When the axis of a bud expands to form a new length of, the fallen bud-scales leave ring scars, or girdle scars, at its base. Thus the girdle scars mark the beginning of a new year’s length of branch and the interval between two sets of scars indicates the amount of growth in any one year. In the older parts of a tree the scars have been shed with the bark, but as long as they persist on a branch its age can be determined by counting the sets of girdle scars.
Girdle scars are very plainly marked on Beech twigs, because here, also, the bud is protected by a large number of paired scale-, some of which are completely circular at the base.
The simple leaves of the Beech grow alternately on the slender twig – therefore the axillary buds and the branches to which they give rise are also alternate. The branching is, moreover, generally sympodial, as in so many of our trees, in the Lime, for instance, and Elm and Hazel.
The reason for this sympodial growth is found in the behaviour of the terminal bud. Very frequently this does not develop properly, but shrivels in the early spring almost as soon as it is formed. It is then the work of the axillary bud immediately below the useless terminal bud to function as a terminal bud and continue the elongation of the. In such a case each new length of stem is always a lateral branch of the axis that precedes it. The slight angularities disappear and the branch appears monopodial.
Both Beech and Lime buds are easily dissected, for their scales are free from resin. There is, however, special protection inside these buds, for most of the foliage leaves have two enveloping attendant scales, called stipules. In the Beech the two or three leaves that surround the tip of the axis are generally not so protected. When the bud expands and the scale-leaves drop off, the stipules still persist, although their work is over. For two or three weeks they remain somewhat loosely attached at the base of the leafstalk. Then they, too, fall.
In Beech and Lime it is clear that the protecting scale-leaves of the buds are stipules which have no related foliage leaves. In the Horse Chestnut and Sycamore the b u d – scales represent t h e bases of foliage leaves. Frequently in the Horse Chestnut the innermost scales are fluffy at the tip, and slightly divided. This morphological character of bud-scales is very clear in the buds of the Flowering Currant, but the gradual transition from bud-scale to foliageis seen at its best in the Bird Cherry. The small red bumps on the -stalks of Cherry trees are extra-floral nectaries. Probably they prevent marauding insects from visiting the . Ants, which would be of little use in bringing about , are satisfied with the honey provided on the vegetative parts and do not enter the flowers.
A study of various leaf-buds is a wonderful lesson in packing. In Beech and Sycamore the individual leaves are folded backwards and forwards, just as we all made paper fans when we were very young. In the Lime the leaf is folded Along the midrib like the cover of a book. In the Plum the blade curls round like a paper spill. In the Apple and Pear the edges curl in towards the midrib of the leaf.
The stem bears its leaves in such a way that they are well placed in relation to light and air. Furthermore, it is the indirect means of supplying them with mineral and Bast Solutions. It is also along the stem that manufactured foods, as distinct from raw materials, travel. There are two separate transport systems in the stem. The raw materials travel in the vessels of the wood, or xylem – the manufactured food is carried by the bast, or phloem. There is no possibility of collision, for the systems are distinct one from another.
The course followed by the raw materials may be seen by allowing the base of a Brussels Sprout to stand in red ink for a day. When the stained base is sliced away the new basal surface is white with, perhaps, a ring of red dots or, more probably, a complete red circle.
A longitudinal section of the bud shows that the apparent dots are the cut ends of long red strands, one appearing on either side of the section. There is just the same appearance in the longitudinal section if the strands are united into a complete ring.
Microscopic examination shows the strands to be made up of numbers of hollow cylinders, placed end to end, whose walls are of wood. It is along these wood vessels that solutions travel to the leaves and along these only. No other part of the axis is stained red. On piecing together the evidence gained from the two sections, it is clear that the strands of wood are disposed in the axis either in an interrupted or in a connected circle of vascular bundles.
This experiment should now be repeated with an elongated stem. The stem of a, or , or Hollyhock gives the same result.
If the cut end of such a stem or, better still, a prepared microscope section be examined with a hand lens, the more intimate structure can be seen.
A transverse section of the stem of the Sunflower, stained and mounted on a slide, shows three regions in the stem. There is : (1) The ring of vascular bundles – (a) The pith that these enclose – and (3) The cortex that surrounds them which is bounded by the epidermis.
The cut ends of the wood vessels of the bundles are oval or round spaces which vary very much in size, but which agree in having thick and deeply stained walls. Beyond each group of vessels there is a roundish mass of cells quite different in shape and arrangement. These make up the bast, or phloem, which conducts the manufactured food.
