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.

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 roots of plants require also a due amount of both water and air if they are to fulfil their functions adequately. An examination of the minute structure of the soil shows that it consists of angular particles of very various size--the larger ones classed as sand and consisting largely of silica; the smaller, which decrease in size beyond the limits of microscopic vision, mainly of clay (silicates) and humus. A film of water clings round each particle, and between the particles the chinks are filled with air. For healthy plant growth a nice balance between these constituents is required. Should sand be in excess, the soil is impoverished, since silica contains no nutriment, and it is rendered too dry, as on account of the relatively small surface of the sand grains in proportion to their mass it retains but little water. Should there be too little sand, percolation of air and water is hampered; the soil tends to become water-logged and badly aerated, and turns sour. Should humus be absent, the nitrogen-producing bacteria cease their activities and the soil is sterile, as may be tested by digging up some _subsoil_, or soil from the deeper levels to which roots or other organic matter have never penetrated. An excess of humus, on the other hand, results in the accumulation of acid products inimical to bacterial growth: in consequence decay is arrested, and a mass of plant débris forms, highly charged (for humus is very spongy) with acid water and badly aerated, which is unsuitable for vegetable growth: we may study an extreme case of such conditions in our peat bogs. Should water be in excess in soils, air is forced out in proportion, and the roots cannot breathe. Too much air means a corresponding diminution of water, and the plants suffer from drought.

“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.”[8]

Such, then, is the nature of the soil in which plants grow, and from which, by means of innumerable elongated cells (the root-hairs) 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 (about three-tenths of its volume) and air (about one-tenth of its volume); 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.

The root of a young plant grows downward into the soil under the influence of gravity. Its tip, which has to force its way through the rough material of sand and clay, is beautifully protected by a special _root-cap_, which covers the growing point as with a cushion. The surface of the root-cap is slimy, to aid it in slipping forward, and its cells, which are being worn away constantly, are replaced by the growth of the interior. Should an obstacle such as a pebble be encountered, a root will bend round it and then return to its former direction. Branch roots are given off on all sides at an angle to the main stem, these also tending in a mysterious way, if their course is disturbed by an obstacle, to resume their former direction of growth; the branches again divide, till at length a complicated root-mass is formed, sometimes of great extent, and capable of extracting water from a large volume of soil. Save for continued growth, the roots show little change in comparison with those exhibited by the aerial parts of plants; safely immersed in the soil, they heed not day or night, storm or calm, but steadily pursue their main function of supplying liquid food material to the green parts overhead.

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 (_Bacillus radicicola_), 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 (see p. 79), 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.

Again, in the roots of most of our forest trees, both hardwoods and conifers, and of many other plants such as the _Ericaceæ_ and _Orchidaceæ_, the root-hairs are replaced by minute fungi known as _mycorhiza_, whose branches take on the function of absorption, while the roots in turn absorb the material which the fungus collects. The fungus obtains from the roots a direct and convenient supply of carbohydrates; the host obtains from the fungus a ready supply of salts and of nitrogenous compounds. In the case of the forest trees and some other plants, the fungus forms a close felt _around_ the roots; but in the Heaths, etc., it penetrates the roots, living in the cells and in some instances, as in the Ling (_Calluna vulgaris_), permeating the whole plant, even to the seed-coat, so that seed and fungus are sown together. Since the higher partner of the symbiosis cannot mature without the lower, this is an obvious advantage to the former, as the two develop together from the commencement of growth. Where the fungus is not present in the seed, the seedling has to rely on its presence in the soil. And so, if we wish to raise any of our common terrestrial Orchids from seed, we try to ensure the presence of the fungus by using soil in which the species has been growing already.

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 (_Neottia Nidus-avis_), 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 (Fig. 31, p. 182).

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 (pneumatophores) through which they may breathe, as in the case of the Swamp Cypress (_Taxodium distichum_) 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 (_Laminaria_) 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 (_Potamogeton_), 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 (_Macrocystis_) 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

beautiful structural adaptations by which it meets the various stresses to which it is subjected. The problem before the plant is to combine a minimum quantity of material with a maximum of strength and rigidity. Strands of toughened fibre, so disposed as to meet the stresses most advantageously, are characteristic of such stems. In the case of many tall annuals, such as the larger _Umbelliferæ_, the principle of the hollow column is largely employed; in proportion to the strength obtained, this is far more economical than a solid column: and economy is particularly necessary in such annual stems, where the time available for construction is short. Transverse partitions at intervals provide stiffening of the whole; and as the efficiency of the toughened longitudinal strands increases with their distance from the centre, the stems are often ribbed, the strands occupying the ribs, with softer substance between. This form of construction may be contrasted with that obtaining in the roots. In the latter the greatest mechanical stress is in the form of a longitudinal pull caused by swaying of the stem under wind-pressure. To meet this the vascular strands are arranged, not marginally, but in a central bundle, where they can best meet stresses of the kind. In most trees the stems are solid; here economy of material is less urgent, as a long period of years is available for their building up; the great amount of cell-space thus made available for food-storage is a valuable asset to the plant, as is evident from a consideration of the vast amount of fresh tissue produced in a brief period by a deciduous tree when it bursts into leaf. As this material, stored in the stems and roots, has to be sent up to the twigs dissolved in water, and as during the whole period of growth vast amounts of water are transpired, an elaborate and complete pipe-system is intercalated with the reinforced-concrete structure of the tree trunk. Pumped up by the roots, and sucked up by the leaves, water and food pass rapidly from the ground to the topmost twig of the loftiest tree.

