Read Ebook: Aspects of plant life; with special reference to the British flora by Praeger R Lloyd Robert Lloyd

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But the assistance which the parent plant gives is often of a more active and even dramatic character, though in these cases it is usually effected not by a movement of living tissue as in the last case, but by mechanical changes taking place in tissues already dead or dying. If we stand by a bank of Gorse on a warm day we may become aware of a snapping sound, and may possibly feel on our faces the impact of small bodies. These are gorse seeds in process of being distributed by the parent. In this shrub the fragrant flowers are succeeded by short tough, hairy pods, formed of two valves joined together by their edges. When the seed is ripe the pod dries, and owing to unequal shrinkage of the valves stresses are set up which at last tear the pod suddenly asunder along its edges, flinging the seeds violently out into new ground, where they will have a better chance of life than if merely dropped into the middle of the parent bush. A similar arrangement is found in the Vetches and many other Leguminosae. In the Cranesbills a very ingenious catapult device may be examined. The fruit is of peculiar structure. We might make a rough model of it by taking five single-sticks and tying them to a broom-handle--firmly at the points, less securely elsewhere--and slipping a tennis-ball into each basketwork handguard before turning its open side in against the broom-stick, so that the ball cannot fall out. Imagine now that unequal drying on the part of the sticks tends to make each bend into a semicircular form, which is hindered by the fastenings at either end. The stress will eventually tear the weak fastenings at the base: the lower end will fly up, bearing with it the ball , which will be projected

A very interesting case, in which the seed is actually buried in the soil by movements of its appendages , may be watched in the case of the Storksbills , Several species of which are British plants of frequent occurrence. Here the young fruit much resembles that of its allies the Cranesbills. The long rod-like axis at the lower end of which the seed is enclosed contracts unequally in drying, so that the upper half assumes a position at right angles to that of the

But most seeds sufficiently light to be capable of extended flights are liberated only a few feet from the ground; they are dependent on upward eddies to raise them if they are to achieve more than a very short migration. That such eddies, both upward and downward, occur on a windy day we all know from experience; and it is they that make or mar the fortune of most wind-borne seeds. Only some local or accidental excess of upward over downward eddies will assist a seed on its journey; and as every upward eddy must be compensated somewhere by a downward eddy, the longer the journey is, the more such eddies tend to neutralize each other. Over the sea--that most formidable barrier to plant migration--eddies do not prevail as they do over rough ground, so that, unless by a series of lucky eddies a seed is whirled up to a considerable elevation before it leaves the shore, the chances of its successful passage across a stretch of water are remote. Discussing the possibility of seeds of Portuguese plants reaching the Azores, lying 800 miles to the westward, H. B. Guppy shows, from observations on the rate of fall of seeds made by several workers, that with a 50 miles per hour horizontal wind the light-plumed seed of the Common Groundsel , for instance, would require to be liberated at a height of 9 miles above the ground if it is to reach the islands: or to express it differently, if liberated at ground-level, the seed would need to be raised 9 miles by upward eddies during its journey, even if corresponding downward eddies were absent--which they certainly never are. It is clear that if even light seeds are to achieve anything more than short journeys, they must depend on exceptional disturbances of the air, such as whirlwinds and tornadoes.

It is now time to examine the devices by which many seeds achieve a more or less wide dispersal by means of the wind. Seeds possessing these adaptations may be divided into three classes: Powder seeds, winged seeds, plumed seeds.

To pass on. Some seeds, many of them of considerable size as compared with those which we have just considered, have coverings which are furnished with a membranous wing , sometimes extending all round the seed, as in the Elm , more often placed at one side, as in the Sycamore . The effect of such wings is to reduce the rate of fall, imparting to the seed an irregular zigzag motion, as in the former case, or a spinning motion as in the latter. A Sycamore seed with the wing removed will fall four or five times as fast as with the wing present. But while a well-developed wing forms a more efficient dispersal device than mere reduction in size as found in Seed Plants, the rate of fall of wing seeds as a whole shows that these appendages do not fit them for anything but short voyages.