Between the wood and the bast there is a layer of extremely delicate cells, the cambium. These cannot be seen with the hand lens, but it is entirely due to their very active division that the stem increases in thickness. The rays of tissue that separate the bundles, extending from the pith to the cortex, are the medullary rays.
Examination of a section of a Lime twig shows a more compact arrangement of the cells, because the activity of the cambium has given rise to a very great increase of tissue in the stem, particularly in the wood.
There is no longer an interrupted circle of vascular bundles. The wood vessels are arranged round the pith in unbroken rings: one, two, or three rings according to the age of the twig. The largest vessels of each year’s growth are towards the centre. These were made by the cambium in springtime when sap was rising vigorously. Later in the year smaller vessels answer the purpose. In autumn the vessel cavities are very small. In winter the activity of the cambium stops dead. It rests until the following spring, when it again takes up its work. The early-formed spring vessels have wide cavities, and it is the abrupt change, as seen in section, from the small cavities of the autumn wood to the large cavities of the spring wood that marks off the stem into the well-knownrings.
The new bast that the cambium forms each year lies towards the outside of the stem. Beyond it is the cortex. Between cortex and epidermis there are now two or three layers of cork. Slender medullary rays glance through the section like darts of varying length. They, too, have been formed by the cambium.
All the tissues of the stem are continuous with similar tissues of the. A different arrangement of the wood and bast gives a young root a characteristic appearance, but sections of old and are practically identical.
The cork layer is impermeable to all solutions, and so all tissue outside the cork dies. Lenticels are patches of more loosely packed cork cells – they allow the oxygen necessary for respiration to pass in and the carbon dioxide to pass out. The green alga, Pleurococcus, that grows so commonly on tree trunks, occurs most densely around lenticels – apparently it benefits by the excess of carbon dioxide that is escaping at these points.
Each year a new cork layer is formed some little distance within the old. Thus the layer of tissue outside the cork, dead because it is cut off from the plant’s food and water supply, becomes each year a little thicker. This dead layer is the bark of the tree.
Similar cork formation heals wounds caused by broken-off branches or cut initials. Owing to the formation of successive layers of cork the initials on a Beech trunk, for instance, appear more and more sunken as the years pass.
Certain details of internal stem structure are seen in the knots of wood in floor planks. A plank is a longitudinal section of a tree trunk. The knot is a section of a branch growing out from it.
In the Botanical Section of the Natural History Museum, South Kensington, at the top of the stairs to the left, are some excellent sections of tree trunks. They are so highly polished that many details of structure can be seen. Moreover, the rings of growth are not only dated, but the dates are related to certain events of historical interest : a most impressive way of giving the realisation of age. Many provincial museums have similar exhibits.
The bast, as seen in section, forms a continuous layer on the outside of the stem, just underneath the bark.
If a ring 2 inches wide is cut round the main stem of a young woody plant so that all tissues outside the wood are removed, the transport service of the bast is at an end. The food made by the leaves travels as far as the cut, but can go no farther. The downward pressure of the food solutions makes a circular bulge at this level. This means that the root is deprived of food just as effectually as in the root-pressure experiment, where the whole leafy crown of the Fuchsia had been removed. As a result of food shortage the root dies, and the rest of the plant shares its fate.
By such a destructive experiment it may be proved that manufactured foods really do travel along the bast. In the country a young tree growing in a tangled thicket may be experimented upon, because a little thinning out is all to the good. Thickets, however, are not commonly available and recourse must be had to a potted sapling of Lime, or Beech, or Maple. A Fuchsia is not much use in this case. It drops its leaves so readily that the experiment, for this reason, is not sufficiently conclusive.
If the stem of Indian Corn, or of a Tulip,, or flower be put into red ink and left for a day or two, the coloration in the cut stem is seen to be different from that of the Brussels Sprout. The red dots, indicating vascular bundles, are scattered without plan against the background of stem-tissue.
A prepared slide of Indian Corn, examined with a hand lens, shows that each bundle has two large wood vessels, a little like motor goggles – two or three small vessels in a row point to the centre of the stem – beyond the two large vessels lies a mass of bast.
