of such markings, as seen in the genus Erodium, is shown in Fig. 14. It is interesting to note the various ways in which flowers render themselves conspicuous in order to attract insects. In the majority of Seed Plants, such as the Buttercup, Pea, Rose, Foxglove, it is the corolla, formed either of separate petals, as in the first three, or of petals fused together, as in the last, which by its bright colour or colours renders the flower noticeable. In other species the calyx takes on the function of advertisement, the corolla being in comparison insignificant—we may study examples of this in the Anemones, Hellebores, and Marsh Marigold (Caltha palustris). It is worth examining this last, to see how its coloured sepals resemble and fulfil the same function as the petals of its cousins the Buttercups. Or, again, sepals and petals may combine in showiness, both sets being brightly coloured in one or more tints—compare the Columbine (Aquilegia), Larkspur (Delphinium), Milkwort (Polygala), and the marvellous flowers of Orchids. In the great group of the Monocotyledons, indeed, to which

the Orchids belong, sepals and petals usually combine in form and colour to form one corolla-like envelope (then called a perianth). In many other plant groups—for instance, the Dipsacaceæ (such as the Scabious), Umbelliferæ, and Compositæ—conspicuousness is obtained by a grouping together of a large number of small flowers. In the Cow Parsnep (Heracleum Sphondylium) the outer petal of the marginal flowers of the large umbel is much enlarged, which enhances this effect. In Astrantia, an interesting genus of Umbelliferæ, the bracts take on the appearance of a ring of large petals, and surround the group of small flowers (Fig. 15). The same thing may be noticed in the outer blossoms of the close flower-head of the Field Scabious (Knautia arvensis). In many Compositæ the process is carried still farther; in the Common Daisy (Bellis perennis) the outer flowers have each a long strap-shaped expansion of the corolla, which is of a different colour (white) from that of the corollas of the inner flowers, which are yellow. In the Dandelion (Taraxacum officinale) all the flowers have a yellow strap-shaped corolla. In the Guelder Rose (Viburnum Opulus) the outer flowers are entirely devoted to advertisement, consisting each of a big white corolla, while only the small inner flowers possess stamens and pistil and are capable of producing the brilliant scarlet berries. In a cultivated form of this, commonly called the Snowball Tree, the advertisement flowers only are present, forming a globe of white blossom, and no fruit is produced in consequence. The Dwarf Cornel (Cornus suecica), a little Dogwood growing on many Scottish moors, bears what looks like a white flower with a purple centre. On examination it is seen that the four white petal-like structures are really foliage-leaves, which have taken on the duty of advertising the group of small purple blossoms which they enclose (Fig. 16). A similar and very gorgeous effect is produced in several Spurges often seen in greenhouses, such as Euphorbia fulgens, E. splendens, and E. punicea; in these the upper foliage-leaves are large and coloured brilliant scarlet, the flowers which accompany them being quite small. These aggregations of flowers with their flaunting flags are in general an invitation to all comers; the nectar in the blossoms lies open to every hungry insect, and pollination is effected in a rather promiscuous and messy way; not only flying insects—bees, butterflies, beetles, and flies of many sorts—but also ants and other creatures which creep up the stems from the ground, assemble for the feast, and incidentally transfer from flower to flower pollen which may adhere to their bodies.

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 (Caryophyllaceæ) produce a sticky secretion which is a very effectual bar to the passage of small walking animals. In the English Catchfly (Silene anglica), Night-flowering Catchfly (S. noctiflora) and the Nottingham Catchfly (S. nutans), 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 (Lychnis Viscaria) 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 (Rubus phœnicolasius), often grown in gardens, the calyx, like the stem, is densely clothed with bright red slender spines (Fig. 17). It opens to allow the inconspicuous petals to expand, and then closes again and resumes its protective rôle till the scarlet fruit approaches maturity.

