study of each one of the factors less interesting.