In these bundles there is no cambium. Therefore even in perennial plants of this class, except in some few anomalous, there is no annual secondary growth in thickness. Each year, up to a certain period, the growing point of the stem develops an increasing number of bundles, so that each year the stem is thicker towards the apex. As the lower part of the stem is still slender this is a top-heavy arrangement. To ensure stability adventitious grow in numbers from the lower part of stem or trunk. It is these prop-roots that, like the guy ropes of a tent, keep the axis erect. Such roots may be seen growing on Palm trunks in any tropical house or in the public gardens of the South of England.
It is because of the absence of secondary growth that plants of this class (Monocotyledons) are useless for timber. The most that can be done with them is seen in the not entirely satisfactory hat-racks and tea-tables of Bamboo.
Among stems there is great variety, both in external form and in habit of growth. Some are always underground and are called roots by the uninitiated. Of those that are above ground, or aerial, some have no power of maintaining an independent, upright. They have, therefore, no alternative but to creep or climb.
The description of the different forms of stems will be taken in the order of the following table of classification:
The different underground stems are all adapted for storing food. The surplus food that is made in the green foliage leaves travels down to the rhizome, tuber, corm, or bulb, as the case may be. In this organ it is held in reserve in some carbohydrate form, such as starch, sugar, inulin, or as protein.
The underground stem is dormant for a considerable period of the year. It bears buds from which, at the proper season, stems, leaves, and flowers grow upwards. These owe their rapid growth to the available food store which has been held in readiness for them in the underground stem, and which is changed into some soluble form, so that it may travel.
The new foliage leaves then make food, which is stored as in the preceding year. Thus a perennating organ is reformed to carry on the growth in each succeeding year.
Shoots that develop from some of the axillary buds break away from the parent and live independently. When new plants are formed in this way, without the intervention of, the process is known as vegetative .
A typical rhizome is that of the. It is a stout stem that grows horizontally in the ground. Its stem-like character is seen in the fact that near the growing point it bears erect, sword-like foliage leaves. They are attached to the stem by such a spreading base that they bestride it, as a rider bestrides a horse. It is for this reason that they are said to be equitant. The whole length of the rhizome behind the tip is marked with rings which are the scars of withered leaves. The dots along their course are the scars of the veins of the leaves.
As they are stems, rhizomes cannot absorb solutions from the soil. Adventitious roots grow from their under surfaces to carry out this work and to help in anchorage.
The tip of the rhizome forsakes the horizontal direction of growth to turn upwards and carry the foliage leaves and flowers above the soil. Thus the direct onward growth of one year’s length of stem comes abruptly to an end. The continuation of horizontal growth is made possible by the development of an axillary bud, a little way behind the growing point before this makes its upward curve. This axillary bud gives rise to a new length of stem that is a branch of the old one, and therhizome, like the Beech branch, grows sympodially.
Other axillary buds grow out into lateral branches. As the rhizome grows forward the hinder part rots away. This causes the lateral branches to become independent individuals. Vegetative propagation has taken place, and there are now two or three plants in place of the original parent.
There is no doubt as to the onward growth of the rhizome, for each year the Iris flowers in the garden are in a slightly different position. Each flowering stem is a little nearer the house, or a little farther from it, than it was the year before. The rhizomes may even creep under the hedge into a neighbour’s garden if the tip is turned in that direction.
Almost all that has been written about the rhizome of the Iris applies to that of Solomon’s Seal. There is, however, this difference. The round scar, or seal, on this rhizome is left by the decay of a whole shoot bearing foliage leaves and flowers.
In each rhizome the constrictions of the stem delimit the growth of one year from that of the next.
Other sympodial rhizomes are the Couchgrass, Lily of the Valley, Mint, and White Deadnettle. The rhizome of Wood-sorrel is monopodial.
In many plants that have a sturdy tap-root, the leaves appear to grow from the root. The delicate leaves of the Carrot and the large leaves of the Turnip grow from the top of Carrot and Turnip alike. But this top is stem, not root. It is a short, thick, vertical rhizome.
The rosette of Dandelion leaves grows from a similar, but slender, vertical rhizome. As there is a slight elongation of this stem, year after year, it would seem natural for the rosette of leaves to rise, by degrees, above the ground-level. This is prevented by the contraction of the lateral roots. The cortical cells of these branch roots shorten, so that the roots themselves become shorter and thicker and draw the whole root system more deeply into the soil – in this way the rosette of leaves still rests upon the surface.
The adventitious roots of rhizomes also contract and counteract the tendency of the whole stem to follow the upward direction of the growing point.