Nevertheless, the present flora of Great Britain is in the long run the result of migration from surrounding areas; so that ease of dispersal has undoubtedly played its part in the building up of our vegetation.

Conditions under which rapid dispersal has obviously an advantage occur when by some exceptional circumstances the natural vegetation is destroyed within an area, as by a flood or landslide. Such conditions are produced artificially each season over much of our own country by the operations of agriculture. Their results will be considered in a subsequent chapter.

SOME INTER-RELATIONS OF PLANTS AND ANIMALS

In the great abysses of the ocean, where vegetable life is absent, the strange creatures which live there in utter darkness prey upon others, and they again on others which belong to lesser depths, the ultimate source of life being again the minute surface organisms which, possessing chlorophyll, can make organic out of inorganic substances by the energy obtained from sunlight. Thus only is life made possible in

the green hells of the sea Where fallen skies and evil hues and eyeless creatures be.

Apart from such special cases, the general dependence of animals upon plants is obvious, and is by no means confined to food-supply. Animals of all grades, from human beings to Caddis Worms, construct houses of vegetable materials; trees are the chosen home of large sections of our fauna, and the herbs of the field are the world for millions of tiny beings.

There's never a leaf or a blade too mean To be some happy creature's palace.

Among certain lower animals and plants symbiotic connection is often most intimate. For instance, in the body-wall of certain Sea Anemones and Holothurians there are small green cells which were long believed to be part of the animal, and which puzzled naturalists because they contained chlorophyll, that remarkable green substance characteristic of plants, which gives to them the power of forming food out of its raw inorganic materials. These cells are now known to be minute seaweeds , which spend their lives in the animal tissues to the benefit of both organisms. The plant, by virtue of its chlorophyll, absorbs carbon dioxide, decomposes it, and gives out oxygen, which is eagerly seized on by the animal. The animal in its turn liberates carbon dioxide, which is required by the plant. Similar relations exist between Algae and some of the lowly Radiolarians and Foraminifera; in these cases, the animals being very minute, the plant partner plays a more conspicuous r?le. It is noteworthy that these Algae are quite capable of living and multiplying separately, free from the body of the animals, and the animals also are capable of pursuing an independent existence.

In a large number of flowers such general feasting is discountenanced, insect traffic is regulated, the visits of insects of little or no service to the plants is discouraged, and special arrangements are made to attract and minister to the needs of those insects whose visits are of most benefit. Except where flowers are borne in clusters, creeping creatures like ants are of no service; for in the course of the journey "by land" from one flower to another, there is a strong probability of any pollen which the insect may be carrying being rubbed off before the next blossom is reached; small flying insects are likewise frequently useless. In many plants the visits of such pedestrians and small fry is very distinctly discouraged. Of different devices which serve this end, the most conspicuous and effective include barriers to the passage of stem-climbers, and devices in the flower preventive of the visits of unwelcome guests. We may take a few instances from among British plants, which the reader may with a little diligence find and study for himself. Several members of the Pink family produce a sticky secretion which is a very effectual bar to the passage of small walking animals. In the English Catchfly , Night-flowering Catchfly and the Nottingham Catchfly , hairs are present all over the leaves and stems, from the tips of which a gummy substance exudes, which is a fatal trap for small insects. Kerner, in his interesting book, "Flowers and their Unbidden Guests," states that on the sticky stems of the last, in the Tyrol, he identified the remains of sixty different kinds of insects--ants, ichneumons, beetles, bugs, flies, and so on. The Red German Campion has an extremely sticky ring below each joint of the stem and inflorescence, which is most fatal to any creature which attempts to climb to the flowers. Other instances, such as the Petunia or Moss Rose, will occur to the reader. Another familiar kind of barrier is the presence on the calyx or involucre of a palisade of stiff hairs or prickles, such as may be studied in the Thistles; in some plants a downward-pointing ring of stiff hairs at each joint serves the same purpose. In the Japanese Wineberry , often grown in gardens, the calyx, like the stem, is densely clothed with bright red slender spines . It opens to allow the inconspicuous petals to expand, and then closes again and resumes its protective r?le till the scarlet fruit approaches maturity.

