Chapter II: Roots

Let us begin our study of roots by considering the way in which plants obtain their nourishment from the earth, and are kept in an upright position by means of their root-fibres. These being out of sight, we may easily not be familiar with this part of the economy of plant life, but we shall soon see what important duties the roots have to fulfil, and how much they vary in character and appearance according to the soil, the climate, and the work they are required to do. The greater number of annual plants (those which live only one year) have fibrous roots, and of these we can find examples almost everywhere. A piece of groundsel or tuft of grass will answer our purpose. On pulling it out of the ground we see a bunch of whitish threads or fibres springing from the crown of the plant (which is the junction between the stem and the root), and on these slender fibres are hairs which are really the active part of the root, for it is only through these hairs that the rootlets are able to absorb the liquid from the soil, the fibres simply acting as channels to convey the watery nourishment to the stem and leaves.

Common earth consists of small particles of mineral substances such as flint, chalk, or iron, and also of such vegetable matter as decayed leaves and rotten wood.

The spaces between the particles are more or less filled with air, each mineral particle being enveloped with a film of water. However dry the soil may appear, this will always be found to be the case. It may be tested by weighing in an agate balance some dry soil on a summer’s day. There is a very delicate instrument called a hygroscope, which can tell us when there is the slightest amount of moisture in the air, and a clever German writer, Von Sachs,[6] has termed this film of water, which gathers round earth-particles, hygroscopic water. It has been ascertained by careful experiment that it is only on this delicate watery film that the root-hairs of plants are able to feed. As these hairs drain away the hygroscopic film it is always being renewed by the free water which comes from rain and dew. The free water of the soil is constantly passing from the surface to the subsoil, and by this action plant-food, in the form of soluble earth salts, is presented to the roots. The passage of the water is of the highest service to the roots, since the warm air follows the water through the soil, and helps to oxidise the mineral particles; these are thus rendered soluble, and are taken up by the fine films of water, and so indirectly the roots are fed. If, however, there is no outlet for the water and the soil becomes water-logged this beneficial action is retarded, and to land-roots the water is hurtful.

[6] Author of “Vegetable Physiology.”

We can now understand why stagnant water in the ground is so injurious to plant-life, as it prevents the needful air from coming into contact with the roots, and this is the reason why farmers are careful to remove the surplus water from their fields by thorough drainage and ploughing. Roots adapt themselves very wonderfully to their situation.

This piece of grass, which we are examining, if it grew in sandy soil, would have its root-fibres covered with a downy growth to enable them the more readily to absorb every particle of moisture in the sand. Dr. Bonar speaks of the date-palm as having this same characteristic. “These palm roots are peculiarly fitted to obtain every drop of water that the sand contains; they consist of long fleshy strings or ropes, shooting straight down into the sand, in numbers quite beyond our reckoning, and extending over a large circle.”

The tendency of fibrous roots to bind sand together is taken advantage of on many of our sea-coasts, where the sand blows inland and renders acres of ground sterile and useless. There, if the _Carex arenaria_ (a kind of sedge) is planted, its roots will spread far and wide, interlacing and creeping through the sandy soil, until in time the latter becomes solid and no longer drifts inland.

An allied species of grass, _Psamma arenaria_ (or marrem grass) grows abundantly at Bournemouth, and wishing to ascertain how far one of its underground stems extended, with some amount of patience I disinterred about six or seven feet of it in a bank on the sea-shore where it was accessible. As it seemed to have no end, I could not ascertain its entire length.

Another instance of root growth adapting itself to situations occurs to me. In visiting the Cheddar Cliffs in Somersetshire I was struck by the beauty of a plant which grew here and there out of the crevices of the rocks. Its tufts of vivid green leaves looked so healthy and vigorous I could not help wondering how it could obtain moisture enough to produce such foliage, placed as it was high up on the dry face of a rock.

Failing to reach its roots in any other way, I climbed up to a spot where I could remove some of the horizontal layers of stone. At last I lifted a flat piece of rock just above one of those plants, and there I saw at a glance the secret of its vigorous growth:

The roots had spread out over the surface of the stone for a distance of eight or nine inches in a perfectly flat layer of fine fibrous rootlets no thicker than a sheet of paper; these would doubtless suck up abundant moisture whenever the rain beat upon the rocks, and there, pressed closely between the two layers of stone the plant has its water-supply stored up, and is enabled to look fresh and green when other vegetation is suffering from drought.