Artichoke begins as a rhizome and ends as a tuber. This is a change that puts an end to any further onward growth, either monopodially or sympodially.
The eyes scooped out in peeling a potato are axillary buds. When a potato sprouts, these buds are shooting. A bud of the tuber sends a leafy stem upwards, and from the base of this stem adventitious roots grow down into the soil.
Many new potato plants are propagated vegetatively, in this way, from one tuber.
The Potato plant bears foliage leaves of two kinds. Those near the ground-level are small and simple. The others are pinnately compound with a varying number of leaflets. The buds in the axils of the compound leaves produce ordinary aerial shoots. From those in the axils of the simple leaves shoots develop that grow downwards into the soil as slender rhizomes, bearing scale-leaves of a triangular shape. Food manufactured by the compound leaves is stored in the underground shoots. These swell greatly to accommodate the reserve supply, and, in swelling, become tubers.
On the Potato the spreading bases of withered triangular scale-leaves have left semilunar scars. These border depressions in which the axillary buds lie. The scars all have the same arrangement with regard to the apex of the tuber. At one end a small roughish scar marks the spot at which the swollen tuber broke away from the rhizome.
The structure of a bulb is more complex than that of a rhizome or tuber.
Thebulb is so regular in its growth that, in spite of its small size, its study gives a very clear understanding of bulb structure.
The scaly wrappings on the outside of the bulb in the autumn should be carefully removed and arranged in a horizontal row. They are really four in number, but the first may come off in pieces or it may be absent because of the rub on the bulb in its various packings and journeys. If it is intact it is completely circular. The second wrapping resembles it. The third is semicir. Cular . The fourth is merely a narrow strip.
The thick white wrappings that follow are identical in number and in shape with the scaly ones. They should be so arranged in a row that each fleshy cover is exactly under the scale to which it corresponds. Those that are circular can only be satisfactorily removed by making a vertical cut downwards, followed by a circular cut around the base. In this second set of structures the narrow strip is tucked inside the incurving edges of the third wrapping.
The removal of the structures shows that they grow on a very short stem, therefore they are leaves. At least they are bases of leaves, because a shrivelled, irregular tip indicates that something has died away at this point.
A dissection of the bud, that now stands alone on the short stem, gives the clue to the morphology of the bulb.
This bud is wrapped up in one bud-scale which is completely circular. Two foliage leaves follow, and a good dissection shows that the first of these has a completely circular base, while the base of the second is ‘semicircular. The Snowdrop flower, enclosed in its bract, is in the centre of the bud.
If these four parts of the bud are arranged to form a third horizontal row, their relationship to the members of the first and second row is clear. Evidently the first thick circular wrapping is the scale-leaf that protected the bud of a year ago. The next two are the bases of the green foliage leaves. The narrow strip is all that remains of last season’s flower stalk.
The scales are similar structures, but a year older still. A year ago they, too, contained a reserve store of food, which they gave up to the leaves and flowers that grew in the spring. If the bulb that is being examined had not been destroyed, the present white fleshy leaf-bases would have been similarly depleted, so that they in their turn would have become thin and brown and scaly.
In the spring, when the bulb is in the soil, adventitious roots grow from the base of the short stem. The water they absorb causes the expansion and growth of the bud-structures, and the whole bud pushes upwards. The bud-scale splits. The leaves elongate and turn green.
The flower stalk lengthens and the flower expands. As long as they live the green foliage leaves are making food. In a soluble form this passes down to the spreading leaf-bases, where it is stored. As a result the bases of the leaves become larger and larger, and when, finally, the green blades above the ground die, the bases remain below the ground as white fleshy structures with a dead and shrivelled tip. Thus the bulb, year after year, is renewed from the centre.
Gardens are apt to look untidy when the flowering time of bulbs is past, because, if the bulbs are to be any good at all next year, the leaves must not be cut off until they have lost the power of manufacturing food. If bulbs grow on a lawn, for the same reason, the lawn cannot be mown until the work of the leaves is at an end.
In the dissection of the bulb axillary buds are found in the axils of both scaly and fleshy leaf-bases. They grow as the central bud does, and in time form daughter-bulbs. Later, owing to the casting-off of old leaf-bases, they are set free as separate individuals. This is another case of vegetative propagation.
