As regards the kinds of insects which visit flowers for food, these are very numerous and belong to almost every section of that large class. In many, such as Neuroptera, Orthoptera, Hemiptera, Coleoptera, there is very little special adaptation for their flower-feeding habits, and these insects visit flowers, such as the Umbelliferæ, in which the nectar and pollen are freely exposed, and lie open to all. Many of the Diptera, or Flies, are in the same case; but in some families, such as the Bombyliidæ, high specialization for securing food from flowers is found: the creatures are provided with elongated probosces for sucking nectar even when it is deeply hidden, and no other food is used by the insects in their adult stage. But it is among the long-tongued Bees and the Lepidoptera (Butterflies and Moths) that the highest degree of adaptation in this direction is found; and the modifications are associated with those flowers which have become most highly specialized for insect pollination, and most completely dependent on it. In the Bees the legs have become much modified for the gathering of pollen, and the mouth is a long flexible sucking-tube which when not in use is carried rolled up in a spiral. The pollen, on which food alone the young bees are fed, is gathered and stored among rows of hairs on the legs, and in the more highly specialized forms it is wetted with honey so as to form a compact mass, easily carried and easily removed when the nest is reached. The balls of pollen thus formed are sometimes nearly the size of the body of the bee, and may contain one to two hundred thousand grains of pollen. The formation of the mouth is beautiful and complicated, adapted to the rapid sucking up of nectar even if deeply placed in the flower. The nectar is stored in the body of the bee, and subsequently transferred to the waxen honey-cells in the hive. In the Butterflies and Moths the mouth parts are also modified for sucking, and as these insects do not build nests or take care of their offspring as Bees do the mouth is formed solely for the purpose of securing the nectar which is their only food. The proboscis varies greatly in length in different groups, according to the kind of flower which they visit. In the Owl Moths (Noctuidæ) it is sometimes only eight millimetres (1/3 inch) long; in many of the Butterflies it is about half an inch. In the Hawk-moths it attains a remarkable development, necessitated no doubt by the habit of these insects of not alighting on or entering a flower, but hovering in front of it as a Humming Bird does, and sucking up the nectar while thus poised. The proboscis of the Convolvulus Hawk-moth measures 65 to 80 millimetres (21/2 to 31/4 inches), and some of the Tropical allies of this moth have probosces twice or even three times that length. These species feed on the nectar of flowers with tubular corolla of corresponding dimensions. Most of the Hawk-moths feed only at dusk, and as the time is short they take advantage of their powers of rapid flight to visit (and incidentally to pollinate) a very large number of flowers in a short period. Moreover, in common with most of the more specialized flower-feeding insects, they do not visit the flowers of different species indiscriminately, but dash to blossom after blossom of whatever single species they have selected. Hermann Müller records watching Humming-bird Hawk-moths (Macroglossa stellatarum) at work at the summit of the Albula Pass; one visited 106 flowers of Viola calcarata in under 4 minutes; another 194 blossoms of the same plant in 63/4 minutes.
The day-flying Butterflies display none of this restless energy. The sunshine is pleasant and the day long. They wander aimlessly in their beauty from flower to flower, sun themselves on the warm ground, or “whirl through the air with the first good comrade that by chance appears.” They are the flowers of the air, and our country rambles are made more joyous by their careless companionship.
CHAPTER V
PLANT STRUCTURES
In the course of the preceding chapters a number of the more striking modifications displayed by the different organs of plants have been described briefly. Reference has been made to the increased length or thickness of the roots in plants of dry places, and the weakness or absence of root-system of many water plants. Corresponding variation in stems has been noted. The remarkable leaves of desert and water plants and of some carnivorous species have been mentioned. The profound alteration in flowers which have adapted themselves to pollination by insects has been sketched; as also the great variety in the shapes of fruits and seeds, correlated to the methods by which they are dispersed. It may be well to consider the question of plant structures on a broader and more systematic basis, and, as before, to connect them where possible with the external factors which have caused their modification and to which they are the plant’s response. These factors are physical, or chemical, or biological, and affect the plant mainly through the agency of the soil, the atmosphere, or living organisms.
“The living plant is a synthetic machine.” Under proper working conditions of heat, moisture, and light it builds up its body by absorption of inorganic material, liquid and gaseous, through its roots and leaves. For the present purpose we may take our typical plant as consisting of subterranean roots and aerial leaves on the one hand, and aerial flowers on the other—the roots and leaves concerned especially with carrying on the life of the individual, the flowers with perpetuating the race. In addition, an aerial stem is usually present, on which the leaves and flowers are displayed, and through which the food materials pass dissolved in water. Of these parts, the lower ones (the roots, and sometimes the stems) are immersed in the soil, while the upper ones (the leaves and the flowers—which are groups of modified leaves—and usually the stems) are immersed in the atmosphere. All the parts have acquired their form and fulfil their functions under control of the particular medium which surrounds them: it becomes necessary to preface any discussion of their characters and uses by a brief survey of the characters of these envelopes.