In the cases of many of these highly specialized flowers, one is no less struck with the perfection of the arrangements made for preventing self-pollination, than those adapted to securing cross-pollination. But in a few, on the contrary, self-pollination is specially arranged for.

It must be pointed out that the insects which pollinate these specialized flowers have in many cases acquired modifications in their structure corresponding to the modifications in the flowers which they frequent. In the more specialized forms, indeed, plant and animal have become entirely dependent on each other; the plants would become extinct in the absence of the special insects through whose agency they are able by pollination to produce fertile seed; and the insects would likewise die out if the flowers to whose nectar and pollen they look for food were not available.

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.

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 are immersed in the soil, while the upper ones 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.

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 d?bris, 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 d?bris, 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 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.

The ordinary soils of our fields may be defined as a mixture of sand, clay, and humus. A soil which is too rich, or too poor, in any one of the three will support plant life with difficulty.

"The soil is not merely a reservoir for the mineral nutrients of plants, but is the seat of complex physical, chemical, and biological actions which directly and indirectly influence soil fertility. These actions are intimately associated with the organic matter of the soil and its bacterial inhabitants. Mineralogy and inorganic chemistry, though helpful, are no longer capable of solving soil problems. Biochemistry and bacteriology, with their modern conceptions of colloids, absorption phenomena, enzymes, oxidizing, reducing, and catalytic actions, etc., are now rapidly extending our knowledge of the soil as a medium for plant growth."

Such, then, is the nature of the soil in which plants grow, and from which, by means of innumerable elongated cells proceeding from near the tips of the roots, food materials dissolved in water are absorbed; these food materials being produced partly by solution of mineral constituents contained in the soil, partly by the action of bacteria in breaking up organic matter. Soil suitable for plant growth may be looked on as consisting of a mineral framework, carrying in its meshes water and air ; mixed with the mineral particles is humus of varying amount; and supported largely by the humus is a vast population of organisms, both animal and vegetable, from earthworms to bacteria, whose activities are often essential, generally beneficial, and occasionally prejudicial to plant growth.

In many instances roots do not accomplish their work single-handed, but only in co-operation with certain lowly organisms; and these cases are so interesting and of so much economic importance that reference should be made to them. The little swellings or tubercles upon the roots of Leguminous plants, such as Clover, are familiar to most of us. These are caused by the stimulation due to colonies of bacteria , which live in the root-tissues as internal parasites. These bacteria feed on the sap and cell-contents of their host, but they supplement this food-supply by absorbing nitrogen direct from the atmosphere, which the host cannot do, though it can and does use the nitrogenous compounds which the bacteria manufacture. It is a case of symbiosis , each organism supplying food useful to the other; but the significance of the phenomenon is that through this agency nitrogen becomes added to the soil as the plants decay, and increases its fertility; and thus the cultivation of a crop of, say, Lucerne becomes a matter of great economic importance in farming operations, and the presence of Clover in pasture is a source of increasing wealth.

The state of mutual dependence existing between seed plants and mycorhizic fungi sometimes ends in the higher organism ceasing to manufacture its food by means of green leaves, and depending wholly on the lower for its sustenance. This is the condition to which some of our Orchids have come, such as the Bird's-nest , which does not produce leaves or chlorophyll, but sends up from its fungus-infested roots merely a scaly brown stem topped with brown blossoms, matching curiously the dead leaves among which it grows .

In contrast to these the case of certain other Orchids may be quoted, which have also lost their leaves, but in a very different manner. In their case the roots, creeping over the bark of trees on which the plants perch as epiphytes, have become green and flattened, like the fronds of some of our native Liverworts; they have assumed the functions of leaves: in them the process of photosynthesis is carried on; and the leaves themselves, thus supplanted, have by degrees disappeared.