The mathematical regularity of the Snowdrop bulb depends upon the fact that, each year, two foliage leaves grow from the central bud. In the Daffodil there is no constant number, therefore the number of leaf-bases varies in different bulbs.
The structure of the Tulip bulb is different. As its foliage leaves grow on an elongated stem, well above the ground, it is plain that they cannot contribute to the renewal of the bulb directly. They do so indirectly, however, by sending the food they make down through the stem to certain short, modified underground leaves, whose work it is, not only to store reserve supplies but, at the proper season, to pass it on to the central bud of the bulb. When this develops, its foliage leaves in their turn make food, which is stored in the modified underground leaves and so a new bulb forms, year after year.
To define a bulb as an underground stem is not altogether satisfactory because it is largely made up of leaf-structures. To call it an underground bud is even more open to criticism. A bud is essentially something new – but most of the parts that make up the bulb are essentially old. They are either the bases of foliage leaves that grew in a time now past or, like the scales of the Horse Chestnut bud, leaf-bases that have never been more than bases.
This must be indicated in the definition. A bulb, then, is a perennating organ that is made up of a number of leaf-bases that grow on a much abbreviated stem and surround a central bud.
Although there are many points in which a bulb differs from a bud such as that of the Horse Chestnut, there are, at the same time, many fundamental points in which they agree.
In both there is an extremely short axis, bearing overlapping leaves – in the bulb this axis is permanently stunted – in the bud it elongates to form a new branch.
The leaves grow extremely close together, so that each internode, that is, the space between the insertion of one leaf and the next, is very short – in the bud the internodes elongate with the elongation of the axis.
In bulb and bud alike axillary buds are produced in the angle between leaf and stem.
To sum up : The bud is an undeveloped branch which only exists by virtue of its attachment to the parent plant. The bulb is a modified branch that contains the bases of foliage leaves that grew in previous years, as well as the leaves and flower of the year that is to come. The bulb is an entirely independent individual, living its own independent life. It is a perennating organ that, by storing supplies of reserve food, makes the growth of new aerial parts possible in each succeeding year.
In a corm, as in the tuber and the rhizome, the surplus food made by the foliage leaves is stored up in the stem, which in this case is round and globular.
On this stem, as is seen in thecorm in autumn, there are a number of fibrous coverings. As these grow on a stem, and as they have buds in their axils, they are evidently leaves. In shape they are all circular. Irregular circular scars persist on the stern when they are removed. The axillary buds are just a little above the level of the scars – evidently the stem has elongated somewhat since their formation.
Two or three larger buds grow at the top of the corm. Each is surrounded at the base by fibrous structures. These are small. They are smoother, too, and paler in colour than those already removed.
It is a good plan to cut away the corm on four sides, so that the largest bud rests on a square pedestal of stem. The numerous bud-scales should be removed with care, so that no other part of the bud is broken in the process. When they are arranged in a row they are seen to approximate in number to the fibrous layers that covered the corm. Like these, too, they are all circular. The little stem, moreover, from which they have been removed, is round and globular – it is, in fact, a corm in miniature .
It is now evident that, apart from the difference in age, the young bud-scales and the old circular fibrous wrappings are one and the same. A year ago the latter were white and living and enclosed the flowers and leaves.
When the foliage leaves of the bud are examined it is seen that they expand a little at the base. It is in their axils that the large buds develop. It is their spreading bases that persist a year later as the little brown frills surrounding the bases of the buds that grow at the top of the corm.
The mode of growth is like that of the bulb. Adventitious roots grow from a stumpy round disk at the base of the corm and, by absorbing water, give the stimulus for the expansion of the buds. The wood vessels up which the water passes to reach the buds are seen as yellow strands in the white bulk of the corm . The bud elongates and the scale-leaves unfold. The pale foliage leaves grow and become green in the light.
The parent corm decreases in size because of the depletion of its food store. The daughter corm increases in size because it is, in its turn, storing food made by the foliage leaves. Thus the new corm is formed above the old one, and it is only the contraction of the adventitious roots that keeps it from rising above the level of the soil.
The old corm diminishes to such a degree that in the following year it is nothing more than a stumpy disk from which spring the adventitious roots .
An unusual method of vegetative propagation occurs in the Lesser Celandine. Some of the axillary buds, instead of BulbiIs developing into ordinary branches, produce an adventitious root. Food manufactured by the foliage leaves is stored in this. The adventitious root then enlarges out of all proportion, so that finally the structure is almost all root, the bud part being scarcely recognisable .