In plant-life there is a marvellous variety in root-structure. Roots differ much, not only in form, but in texture and duration of life, so that to gain a true knowledge of them we must carefully examine those of herbs, shrubs, and trees, and observation will soon teach us the fact that there exists a close correlation between the form and texture of the root and the size and character of the plant. The external shape will depend principally upon whether a tap-root is developed or no. Such, for instance, as the carrot and the dock are those of the true tap-root character. Of branching roots we may find endless modifications amongst ordinary field or garden flowers from the fibrous roots of the little groundsel to the large fleshy tubers of the dahlia. Between these two types there are others of an intermediate kind, but it is possible to recognise amongst common plants the roots belonging to one or other of the types I have described. For the purposes of study we may broadly group roots into classes according to their method of collecting and absorbing food. Thus we find one group growing in soil and feeding upon the soluble earth salts and moisture of the soil. Another group will be found growing in water, like the water-lily and pond weeds. A third group simply hangs down in space from some perching plant like the tropical orchid, whilst a fourth and very small group consists of parasitic roots, of which a very common example is the mistletoe. We will now study each of these groups separately.

I have already spoken of some kinds of fibrous roots, and may add that if the root of a land plant is immersed in water, it will after a time develop a different kind of fibre, capable of receiving nourishment from water instead of earth. I remember seeing an instance of this in the case of a laurel bush which grew near a well in our garden. We had occasion to examine the water, and found that the laurel had thrown down its roots below the surface, where they grew luxuriantly, finely subdivided, of a delicate ivory white, owing to the absence of light, and more than a yard in length. They had adapted themselves to the duty of absorbing water only, but had we replanted them in earth they would have withered, from their unfitness to take up the hygroscopic water of which I have already spoken. On the other hand, if the seeds of a plant formed to live in the water, such, for instance, as the water-lily, are sown in ordinary soil, they adapt themselves to the new conditions, and are able to live on the hygroscopic water they find around the particles of earth.

Some plants send out a horizontal stem (culm) along the ground, with a bud and some roots growing out of it at regular intervals. Each of these joints (or nodes) takes root and forms a separate plant. What are called strawberry runners are stems of this kind, and so are the creeping stems of _Potentilla reptans_.

I once found a plant of the latter growing on a low wall, and, as I imagine, because it desired to reach the ground and root itself there, it had thrown down a stem a yard and a half long with eight young plants growing upon it at intervals ready to form so many colonies when they should reach the ground.

One may frequently find stems of various grasses running along the ground, and taking root at each joint. I have one such spray in my herbarium, with twelve young plants upon it at regular intervals.

Some plants store up nourishment in their roots, as may be seen in one of our common seaside grasses (_Poa bulbosa_); this soon withers after flowering, and becoming uprooted, its bulbs, which are like small round cheeses strung together, may be seen blowing about in the wind.

With such a provision as this, the parent plant is able to bear extremes of cold and drought.

It is well for us that plants have this power of storing up their food underground, for to it we owe such useful tubers as the potato and Jerusalem artichoke.

One of our native plants, the earth-nut (_Bunium flexuosum_), has a single round tuber which is eatable when roasted, and is often dug up by children. Long ago, when England was liable to famines, even this small tuber was valued as a means of eking out the labourer’s daily meal. It is worth while to examine the curious divided tubers of some of our common orchises, such as the spotted orchis (_O. maculata_), or the meadow orchis (_O. morio_). The tuber which produces the leaves and flowers withers away at the end of the summer, but it leaves behind it a second tuber in which is stored up the nourishment required to enable it to bring forth leaves and flowers in the following spring.

Tubers are in reality underground stems which have thickened into rounded balls to contain plant food.

If we examine a potato we shall see that it contains true buds in the little hollows on its surface; these are called “eyes,” and each of them if sown in the ground will produce a new potato plant. If a potato is left in a damp cellar, each of these eyes will send out a stem, thus proving that the “eye” has the nature of a bud. If we cut the potato in half we shall see it is of an even substance mainly composed of starch, but if we halve an onion it will be found to consist of rings or layers of a thick fleshy nature, which proves it to be a bulb and not a tuber. The onion is like a large bud growing underground, instead of on a tree branch. We can prove how similar the onion and the bud are, by searching on a lily stem for buds or bulbils, which are often produced in the axils of the leaves; if we plant such a bud it will throw out fibres and become a bulbous-rooted plant. Some of our native grasses seem to have a singular power of adapting themselves to their position. For instance, the common Timothy grass (_Phleum pratense_), which usually lives by means of a fibrous root, can, if needful, produce a bulb which enables it to keep living in a very dry place, but if removed to a wet soil it returns to a fibrous root. Other grasses have been observed to alter their root-growth in the same way, adapting themselves to their surroundings.

+Air Roots.+

These absorb the watery vapour of the air; they cannot adapt themselves to live in earth, but under certain conditions they can put forth other kinds of roots that are partially adapted for growing in soil.