While the atmosphere is familiar to us as the medium in which we ourselves live and move and have our being, and while its chemical and physical properties are known in outline to every schoolchild, it is different with the soil; not only because, unlike the atmosphere, soil varies much in composition and character, but also because the soil is in fact a very complex product, offering many difficult problems to the investigator; it is only of late years that the scientific study of the soil has been placed on a sound basis; our knowledge of it is still far from complete.
Whence does soil arise? How is it that the surface of the land is usually covered with a layer of fertile material? The answer is to be found, in the first place, in the decay of rocks under the influence of natural agents. Heat and frost, rain and drought, by slow degrees break up the surface of the hard material of which the solid crust of the Earth is built up. The débris thus formed is washed into streams by rain, or scattered by wind. A stream flowing into the sea, and charged with the débris of the land, deposits the coarser material near its mouth, while the finer particles are carried farther. In dry regions wind plays a similar part. And so, while the materials which composed the surface layer of the cooling primitive Earth may have been tolerably uniform in composition, the débris derived from them has ever tended to get sorted out, as, for instance, into sand and mud at river mouths, or sand and dust in dry regions. In the course of ages the sorted materials, buried beneath subsequent deposits, have been formed through heat and pressure into rocks, which, when at length again brought to the surface by earth movement and exposed to the agents of disintegration, have been resolved once more into sands, clays, and so on. In the long history of the Earth this sorting process has been repeated till now large tracts of rocks and of soils are composed mainly of sand or mainly of clay. The prevalence of these two kinds of material arises from the abundance in the primitive crust of the substances of which they are composed. Silica (oxide of silicon), the material of which ordinary sand, as well as quartz, flint, etc., is composed, is of extreme hardness and insolubility, and its small crystals and fragments, disintegrated from the rocks, remain almost indestructible as grains of sand. Clays, on the other hand, are derived from silicates (compounds of silicon and oxygen with various metals such as aluminium, calcium, magnesium, potassium, sodium, or iron). These substances mostly disintegrate more completely into very small particles, which when wet cohere into a sticky mass and form clays. Along with the humus matter they include all the colloids of the soil. These latter bodies consist of the extremely minute—indeed, ultra-microscopic—particles, having in consequence of their small size a great total surface in proportion to their mass. In virtue of this, they function as the chief absorbents of the soil, holding water in enormous quantities, and abstracting and retaining till used by the plants the bases of the various substances applied as manures. Another constituent of the primitive crust was lime (oxide of calcium). Unlike the preceding substances, lime is readily soluble in acid water, and so is washed out of the rocks and carried in solution to the sea. Marine animals of many kinds—such as Molluscs, Corals, Foraminifera—extract the lime from the sea water and use it in large quantities to build up their shells or skeletons. This material slowly accumulates at the bottom of the ocean as generation after generation of animals passes away, becomes at length consolidated by heat and pressure, and through earth movements may eventually appear above the sea to form land, in the form of limestone or chalk. Exposed to the weather, it is once more slowly disintegrated; the lime passes off again in solution, the impurities being left behind; a limy soil results.
On a great plain, devoid of hills or rivers, composed of different rocks, and subjected to the agents of disintegration, we can conceive that over each kind of rock a soil would be formed corresponding closely to the materials of which that rock is composed. In sections formed by quarrying, by the cutting action of rapid streams, and so on, we may often see this. Below is the solid rock. Its upper layers tend to be loose and rotten owing to the action of percolating water, etc. They merge into a layer of stony débris, where the harder portions still retain their rock character, while the softer are disintegrating into clay or sand. Above this the rock is wholly disintegrated into a soil, the upper layers of which, mixed with plant débris, and consequently of darker colour, are full of the roots of living plants descending from the sward which covers the surface of the ground. In practice, however, such close conformity of soil to underlying rock is not always found.
Various distributing agents are ever at work—wind, water in an especial degree, and on sloping ground the action of gravity. In northern countries, besides, the ice of the Glacial Period has in its passage caught up all the loose surface material, added immensely to its volume by grinding down the rocks, and flung the products broadcast over the country, so that old sea bottoms may be strewn over coastal lands, sands and gravels over clayey rocks, and limy soils over areas where no limestone exists. The soil over much of the British Isles is formed from the surface-layer of these glacial deposits, which—tough, intractable, sterile—underlie the soil often to a great depth, where they rest on rock. In southern England the covering of glacial deposits is absent, since the ice-cap did not extend beyond the Thames valley; beds much older than the Ice Age, often of a gravelly or clayey nature, occupy the ground, and from these the present soils are derived.
There is another constituent of soils of primary importance for vegetable life, which results from the decay of the generations of plants which have gone before. When plants die, their bodies are decomposed by the agency of bacteria. Some of the constituents pass off as gas or water, but there remains an amount of solid matter (humus) which mixes with the soil and is of the utmost importance for plant growth. Nitrogen, which forms the greater part of the atmosphere, cannot in the gaseous state be absorbed by plants, although they spend their lives surrounded by it. It is a necessary substance in the plant’s economy, and through the action of soil bacteria, which change the nitrogenous matter in humus into soluble nitrates, plants are able to utilize this store.