But it is in the flower itself that we find the most ingenious arrangements to encourage useful and discourage useless visitors, to assist the former to pollinate the flower, and while offering nectar to the welcome guest to deny it to the unwelcome. The first stage in this specialization is that the flower, instead of having its axis vertical, and facing the sky, is turned on its side by the curving of its stalk, and looks out horizontally. The effect of this is to cause a flying insect on approaching the flower to alight in a particular position—namely, on the lowest petal. Following on the adoption of this attitude the next stage in development is seen in the parts of the flower beginning to alter their shape and position relative to each other and often also their colour. Thus, beginning with a quite regular flower, we can arrange a series showing more and more asymmetry. The tendency is generally for the lowest petal to become enlarged and often conspicuously marked, providing a broad, convenient platform on which insects may alight, while the remainder form walls and roof, protecting the important parts within and by their shape, which is often narrowed and tubular behind, barring access to all but chosen visitors. To find a full series illustrating these transformations we do not need to go to plants widely separated in their affinities. In the Buttercup order (Ranunculaceæ) alone every gradation may be found. The flowers of the Buttercups themselves are upright and quite regular. In the Larkspur (Delphinium) the flower is turned on its side, and a puzzling combination of coloured sepals and petals—five bright blue unequal sepals and a single large purplish petal of peculiar shape with a long hollow spur behind—produces a quite irregular blossom. The process is carried farther again in the Monkshood (Aconitum), in whose well-known blue flower the sepals and petals combine to produce a strikingly irregular blossom, with the upper sepal arching over into a great hood protecting the rest of the flower. In such irregular flowers the essential parts—the pollen-producing and pollen-receiving portions, or stamens and stigma—also alter their position and form, and are so placed that an insect, visiting the flower to obtain nectar (which is generally stored at the back, well out of the way), must of necessity receive pollen on its body, and probably deposit pollen on the stigma. To describe the variety and ingenuity of these devices as found in different flowers might well occupy several chapters, and only one or two examples can be quoted here; familiar wild flowers are chosen, and the reader should examine them for himself to understand their structure. In the well-known Pea type, one great petal arches over the flower; two narrow ones stand one on either side; the remaining two stand on edge below, with their margins in contact, enclosing the stamens and pistil. An insect visiting the flower alights naturally on the keel or pair of lower petals. Pressed down by its weight, these open, often with a sudden movement like bursting, and dust the insect with pollen. Compare also the flowers of the Snapdragons (Antirrhinum) and Toadflaxes (Linaria), in which the upper and lower lips of the corolla meet like a closed mouth, which can be forced open only by a strong insect like a bee, and is safe from predatory visits of smaller fry (Fig. 18). In the Sages (Salvia) the corolla is tubular at the base; there is a large lobed lip on which visiting insects alight, and a hooded roof above arching over the stamens and pistil, which are placed close against it, overhanging the entrance to the corolla-tube, at the base of which the nectar is stored. The stamens, only two of which are developed, have each a hinge near the top, the part above the hinge being like a curved rod supported near its middle. These two curved rods stand normally in a vertical position, so that their lower ends partly block the entrance to the tube; the pollen is borne at their upper ends. Should a bee insert its head down the tube in search of nectar, it pushes the lower ends of the hinged rods upwards, with the result that their upper ends swing downward against the bee’s back, dusting it with pollen just at that part of its body which, if the bee should visit a rather older flower, would come in contact with the stigma, the slender stalk of which (the style) increases in length during the period of flowering, and is in consequence the more liable to be encountered.

Only one more instance can be referred to, which can be tested by the reader any summer day wherever any of our native Orchids grow. In these, the most highly specialized of all plant groups as regards pollination by insects, the general arrangement of the flower is often somewhat similar in a general sense to the last case; but here the sepals and petals which between them form the platform, tube, sides, and roof of the flower, are all separate and often differently and elaborately coloured. The essential organs are greatly modified and hardly recognizable at first. There is only one stamen, producing two clusters of pollen, which are embedded in the roof of the flower. Each possesses a slender stalk which terminates in a little sticky disc which projects from the general surface. The pollen grains are held together in a mass by fine threads, and the whole with its stalk—the pollinium—resembles a lemonade bottle in shape. The stigma is also embedded, forming a sticky surface in the roof of the flower behind the stamen. When an insect inserts its head into the flower, its forehead comes in contact with the sticky ends of the pollinia, which adhere, so that on leaving the flower the insect flies away with the pollen sticking to its forehead like two little horns. And now a remarkable thing happens. The stalks of the pollinia, drying rapidly in the air, contract unequally, and become curved, so that the pollinia bend forward into a horizontal position. When the insect visits another flower and thrusts in its head, the pollen consequently comes in contact with the sticky stigmatic surface farther down the tube, and cross-pollination is effected.