I may here give some personal observations about a certain _Hoya_ plant that came into my possession so long ago as 1855. This muchenduring plant lived in a hanging basket for many years, in the dry air of a sitting-room. Its leaves were sometimes shrivelled from lack of water, and it never had vigour enough to produce flowers. At last, after enduring this life for twenty years, it was placed in a stove-house where the moist heat suited its requirements. Then it flowered charmingly, and even now is showing a further degree of enterprise by growing a bunch of fibrous roots at the end of one of its stems. I imagine it intends to plant itself into another pot standing near. I am watching it with much curiosity, because if it does this, the old plant will prove that it has a high degree of intelligence, and that although it remained quiescent for so many years, it was only from lack of opportunity to do more than quietly endure its privations.

In tropical countries, some plants and trees such as _Monstera_ and _Philodendron_ send down slender aerial roots called lianes, many hundred feet in length.

In the Aëroid House at Kew, I remember seeing these lianes coming down from the roof of the house in search of water and earthy nourishment. It seemed like actual intelligence that directed these roots to a tank of water twenty-five feet distant from their starting-point above. Whilst we are considering this subject, I may mention the curious root action of a kind of fig-tree growing in the tropics which is sometimes known by the name of the “Murderer.” Its seed often falls, or is dropped by birds, amongst the leaves in the head of a palm-tree, there it begins to grow and forms root after root, gradually descending the stem of the tree and clasping it so tightly that at last the palm is strangled and falls to the ground carrying its destroyer with it, where it roots and grows into a tree.

+Parasitic Roots.+

As in human society there are thievish characters who live by preying upon their neighbours, so in vegetable society we find quite a number of different plants growing at the expense of others, inserting their roots into the stems and roots of trees instead of drawing their nourishment from the ground. Careful distinction must be drawn between such plants as ivy, virginian creeper, clematis, lichens, &c., which simply grow and climb on the bark of trees, and the true parasites which are nourished by the juices of the trees and plants into which their routs penetrate.

Some plants are only partially parasitic, such as the cow-wheat (_Melampyrum_) and the yellow rattle (_Rhinanthus_). These represent a very deceitful kind of growth. To all appearance the plants are getting an honest living, the leaves are perfectly green and capable of performing all the duties of leaves, and yet, if we remove a little of the soil the plant will be found to be attached to, and growing from the roots of some strong kind of grass, and is deriving its nourishment from the food collected by those grass roots.

Yellow rattle grows abundantly in undrained marshy fields, where it is easy to obtain the plant, so as to examine its mode of growth. We may then go on to a clover-field and seek for that true parasite and most troublesome enemy to the farmer, the clover-dodder (_Cuscuta trifolii_). Its seeds are frequently mixed with the clover, and when sown they germinate on the surface, but the little thread-like stem, instead of entering the ground, feels about in the air until it reaches a young clover-plant. It soon clasps its victim with its fast-growing stem; as the clover grows the dodder coils around it and is carried away from the ground.

As the wiry stem gains strength, it developes a series of suckers that eat into the clover stem and rob it of the food it has collected; it lives, flowers, and grows at the other’s expense. The rate of growth of the dodder exceeds that of the clover, so that the latter is both exhausted and choked by its snake-like enemy.

I once sowed a patch of flax in a garden, and not knowing that it too had a parasitic enemy, I was greatly puzzled to find quantities of pinkish threads growing out of the flax stems. These threads bore round bunches of tiny flowers. All this was very pretty and interesting, but it resulted in my patch of flax becoming a mass of interlacing threads and dying a miserable death, fairly strangled by the flax dodder. Another species of _Cuscuta epilinum_, grows on furze and also on heather, it having the twine-like stems by which dodder may readily be known.

We are all familiar with the mistletoe, its leathery leaves and its white berries.

This plant grows out of the branches of poplar, hawthorn, and apple, and very occasionally upon the oak.

In France and Belgium the custom of bordering the fields with single rows of Lombardy poplars seems to favour the growth of mistletoe, for its large green bunches form quite a feature in the landscape, and cannot fail to be observed by the traveller as he journeys in the railway train. I have been told that mistletoe is sufficiently abundant to be used in Normandy as cattle-food.

If a mistletoe-berry is gently pressed upon a young branch of an apple-tree, its own viscid juice will cause it to adhere, and before long it germinates and sends its roots into the tissues of the tree; as it grows, it fuses with them, and derives all its root nourishment from the substances in the branch. Of course the tree is weakened by this parasite, the sucking roots of which disturb the flow of the sap; woody knots are apt to form, and not unfrequently the branch is killed by the intruder which has fastened upon it.