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.

Climbing stems represent efforts on the part of plants to economize material by utilizing the rigidity of neighbouring plants, and by reaching to the light on their shoulders. Here, as in aquatics, the _rope_ type of stem is in evidence; it resembles a garden hose, offering great flexibility and conducting capacity, but without rigidity to support its own weight, much less that of the leaves and flowers which it bears. To secure support, the stem itself (or branches of it), the leaves, or the stipules (leafy projections on either side of the junction of leaf and stem), are used. Sometimes support is obtained by twining (compare Convolvulus, Grape-Vine, Vetch), sometimes by adherent discs (Virginia Creeper), or aerial roots (Ivy), often by mere scrambling, often aided by reflexed hooks on leaf and stem (Bramble, Cleavers). The mechanism by which twining is accomplished is of great interest. It is an effect of unequal growth of the different sides of the stem. If the unequal growth were confined to one side, the stem would eventually form a coil, or series of circles. But the region of greatest growth keeps shifting round the stem, with the result that the tip of the shoot describes a circle or ellipse, like the hand of a clock pointing successively in all directions. The stimulus is due, as in the case of the erect growth of ordinary stems (which usually display similar movements in a less degree) to gravity. Sometimes the movement, or _nutation_, is in the same direction as that of the hands of a clock (_e.g._, in the Hop); more frequently it is in the opposite direction, as of a clock-hand moving backwards. The result of this movement is that if the shoot encounters, say, an upright stem, it will lap round it in a spiral manner, and unless the said stem be quite smooth and unbranched, the twining shoot will be eventually supported by it. How effective the twining habit is as regards economy of building material may be seen from comparing the weight of the stem of a Hop with that of some tall herbaceous plant of the same altitude, and bearing an equal weight of leaves and flowers. The tendril-climbers are still more efficient, for they avoid the increased length of stem which arises from a twining habit. They grow straight up towards the light. Both the top of the growing shoot and the spreading tendrils which arise from it are continuously revolving in search of a support. When a tendril encounters one (such as a twig), the contact produces a stimulus which results in the tendril taking several close turns round the support. Nor does the action stop there, for usually the lower unattached portion of the tendril contracts into a spiral, drawing the stem closer to the support, and woody growth ensues, by which the tendril becomes exceedingly tough, often stronger than the stem itself.

One other point concerning climbers may be noted. Did they exhibit in a marked degree that bending towards the light which is characteristic of most plants, they would often defeat their own object, as they would grow _away from_ possible supports. But they grow boldly up into an overhanging canopy, apparently confident of their power to ascend into the light and air which exist above. In the root-climbers, such as the Ivy, this bending away from the light is very marked; the stem presses closely to the bark or stone on which it creeps, probing every cranny, and the numerous rootlets by which it is attached are developed only on the dark side. But when the plant is old enough to flower, then branches devoid of roots grow out _towards_ the light, so that the blossoms may be borne in the open, where they may be seen and visited by the numerous insects which, in their search for nectar, pollinate them.

In contrast to the extreme development in length found in the stems of climbing plants the extreme reduction of stem found in many plants of dry places may be referred to. The Crocus, for instance, has an abbreviated upright stem of which each year’s growth is distended for the storage of food: one year’s growth dies away as the next enlarges, so that the well-known bulb-like _corm_ is produced. Compare the “roots”--really the stem--of Montbretia, in which the annual growths remain, the result being a knobby structure like a string of onions. In bulbs reduction in length is carried still farther, the stem forming a broad cone from the surface of which spring a number of modified leaves, forming fleshy scales swollen with food material; these surround and protect the bud, which when it grows produces green leaves and a terminal flower-shoot; growth is continued by axillary scale-leaved shoots situated among the scale leaves, which in due course themselves produce green leaves and flowers. These compact food-charged stems take up their position well below the ground, out of reach of intense heat or drought, and during the favourable season send up rapidly into the air their leaves and flowers, after which they remain dormant till the following year.