These curious bulbils separate from the parent and give rise to new and independent plants. In appearance they resemble exceedingly small new potatoes. Often they are washed from the parent plant in numbers by a heavy shower and lie in collections on the slope of a Celandine bank. In many country districts they are, for this reason, spoken of as potato rain.
The upright growth of stems is a tropic response to the stimulus of gravity. Stems are nega- tively geotropic. They are able to maintain the erect position partly because they are provided with internal mechanical, or strengthening, tissue, such as the vessels of the wood – partly, also, because the individual living cells are extremely turgid, that is, so full of cell-sap that the walls are extended to their utmost capacity. The result is that all the living tissues are taut.
In spite of possessing these apparent advantages, certain shoots forsake the upright position and creep along the ground. Contact with the soil induces the creeping stem to put out adventitious roots, which always grow downwards from the nodes, that is, from those parts of the stem where the leaves arise. As the buds in the leaf axils at the same time develop into upward-growing shoots, the result is that a number of young plants are formed, attached by slender stems to each other and to the parent plant. Rotting of the connecting stems brings about the separation of the daughter plants, which thus have a vegetative origin.
This formation of runners occurs in Creeping Jenny and Ground, in both of which the growth is monopodial.
When the runner is sympodial, as it is in theand Cinquefoil, certain individual differences obtain . The growing point turns upward to carry the leafy flowering shoot above the ground. Where the stem curves adventitious roots grow down into the soil. A bud in the axil of one of the leaves develops into a shoot that grows forward along the ground. This is a branch of the original runner. In due season its growing point turns upward – an axillary bud continues the onward growth, and so a long, sympodial runner, often branching freely, spreads over the surface of the soil. By the severing of the connecting stems, which bear small scale-leaves, a number of daughter plants break away from the original parent.
By means of their adventitious roots such plants get a ready supply of water and stake a claim by covering a considerable area of ground, thus interfering with the growth of would-be rivals. They are, however, often at a disadvantage with regard to light and air.
Other weak-stemmed plants overcome this disadvantage by adopting the climbing habit, and making use of sturdy neighbours to help them to raise their stems and leaves well above the undergrowth. They are generally extraordinarily successful and really gain the light that is their object.
Pink flowers of the Blackberry have been seen forming a crown to a high Elm, and the hairy fruits of Travellers’ Joy (Old Man’s Beard) hang from many high trees in the autumn hedgerows.
Various devices adopted by plants have led to their success as climbers.
In some cases the main stem twists round a support . This support may be nothing more than a thin string fixed for a Runner Bean, or it may be a sturdy forest tree, round which the Honeysuckle twines.
The Honeysuckle is the only British plant with a twining stem that can encircle a tree. There are many such climbers in South America – thick-stemmed lianes that twine around trunks and branches often exert so much pressure, that the limb of the tree becomes malformed as a result of the support it gives. The curious spiral walking-sticks sometimes seen are branches from which such a liane has been cut away.
A twining stem gets a hold on its support because of inequality in its growth. At first the stem is erect, as seen in the Convolvulus, the tiresome bindweed of the garden. Then the upper part waves out practically at a right angle from the vertical. Growth is more rapid on one side of the stem than on the other. The inequality of growth is an irritable movement, or circumnutation, induced by the force of gravity. The greater growth of the one side produces a curve in the stem, but only in its young growing region. When this region comes in con- tact with a support the inequality of growth on the two sides of the stem induces further curvature, with the result that the climber clasps the support and twines around it as long as growth persists.
The turn of the spiral made by a twining stem depends upon which side grows the more rapidly. Honeysuckle, Hop, and Black Bryony twine in a clockwise direction. The turn of the spiral in the Runner Bean and Convolvulus is counter clockwise.
In other cases plants climb by means of modified lateral appendages. In some plants, as inand Garden Nasturtium, the appendage so modified is the leaf-stalk, or petiok . In others, as in the Vine and , the climbing organs are tendrils. The grip of both petiole and tendril on the support is governed, as in the case of twining stems, by inequality of growth on the two sides.
In the Sweet Pea the climbing habit has led to a sacrifice of some of the leaflets of the compound leaf. All but the lowest pair have been modified to form tendrils.
For this reason the tendrils are in pairs and an odd one terminates the leaf.
As leaves are the great food-manufacturing centres of plants, this reduction in their number is a very real sacrifice.