Like many other parts of plants, roots are often used for the storage of reserve supplies of food or of water. For this purpose they become much thickened, and this thickening is the most conspicuous change which roots usually undergo. Note the fat roots of many plants which grow in dry or arid places, such as the Sea Holly, Dandelion, and many desert plants and alpines. The thickening is often accompanied by increase in length, as the roots range far in search of water. Another point to notice is that though normally roots differ considerably from their associated stems in general appearance, and also in their minute structure, as in the arrangement of the vascular strands, the two are related. Stem structures are often produced at various points on roots; the suckers sent up by many kinds of trees offer an example. Conversely, roots are readily produced even from the upper portions of many stems--else how could we grow cuttings? Where roots are succulent--that is, when they have a reserve of food stored in them--cuttings of them will conversely produce stems. A classical instance of such interchangeability of function is the young willow which Lindley bent down and buried the top till it rooted; the original roots were then dug up and raised into the air, when they produced leafy branches, and the tree grew upside down henceforth. Underground stems, also, of which there is a great variety, take on many of the characters of roots, and from an examination of a small piece of one it is often difficult to tell whether we are dealing with a root or a stem. The point at which root joins stem is, in fact, in many instances, so far as function is concerned, fixed only so long as the level of the surface remains fixed: we can often alter it by "earthing up" or by stripping away the soil. In Tropical forests, where the air is moist, hot, and still, roots--or branches which serve only as roots--descend through the air from heights almost equalling those to which stems ascend; while, on the other hand, in hot, poorly aerated swamps, roots send up from the mud into the air stem-like structures through which they may breathe, as in the case of the Swamp Cypress of Florida. The primary differences between the two, in fact, do not prevent the one from taking on the general characters of the other, and from functioning as the other, when the environment changes.

The STEMS of plants may be looked on from two points of view--as a framework devoted to the display of the leaves and flowers, and as pipe-lines connected with the nutrition of the plant, conveying raw materials from the roots to the leaves, and manufactured products from the leaves to all growing parts. It is the former relation which has mainly determined the forms of stems. Even a very slender stem can convey a vast amount of water and food to a plant which is transpiring or growing actively, as we can test roughly by weighing a pot shrub as it begins to come into leaf, and again a week later, or comparing the growth of a pea with the size of its stem at the base. The surprising variation in length, thickness, form, position, and branching of stems is the plant's response to external conditions--such as exposure, the competition of neighbouring plants, and so on--which resolve themselves ultimately into questions of wind-pressure, of temperature, of moisture, and in particular of light. The first duty of most stems is to spread out the leaves so that they may receive a maximum share of sunlight, and the complicated systems of branches with which we are so well acquainted are devoted to this object, the leaves themselves helping materially by the positions which they assume. This familiar and typical kind of stem, upright and column-like, beautifully constructed to bear the weight of leaves and branches, and to resist wind-pressure, alone furnishes a delightful study; but it can be dealt with only very briefly, as also some of the modifications which it undergoes under special circumstances.

To plants which have not taken to a terrestrial existence, and which still inhabit their ancestral home in the water, the stem problem is comparatively simple. A flexible shaft capable of withstanding wave and current action suffices so far as mechanical considerations go; such shafts--as we may observe by watching the Oar-weed on an exposed coast--are effective under very arduous conditions. Those Seed Plants which, evolved on land, have later returned to the water, such as the Pondweeds , have often redeveloped a stem of a similar kind--a flexible shaft possessing a sufficient tensile strength. The specific gravity of such plants does not exceed that of the medium in which they are immersed, and the stem has not to support the weight of leaves and branches. It is, therefore, not surprising to find that the longest, though by no means the bulkiest, of all plants, are found in the sea. Some of the Oar-weeds of the southern and western oceans attain lengths which have been estimated at 500 to 1,000 feet; but these gigantic Seaweeds are nevertheless slender plants, suspended lightly in the water. But after the colonization of the land by the aquatic flora numerous serious problems had to be encountered and solved before plants in an aerial environment could rise boldly into the air. Extremes of temperature unknown in the water had to be faced. Along with a greatly increased loss of water owing to the presence of air and direct sunlight, the area over which water might be absorbed became largely reduced, the roots alone being now available. The whole weight of branches and leaves and fruit had to be borne by the stem, not only in calm but in storm. No wonder that to meet these conditions, or to avoid such extremes as were avoidable, aerial stems often display great complexity and diversity of structure and form. From the mechanical standpoint the tall stem is especially interesting on account of the