Having touched upon the four principal kinds of roots, we will now take a single root-fibre and examine it more closely. It seems scarcely possible that such a brittle, feeble thread should be able to penetrate into the ground and make its way amongst stones and sharp-edged fragments of earth without being bruised or torn. The chief friction is borne by the growing-point, and this always has for its protection a root-cap; the section of the growing-point of root-fibre given in the plate shows the outer skin, called the epidermis, and over that is the root-cap shaped like a thimble formed of small cells. As they are worn away outside and become dead tissue, owing to friction with the soil, the cells are constantly being renewed from within. The root is thus enabled to grow and perform its part in maintaining the life of the plant. The presence of this root-cap and the absence of leaves are the marks by which a true root is known and distinguished from an underground stem. With a small lens one can see this extinguisher-like cap protecting the extreme point of the root, and it is well to examine a variety of specimens, and see how they differ slightly in size and shape.

The one especial office of the root is to absorb liquid nourishment from the soil for the benefit of the plant, and, as I have already explained, this is done mainly through the hairs which grow upon the fibres of the roots. For instance, there is no absorption in old tree-roots, such as we sometimes see above the ground, nor in carrots, turnips, and parsnips, but thrown out from such bulbous plants are the fibres and their hairs which enable them to grow to maturity. We may naturally inquire how the solid materials in the soil, which are needful to the growth of the stem and leaves, can possibly be taken up by these extremely minute hairs.

We may look upon the earth as being a sort of storehouse of indigestible, unprepared plant-food which must be altered in its character before it will be fit for absorption by the roots. Some substances, such as sugar, will readily dissolve in water; others, such as starch and sand, are insoluble, but the effect of rain-water and atmospheric air passing through the soil, converts this insoluble dormant food into soluble active food.

The root-hairs convey this food to the small fibres, and through them as channels it passes on to larger ones, until it reaches the stem and goes to feed the growing leaves and flowers.

In order to remain in a healthy state, roots must absorb oxygen gas, and for this reason gardeners, when they find the soil growing caked and hard on the surface, dig and rake the flower-borders in order that air may freely permeate the soil and find access to the roots of the plants.

Roots appear to be endowed with certain remarkable attributes, about which learned books have been written of late, giving the result of patient investigation as to their power of movement, the way in which they are affected by gravitation, the influence of light, and other forces.

The experiments of Darwin and other scientists have revealed very singular facts about the movements of plants. The term used to describe their motion is one we must learn, as it frequently appears in botanical works. Circumnutation we may translate as wavering around, and it well describes the curious way in which rootlets, for instance, are always moving slowly from one side to the other, describing a kind of oval zig-zag track through the earth. The fibres appear to have a discriminating power, enabling them to choose convenient crevices through which to penetrate hard soil, to avoid stone, and to seek out any attractive food which lies in their way.

As soon as roots emerge from the seed they at once turn from the light and seek to bury themselves in the earth; the plumule from which the leaves will spring has exactly the reverse tendency, and invariably seeks the light and grows upwards. This can be proved by growing some mustard seeds on a piece of flannel about the size of a shilling, floating it on water in a saucer exposed to light from a single window; as soon as the leaves appear, they will lean towards the light, whilst the roots will point towards the dark part of the room. If a germinating seed is even placed with the root uppermost, and the plumule pointing downwards, it will very speedily right itself, the stem will turn and grow up, and the root will seek the ground.

The amazing strength of growing tree roots can be imagined when we watch a tree in full leaf during a high wind. As the terrific force of the gale sways the trunk backwards and forwards the roots are subjected to an enormous strain. Like great india-rubber cables they give and retract, and when the wind subsides we find the trunk as rigid as ever.

If my readers will seek for the specimens enumerated below, and compare them with the remarks made in this chapter, they will have such a general idea of the functions of roots as will, I trust, enable them to enjoy the study of more advanced works upon the subject.

Specimens to be obtained and compared with the descriptions in this chapter:—Sedge or marrem grass growing on a sandy sea coast; plants growing between layers of stone; tree roots at the edge of a pond; strawberry runners; a plant of _Potentilla reptans_; creeping grasses; _Poa bulbosa_ roots from the seaside; potato. Earth nuts; lily bulbils; Timothy grass; cow-wheat (_Melampyrum_); yellow rattle (_Rhinanthus_); clover dodder (_Cuscuta trifolii_); flax dodder (_Cuscuta epilinum_); mistletoe. Root fibres of various plants. Mustard seed sown on flannel.

CHAPTER III

_TREE STEMS_

“If thou art worn and hard beset
With sorrows, that thou wouldst forget,
If thou wouldst read a lesson, that will keep
Thy heart from fainting and thy soul from sleep,
Go to the woods and hills! No tears
Dim the sweet look that Nature wears.”

+Longfellow.+