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 (_i.e._, chlorophyll-bearing) 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 (_Genista_) the leaf-development is often weak, but the stems sometimes make up for this by bearing green flattened wings. In the Spanish Broom (_G. sagittalis_), 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 (two to four) 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 (_Sarothamnus scoparius_) 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 (_Ruscus aculeatus_), 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 Leguminosæ and Rosaceæ, 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 (_Eucalyptus_), 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 (_Carduus lanceolatus_) and Herb Robert (_Geranium Robertianum_), as also especially in winter by perennials like the Dandelion (_Taraxacum officinale_) and Ribwort (_Plantago lanceolata_). 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 (Fig. 21). This is well seen in horizontal branches of the Elm and other familiar trees. In the plant chosen for illustration (_Azara microphylla_, a Chilian shrub), 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 (the larger ones) and stipules (the smaller alternating ones). On all stems the leaves are arranged not at haphazard, but according to definite rules. Sometimes they

are grouped in circles (_whorls_) at certain points of the stem, as in the Bedstraws; often in opposite pairs, arranged criss-cross, as in the Sycamore; most frequently in a series of spirals. The result in all cases is the same--it allows of as great an interval as possible between any leaf and the one immediately below or above it, and gives to all an equal share of light. The indenting of leaves, as in the Sycamore, or their division into separate segments, as in the Ash and Horse Chestnut, is of undoubted advantage as allowing light to pass through to lower layers of leaves; it also materially diminishes the danger arising from excessive wind-pressure. In the former case there is often a wide space between the divisions of the leaf; but where this is not required, the parts of the leaf fit closely together, to secure a maximum of surface. A particularly pretty example is seen in the Chilian shrub _Weinmannia trichosperma_ (Fig. 22). Here, to avoid the loss of the area between the leaflets, the mid rib steps in, developing triangular wings which fill the spaces. It might be objected that the plant might have saved itself much trouble by producing, while it was about it, a simple undivided leaf covering the whole area. It is difficult to answer such suggestions. Probably the present form of the leaf best meets the conditions of wind, rain, and light under which it lives. Possibly its present form is bound up with its ancestral history. “It must be acknowledged,” says D. H. Scott, “that nothing is more difficult than to find out why one plant equips itself for the struggle with one device and another attains the same end in quite a different way.”

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 (the trees), 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.

Stipules and bracts need not delay us in this sketchy survey of plant organs. They are leaves, generally of rather small size, placed, the former one on either side of the point where a leaf-stalk emerges from the stem, the latter singly below a flower; they are present in some plants, absent from others. They function in the same way as ordinary leaves, and in the earlier stages of growth are of use protectively. Occasionally the stipules exceed or even replace the leaves, as in the native _Lathyrus Aphaca_, where the leaf is reduced to a tendril, and the pairs of broad “leaves” are really the stipules. The bracts, in their turn, sometimes take on the “advertisement” function of the petals, as we have already seen (p. 87) in the case of certain Euphorbias.

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 (_Sagittaria sagittifolia_). 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.

Coming now to FLOWERS, it is possible here to refer only to a few macroscopic or “naked-eye” characters and modifications; the full study of the flower and its essential functions being a matter for the laboratory and the high-power microscope, as very minute structures are involved. As briefly described in Chapter IV., flowers are groups of modified leaves arranged mostly very close together at the ends of branches, the tip of the shoot being often expanded into a _receptacle_ (very well seen in the Compositæ--_e.g._, Dandelion) for the accommodation of the crowded floral leaves. Just as the foliage leaves have become modified to carry on to the best advantage the process of assimilation, so the different series of floral leaves are specially adapted to their several functions. The sepals, which compose the _calyx_, having usually a protective rôle, in most cases enclose the young flower with a tough envelope; they usually retain their primitive green colour, and take part in the process of assimilation. They may drop off as the flower opens (_e.g._, Poppy), or wither as the petals wither, or remain fresh until the fruit is ripe. Sometimes, as in many Ranunculaceæ (compare _Anemone_, _Caltha_, _Helleborus_), they take on the advertising rôle usually assigned to the petals, being large and coloured, while the petals themselves are minute. In the Monocotyledons they usually join with the petals in adorning the flower. The next whorl, lying inside (that is, above) the sepals, is formed of petals, constituting the _corolla_. The connection of colour and form of petals with the visits of insects, and their relative insignificance in wind-pollinated flowers, has already been referred to (p. 81). The marvellous variety of colour and form observable in the corolla has for its main object the attracting of insects to the flower. The petals have departed much farther from the ordinary leaf-form than the sepals. They assume brilliant hues of every tint, the pigment being due either to colouring matter dissolved in the cell-sap (pinks and blues) or to small coloured solid bodies (_chromoplasts_) contained in the cells (reds and yellows). Chlorophyll being absent, the coloured petals do not assist assimilation: they are purely advertisements, though incidentally they often fulfil a useful protective rôle for the important organs which they surround. In this latter connection their sensitiveness to changes of light and temperature, which causes them to close in dark or cold weather, is a very familiar phenomenon; as is also the excellent protection which they provide in flowers such as those of the _Labiatæ_, where, fused together into a tube, they form a kind of cave in which the stamens and pistil nestle securely.