To explain the massiveness of a tree trunk we have to remember that, while the cross-section of any structure varies as the square of its linear dimension, the volume varies as the cube of the same. If we double the dimensions of a tree, we increase its weight eight times, but the strength of the trunk is increased only four times. If a tree 100 feet high is supported on a stem 6 feet in diameter, a tree 200 feet high of the same proportions would need a stem not 12 feet, but over 17 feet in diameter, to be supported equally efficiently. This proportion increases rapidly: a similar tree 300 feet high would need a stem 30 feet in diameter; a tree 1,000 feet high would require a stem 180 feet in diameter, or 32,400 square feet in cross-section. We see, then, why a limit of tree growth is rapidly reached, at about 300 feet, and why the trees which grow to that height have trunks which are one of the wonders of the world, exceeding 30 feet in diameter, or about 100 feet in circumference.

It has been seen that unless a plant is a parasite or saprophyte, using as food ready-made organic material, it is necessary that it should possess a sufficient expanse of green tissue for the purpose of assimilation. This is the essential function of the leaves; but before leaving the study of stems it should be pointed out that they usually assist, and sometimes entirely replace, the leaves as organs of food-manufacture. We have seen how in dry places--whether physically dry, from direct scarcity of water, or physiologically dry, owing to reduced activity on the part of the plant due to unfavourable conditions, such as obtain in cold regions, or on poisoned ground like salt-marshes or bogs--leaf surface tends to be reduced, to avoid excessive loss of water. In such plants as the Cacti, and the Euphorbias which so closely mimic the cactus form, this reduction is carried to its limit. Leaves are absent, and the stems, greatly swollen so as to store water, take up the process of assimilation, and perform it satisfactorily. In more rapid-growing plants, a sufficient area for assimilation may be obtained by abundant branching, as in the Gorse, in which leaves are present only in the seedling stage. In the Brooms the leaf-development is often weak, but the stems sometimes make up for this by bearing green flattened wings. In the Spanish Broom , a straggling shrub inhabiting dry places in south-west Europe, the few ovate hairy leaves, produced in spring, soon fall; but the slender branches bear several broad green wings, which act as

leaves, and persist for a couple of years, when they pass away, leaving slender, round, brown stems. In our native Broom a similar modification may be observed, though of less degree. Sometimes stem-structures assume a very leaf-like form, as in the Butcher's Broom , where the ultimate branches are ovate and quite flat, and might be taken for true leaves but for the fact that they bear on their surface flowers, and subsequently berries. The leaves themselves are in this plant reduced to minute scales, and from their axils these flattened branches spring. In fact, where leaf reduction takes place, the process of assimilation is often shared in varying degree by the leaves, the stipules, and the stems. Among our native plants, as, for instance, in the Leguminosae and Rosaceae, the reader may find for himself many interesting examples for examination.

But the large majority of the Seed Plants bear well-developed leaves, to which the process of assimilation is practically confined.