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.

As regards the kinds of insects which visit flowers for food, these are very numerous and belong to almost every section of that large class. In many, such as Neuroptera, Orthoptera, Hemiptera, Coleoptera, there is very little special adaptation for their flower-feeding habits, and these insects visit flowers, such as the Umbelliferæ, in which the nectar and pollen are freely exposed, and lie open to all. Many of the Diptera, or Flies, are in the same case; but in some families, such as the Bombyliidæ, high specialization for securing food from flowers is found: the creatures are provided with elongated probosces for sucking nectar even when it is deeply hidden, and no other food is used by the insects in their adult stage. But it is among the long-tongued Bees and the Lepidoptera (Butterflies and Moths) that the highest degree of adaptation in this direction is found; and the modifications are associated with those flowers which have become most highly specialized for insect pollination, and most completely dependent on it. In the Bees the legs have become much modified for the gathering of pollen, and the mouth is a long flexible sucking-tube which when not in use is carried rolled up in a spiral. The pollen, on which food alone the young bees are fed, is gathered and stored among rows of hairs on the legs, and in the more highly specialized forms it is wetted with honey so as to form a compact mass, easily carried and easily removed when the nest is reached. The balls of pollen thus formed are sometimes nearly the size of the body of the bee, and may contain one to two hundred thousand grains of pollen. The formation of the mouth is beautiful and complicated, adapted to the rapid sucking up of nectar even if deeply placed in the flower. The nectar is stored in the body of the bee, and subsequently transferred to the waxen honey-cells in the hive. In the Butterflies and Moths the mouth parts are also modified for sucking, and as these insects do not build nests or take care of their offspring as Bees do the mouth is formed solely for the purpose of securing the nectar which is their only food. The proboscis varies greatly in length in different groups, according to the kind of flower which they visit. In the Owl Moths (Noctuidæ) it is sometimes only eight millimetres (¹/₃ inch) long; in many of the Butterflies it is about half an inch. In the Hawk-moths it attains a remarkable development, necessitated no doubt by the habit of these insects of not alighting on or entering a flower, but hovering in front of it as a Humming Bird does, and sucking up the nectar while thus poised. The proboscis of the Convolvulus Hawk-moth measures 65 to 80 millimetres (2¹/₂ to 3¹/₄ inches), and some of the Tropical allies of this moth have probosces twice or even three times that length. These species feed on the nectar of flowers with tubular corolla of corresponding dimensions. Most of the Hawk-moths feed only at dusk, and as the time is short they take advantage of their powers of rapid flight to visit (and incidentally to pollinate) a very large number of flowers in a short period. Moreover, in common with most of the more specialized flower-feeding insects, they do not visit the flowers of different species indiscriminately, but dash to blossom after blossom of whatever single species they have selected. Hermann Müller records watching Humming-bird Hawk-moths (Macroglossa stellatarum) at work at the summit of the Albula Pass; one visited 106 flowers of Viola calcarata in under 4 minutes; another 194 blossoms of the same plant in 6³/₄ minutes.

The day-flying Butterflies display none of this restless energy. The sunshine is pleasant and the day long. They wander aimlessly in their beauty from flower to flower, sun themselves on the warm ground, or “whirl through the air with the first good comrade that by chance appears.” They are the flowers of the air, and our country rambles are made more joyous by their careless companionship.

CHAPTER V

PLANT STRUCTURES

“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