An exceptional use of petals, where indeed they are used for the purposes of advertisement, but to secure the dispersal not of the pollen, but of the seeds, is illustrated in Fig. 24. In the genus _Coriaria_ the staminate and pistillate organs are borne on separate flowers. The flowers of both kinds are small and inconspicuous. But in the “female” flowers the petals persist after flowering, and, becoming fleshy and comparatively large, enclose the seed in a pulpy berry-like envelope, which no doubt serves the same purpose as a true berry in securing seed-dispersal by being devoured by birds. In _C. terminalis_, which comes from the Himalayas, the “ripe” corolla is bright orange; in _C. japonica_, from Japan, it is at first coral-red, and when mature velvet-black.

The _stamens_, which form the next ring (sometimes a double ring or a close spiral), are much less leaf-like than the sepals or petals, yet there can be no doubt that they are descended from leaf-shaped organs; this is especially clear from the study of certain primitive fossil types, in which the corresponding organs which bear the pollen are actually leaf-like. In most of the present-day Seed Plants the stamens conform to a uniform type--a slender stalk (_filament_) bearing a head (_anther_) containing four chambers, in which are produced _pollen grains_, which escape when the flower is mature by the splitting of the enclosing walls. The ways in which the pollen is then conveyed to the pistil of other flowers have been referred to briefly on a previous page (p. 82). The stamens in many flowers are few, and their number usually bears a relation to the number of the other floral parts; in other flowers, for instance Rose and St. John’s wort (_Hypericum_), they are of large and indefinite number. The peculiar arrangement of the pollen in Orchids has been already noted (p. 94).

The final ring of modified leaves in our typical flower constitutes the _pistil_, formed of one or many _carpels_, the essential structure of which has been touched on already (p. 82). In the present place it is desired only to point out some of the leading modifications which the pistil undergoes, so that its structure as seen by the naked eye may be understood. In the simpler forms of carpel, the affinity to leaves is still evident, though in forms of pistil made up of a number of carpels this may be very difficult to trace. With the Pea, for instance, we may begin, as presenting a very simple example. Take an oblong leaf like that of a Laurel, and fold it down the mid rib till the two edges are in contact. There is our pea-pod complete. The young seeds, or _ovules_, are borne in a row along the mid rib, a very usual arrangement. Examine next the young fruit of a Columbine (_Aquilegia_). Here there is a group of five separate erect carpels, but each is essentially like a pea-pod in structure. Compare the fruit of a Saxifrage. This clearly consists of two carpels which are grown together save at the tips, where the two styles stand out like little horns. From this we may go on to other pistils in which several carpels are completely fused together. Next, the compact body thus formed may be sunk down in the expanded top of the stem (the receptacle). Or the other parts of the flower--sepals, petals, stamens--may in their lower part be fused with the walls of the pistil, and may thus appear to spring from the top of it. In such cases the structure of the flower may easily be wrongly interpreted, and reference to a work on systematic botany is necessary if pitfalls are to be avoided. It is indeed to be noted that in flowers, as in other parts of plants, complicated structure or multiplication of parts is not necessarily an indication of advanced evolution; on the contrary, it is often indicative of a primitive condition. Just as in machinery or in organized human effort simplification often accompanies improvement, so it is with plant structures. Many of the more primitive types of flowers, such as Buttercups or Water Lilies, have a multitude of petals or stamens or carpels, while in many of the most specialized, such as Composites or Campanulas, the number of parts is much reduced. The primitive wind-pollinated flowers produce large quantities of pollen; in those which have adopted the improved method of utilizing insects, the amount of pollen is much less; in the highly specialized Orchids, a most successful group, the pollen is reduced to two small bundles.

Once the act of pollination is effected, the duty of the petals and stamens is finished, and they generally fade. The sepals often remain, as in the Rose. By the growth of the pollen tube from the stigma into the ovary, fertilization is effected, and mature seed is produced. The fruit--that is, the seed and its coverings or appendages--offers the most varied forms of any of the plant organs--compare Hazel, Strawberry, Pea, Apple, Cranesbill, Dandelion; the variety is endless. Many of these forms are connected with the means by which seed-dispersal is effected: this subject has been touched on in Chapter III. But in numerous instances we can no more assign a reason for their beautiful or fantastic forms than we can account for the infinite variety of shape assumed by leaves and flowers.

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.[9] 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.