LEAVES vary surprisingly in size, shape, and arrangement, features which are closely related to the characters of the stems which bear them, the object being the most advantageous display of the chlorophyll in relation to the light-supply. In general they naturally take the form of a broad thin blade, protected as may be necessary against extremes of weather, and guarded against the obvious danger of being dried up by a thin waterproof covering or cuticle outside the epidermal layer of cells. In leaves we find the same beauty of mechanical construction as is seen in stems. The problem is again that of securing maximum efficiency with minimum expenditure of material. To give as great a surface as possible, the leaves are as broad and thin as is consistent with safety, the question of damage by wind being an important controlling factor. The veins, or vascular bundles, act efficiently as strengtheners of the thin surface; to prevent tearing at the leaf-edges the veins are often looped along the margin; while in indented leaves the extremities of the indentations are strengthened with special tissue. When one surface of the leaf faces the sky, as in most cases it does, this surface is strengthened against the weather, and the stomata are arranged mostly on the lower surface. Where occasionally the leaves hang normally in a vertical position, as do the mature leaves of the Gum Trees , both sides are protected, and the stomata are borne on the two faces equally. In the Water Lily, again, whose leaves float, the upper face, which alone is exposed to the air, bears the stomata, which are present in unusual numbers--nearly 300,000 to the square inch; the leaf surface is toughened to resist rain and wind, and waxy to prevent water from lying on it and so interfering with transpiration. The presence or absence of a leaf-stalk, again, is often clearly related to the light question. In the Water Lilies the continued lengthening of the elongated petiole causes the older leaves to float clear outside of the younger ones. In many biennial herbs, where food is stored up during the first season in preparation for the flowering effort in the second, a similar arrangement prevails--note the leaf-rosettes displayed by Spear Thistle and Herb Robert , as also especially in winter by perennials like the Dandelion and Ribwort . Where stems spread horizontally, as the lower branches of trees, the leaves are arranged more or less in one

plane, in such a manner that overlapping is reduced to a minimum . This is well seen in horizontal branches of the Elm and other familiar trees. In the plant chosen for illustration , an interesting arrangement obtains. One of the pair of stipules which subtends each leaf is itself leaf-like, and stands at an angle, so that a mosaic is formed of true leaves and stipules . On all stems the leaves are arranged not at haphazard, but according to definite rules. Sometimes they

During cold and tempestuous weather the presence of leaves may be a danger to the plant rather than a help; and where seasonal variations are such that strongly contrasted periods of favourable and unfavourable weather occur, such as the summer and winter of our own climate, many plants have adopted the device of shedding all their leaves: this is especially characteristic of the largest plants , which would naturally suffer most from unfavourable weather. The fall of the leaf is accomplished by means of the formation of a transverse layer of corky tissue across the base of the leaf-stalk, combined with a weakening of the layer of cells immediately above. Prior to the perfecting of these arrangements for dropping the leaf, all the useful materials in it are withdrawn down the stem, so that only an empty skeleton is shed; the scar that remains is not an open wound, but is well protected by the corky layer before mentioned.

The leaves of water plants offer several points of interest. Where they are entirely submerged, and, protected against the drying influence of wind and sun, they are of filmy texture. Broad blades are seldom met with, the leaves being usually either finely dissected or strap-shaped. The floating leaf, on the contrary, as already described in the Water Lily, is strongly built up, to withstand wave action and rain; it is usually broad and entire, which simplifies the

problem of avoiding submergence; and the stomata are confined to the upper side, which alone is in contact with the atmosphere. Those water plants which raise their leaves into the air, on the other hand, have leaves of a variety of shapes, which in most respects approach those of land plants. An interesting progression of leaves illustrating all three stages may be watched in spring in the Arrow-head . The first leaves produced are entirely submerged, and conform to the usual ribbon shape and delicate texture. Those which follow float on the surface. In them the lower part is contracted into a flaccid winged petiole, the upper part being expanded into an oblong floating blade with a waxy surface to keep the leaf dry on the upper side. These in turn give way to the characteristic aerial arrow-shaped leaves of summer, which approach in character the leaves of land plants, and are borne on stout, stiff petioles capable of resisting wind and wave.