By way of compensation there are wings, or flaps, on either side of the stem, and these carry on the work of carbon assimilation which is the normal function of foliage leaves.
In Vetches, where the number of leaflets is great and the number of tendrils few, there are no assimilating wings on the stem, because the leaflets are present in sufficient number to make all the food the plant requires.
At the base of the compound leaf of the Sweet Pea, near the axil, are two stipules which enclosed the leaf when it was very young.
These stipules also compensate to some extent for the loss of assimilating area, as, unlike the stipules of Beech and Lime, they remain in permanent con- nection with the foliage leaf and are actively engaged in the manufacture of food .
The tendrils of the Vine are morphologically distinct from those of the Sweet Pea. A Vine tendril always occurs opposite a foliage leaf, as do the clusters of Vine flowers and, of course, the bunches of grapes that follow the flowers. Judging, then, by analogy, the Vine tendril is a branch.
It has been suggested that the tendril is really a continuation of the main axis, and that it has been pushed to one side by a lateral branch which developed in the axil of a foliage leaf and then grew vertically onwards. If this be so, the growth is sympodial, as in Beech and Lime. It is, however, a difficult question, and other views as to the morphology of the tendril have been advanced.
In Ampelopsis, as in the Vine, the foliage leaves grow alternately on the stem. Opposite each leaf is a modified shoot, which is much more branched than the Vine tendril. The little branches end in adhesive disks which cling to flat surfaces. Where these structures occur on the stem the opposite leaf has no axillary bud. Where there are no disks a bud, or a leafy branch that has grown from a bud, is present in the leaf-axil . If the flattened disks meet no support, the individual branches curl, tendril-wise.
In the White Bryony the tendrils are unbranched. Furthermore, they do not merely curl where they are in contact with a support. The stimulus to curve is transmitted from the apex along the tendril. As the tendril is now fixed at two points, not only is a direct corkscrew spiral produced, but the part of the tendril attached to the parent plant turns in the opposite direction. Thus one half of the tendril coils to the right, the other to the left. Between these two parts is a neutral length . The regular coiling draws the climber nearer to the support. The numerous coils and the reversed spiral allow for considerable play, so that in a wind the White Bryony may sway considerably, but the tendrils do not break.
In Smilax two tendrils occur at the base of each foliage leaf . In this case they are modified stipules.
The Ivy climbs by means of adventitious roots which grow from the shaded side of the stem in stiff, brush-like. The sole function of these roots is to cling, and for this purpose they secrete a sticky fluid. They absorb no food, therefore the Ivy is in no way parasitic. It does, however, kill the tree up which it climbs, because it effectually blocks the lenticels of trunk and branches. The more freely the Ivy grows, the greater is the area of the tree cut off from access to oxygen. Eventually the tree loses in the unequal struggle and dies from strangulation.
The hooks of scrambling climbers of the bramble type are outgrowths of the epidermis. In thethe hook is very readily separated from the stem. Its removal affects nothing more than the external skin, and a layer of cork quickly heals the slight damage caused. Hooks are found on the underside of the midribs of the compound leaves as well as on the stem .
In such scramblers the growing tips of the shoots follow an intricate path, so that the stems weave in and out among the branches of the support. The hooks are slightly curved in most species ofand this gives the weak stem a definite advantage. The hold gained by the combination of the interweaving, together with the form of the epidermal hooks, adds greatly to the gardener’s work in ramblers.
It is by means of epidermal outgrowths that the Goose Grass, or Cleavers, climbs. These are very numerous and occur as decurved hairs covering the surface of stems and leaves alike .
When, Geraniums, and similar plants grow in windows they are quite one-sided. The part of the plant that is towards the room appears all stems, and these are not erect, but inclined towards the window. The leaves all turn to the light. The sides of the stems towards the room, that is, the sides in the shade, grow more quickly than those that are in the full light, hence the inclination of the axes. Light has had a retarding influence upon the growth of the sides of the stems that face the window.
This retarding influence of light is well seen when two Broad Beans are allowed to germinate, one in a window, the other in a dark room or cupboard.
The latter grows much more rapidly than the former, but this is not the only difference between the two plants. Thegrown in the dark is whitish, because the presence of chlorophyll depends upon light. Apart from this the plant in the dark shows other definite peculiarities. Its rapidly growing stem is very weak. The intemodes are unusually long. The leaves are unusually small. Such plants are said to be etiolated.