Summing up, then, what has been sketched in this chapter, we must think of our plant as a very complicated and wonderful machine, of which the terrestrial Seed Plant is the highest expression. Water is the basis on which its activities are founded--the currency in which all business is transacted. The amount of water contained in a growing plant is seldom realized. Even solid timber, when growing, is half wood, half water. A fresh lettuce loses 95 per cent. of its weight if the water is driven off by drying. Living in an aerial medium which tends to deprive it of moisture continually, and which furnishes water to the soil only intermittently in the form of rain, and often in sparing quantity, the plant envelops itself from end to end of its exposed portions in a waterproof cuticle; the only openings in its surface layer are the spongy tips of the root hairs on the one hand, and in the stomata on the other. These minutest of openings--so small that the number on a square inch of leaf surface often far exceeds a hundred thousand--might prove danger-points were they not most jealously watched over. But each is provided with a pair of guard-cells ready to close the opening at any moment; and where drought threatens, the whole of the stomata are found in concealed positions. An ample pipe-system extends from root, through stem, to leaf, but it does not communicate directly with the openings at either end. All material, whether liquid or gaseous, absorbed or given out, has to run the gauntlet of the living cells, which are jealous watchmen, and allow only selected substances to pass through them. The crude building materials and food materials are assembled in the leaves, where in cells spread out to the light the chlorophyll is massed. Under the microscope, the chlorophyll is seen to be located in minute granules embedded in the semifluid contents of the cells. Well may we gaze in wonder at these tiny green specks. Each is so small that although a couple of hundred of them are often present in each cell, they occupy but a very small proportion of its volume. The cells themselves are of microscopic size. The chlorophyll itself occupies only quite a small portion of the corpuscle in which it is immersed; yet on its activity as spread in this infinitesimal quantity through the leaves the whole organic world, animal as well as vegetable, depends. Utilizing the energy which comes through space from the sun, it builds up organic compounds; from the energy thus stored comes all the varied life and vital movement which fill our world--the opening of flowers, the hum of insects, the march of armies, and our own restless thought; while its work in the distant past, laid by in coal and oil, warms our houses and drives our trains, factories, and steamships.

The work of the living chlorophyll accomplished, the food materials produced by its agency are sent by the pipe-system to all parts of the plant, for present use, or to be stored in root, stem, or leaf for future requirements.

Nor is our plant the passive, motionless thing that it may appear to be in comparison with animals and their larger movements. Active motion, local and general, though usually of relatively small amount, accompanies all plant-growth. Throughout root, stem, leaf, and flower transference of material is going forward vigorously. The root hairs and stomata are working at high pressure; the chlorophyll never ceases its activities while daylight lasts. Externally, the growing branches, leaves, and flowers also display incessant movement, sweeping the air in small circles, or in the case of climbing plants in curves of considerable amplitude. Alterations of illumination or of temperature produce other movement--bendings towards or away from light, the drooping of leaves and closing of flowers at nightfall, and so on.

All these phenomena of growth and movement culminate in the production of flowers, and in the remarkable developments by which, through the agency of pollen and ovule, a new generation is produced.

PLANTS AND MAN

The appearance of man upon the Earth is an event of very recent occurrence, not only in terrestrial history, but in the history of organic life in the world. In the life-story which began somewhere in far pre-Cambrian times, the record of the whole of human activities occupies but the last paragraph of the last chapter. For millions of years--ever since the larger animals first abandoned the aquatic haunts of their ancestors and took to a terrestrial life--creatures great and small, of myriad kinds, including huge reptiles and amphibians, and later on a crowd of birds and mammals, have fed on land plants, without effecting any profound changes in the appearance of the mantle of vegetation which covered so much of the Earth's surface. It has been left for the human race, in the course of the few thousand years that have elapsed since it emerged from an existence comparable to that of the beasts and birds, and learned the arts of peace and war, to effect such sweeping changes in terrestrial vegetation over wide areas, that its influence in this respect requires a separate chapter for its consideration.

The changes referred to are largely--though by no means wholly--due to the requirements of the art of husbandry; and to the history of agriculture we may look for information as to the time and place and nature of man's conquest of the surface of the globe. At the period of the earliest human civilizations, such as those of Egypt and Mesopotamia, the domestication of plants and animals had already reached an advanced stage. Its origin lies far behind the historic period. We can picture in imagination the time when in all inhabited parts of the globe man wandered with no fixed abode, seeking food when he was hungry, and making no provision for the morrow. Residence in a spot which afforded a valued supply of food, such as an abundance of buckwheat or millet or dates or bread-fruit, might lead to a desire to encourage the growth of such useful plants by protecting them and their offspring; following on which might arise the practice of assisting their growth, and thus eventually of cultivating them. Selection of the most productive strains would gradually follow, and barter would cause the spread of useful plants over wider and wider areas. We can picture development from such rude beginnings into the regular cultivation of the soil and the enclosing of the cultivated areas for their protection. It is clear that such practices would not readily arise among nomadic tribes, nor among those inhabiting forest regions where the ground was densely covered by trees. An abundance of animal food would produce a race of hunters rather than of tillers of the soil; and as for forest regions, they are unsuitable for human development; forest races have never been pioneers of civilization. Before agriculture--indeed, before civilization in any form--could make much progress, a settled life was necessary, free from migrations in search of food or for the avoidance of enemies. Hence the earliest civilizations tended to arise in areas which were protected by natural ramparts from the irruption of rival tribes. Egypt had the desert on three sides, and the sea--an impassable barrier to early peoples--on the fourth. The valleys of the Euphrates and Tigris presented similar features. In both areas rich alluvial soil offered a full reward to attempts at agriculture, and the alternation of summer and winter encouraged the making of provision for the non-productive period by the taking advantage of the period of growth: conditions not present under the "endless summer skies" of Tropical lands, where an easy and perennial food-supply tended against the development of industry.

The basin of the Mediterranean--the cradle of the earlier Western civilizations from the time of Egypt down to Rome--was, then, also the cradle of European agriculture. These lands, with their wet winters and dry summers, the latter inimical to the development of tree growth, lent themselves to cultivation more readily than the great forest-belt which lay to the northward, sweeping across Europe from Britain to the Urals. Although there is clear evidence that grain was cultivated in Europe as far back as the Neolithic Period , it seems established that when Roman agriculture stood at its perfection the peoples to the north were still mainly nomads, dependent for their food-supply on their flocks or on the chase. In Britain, Caesar found corn grown in Southern England, but the centre and north were largely forest land tenanted by tribes living on flesh and milk, and clothed in skins. The vigorous colonization of the Romans may well have been accountable for the introduction into Britain of many of the farm plants still grown there. The wars of the next fifteen hundred years on the one hand, and the spread of agriculture on the other, caused the steady destruction of the forests, till at length England and Central Europe began to assume their present appearance. The draining of marshes and fens, the enclosing of land, went on steadily, and to a slight extent is going on still; within recent years, the European War has resulted in the disappearance of many of the remaining woods, and in the breaking up of fresh land.

From the point of view of the botanist, agriculture consists of the destruction of the plant associations which for some thousands of years have occupied the ground, and their replacement by other plants which are useful to man. The natural plant associations being the result of the survival of the fittest through a long period of time, while the farmer's crops represent plants which do not grow naturally on the ground, nor often indeed in the country , it follows that the latter cannot compete with the former, and can be maintained only by the most careful protection. The native plants are always striving to reoccupy their legitimate territory, and the farmer is incessantly engaged in trying to keep them out. Agriculture, indeed, has been defined as "a controversy with weeds." Incidentally, the suppression of the natural flora allows many weaker plants an opportunity of which they are not slow to take advantage. These may be natives, but are often annuals which have followed the spread of farming operations, or which are directly--though unintentionally--introduced by man as impurities in the seed which he sows.

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