Certain characters associated with the climbing habit in plants resemble those of etiolated plants.
Climbing plants have weak stems that grow rapidly, with the result that the nodes are far apart. At first sight the leaves do not appear to be particularly small. When, however, they are contrasted with those borne on erect stems their small size is apparent. Climbing, Clematis, and Convolvulus never bear leaves comparable in size to the large radical leaves of the Foxglove, or to the stem-leaves (cauline leaves) of , , Lime, Horse Chestnut, or Ash.
In allthe root is relieved of a large part of its work. The climber depends for its support upon a sturdy neighbour. For this reason there is seldom any great development of the root system in climbers. Climbing Roses, such as the very freely growing American Pillar and Jersey Beauty, have roots that branch very sparsely in comparison with the spread of the aerial parts of the plant.
It is the growing region of a root that responds to the stimulus of moisture, of light, and of gravity . Judging by analogy, the revolving movement of the tips of a twining stem suggests that here, too, the growing region may be just behind the apex. That this is so is proved by marking off equal intervals along a stem near its tip. This is not so simple a matter as in the root, because young leaves fold over the growing point. The lines, a centimetre apart, should be made with Indian ink behind the tip of a young Broad Bean or Sunflower stem, both of which grow rapidly. In the case of the bean, growth is found to take place along a distance of about five centimetres, the region of most active growth being about I.5 centimetres behind the apex.
Generally speaking, the response that main stems make to the stimulus of light and gravity is diametrically opposite to that of main roots. No matter in what position soaked beans are placed between gas-jar and blotting-paper, all the shoots grow upwards. When a young plant is inverted, the stem makes a curve in the growing region and grows vertically upwards.
The main stem, then, is negatively geotropic. Its different orders of branches, like the branches of roots, take various positions in relation to the earth’s surface. Branches of the first order are often diageotropic, like creeping stems which grow parallel to the surface of the soil.
Aerial stems are positively heliotropic. To show that this is so, one of the beanshould be potted and trans- ferred to a box that stands on end and is completely covered with black paper. A sheet of black paper, fastened with drawing-pins, replaces the lid. A hole, half an inch square, is cut in this covering paper, near the top. To do away with the necessity of , the plant-pot should stand in a saucer of water.
The box is then placed with the paper cover facing a window, so that rays of light enter through the square hole. The bean stem grows upwards, but not vertically so, for the growing region inclines more and more to the rays of light, and eventually the leafy tip of the stem appears in the aperture. A day or two later it has escaped from the darkened box and pushed its way through to the full light of day.
As regards the extent of their elongation, stems fall into one of two classes. A growing point may add to the length of the stem each year. Such a stem is said to be of unlimited, or indefinite, growth. In other stems the activity of the growing point stops abruptly at the end of a short period of growth, so that a dwarf branch, or branch of limited growth, is produced.
Dwarf branches serve different purposes. The special function of certain dwarf branches is to bear fruit.
All children in our country have, at one time or another, hung paired Cherries over their ears.
The short brown stem to which the two Cherry stalks are attached is a dwarf, or fruiting, branch of the Cherry tree.
A cluster of Black, White, or Red Currants grows from a smaller but similar dwarf branch. In topping and tailing Gooseberries the brown, shrivelled calyx is cut off from one end, but very often it is more than just the old flower stalk that is snipped off from the other – the presence of a tiny bud or of a leaf shows it to be a much abbreviated branch.
InApple and Pear trees it is the long vegetative branches that are cut off in late autumn. The gnarled branches are left, because the gnarls are dwarf branches, which will bear the Apple or Pear blossom in the spring and the fruits in summer and autumn.
In these cases, once a dwarf branch always a dwarf branch holds good.
In the Beech this is not so.
A branch that has borne Beech nuts for years may forsake what appears to be its fixed habit and grow indefinitely for several years in succession.
After this it may again revert to fruit-bearing .
The circular lines on all fruit branches are, of course, the scars of bud-scales that have been shed year after year. The growth in length of the stem each year has been so very slight that the girdle scars of one year almost touch those of the next, so that an almost continuous series is formed .
In many cases dwarf branches are developed to serve an entirely different purpose, as, for instance, to make up for reduction in leaf-surface.
In no plant is this more clearly seen than in the Butcher’s Broom. Its leaves are reduced to mere scales on the branches of unlimited growth. Some of the buds that occur in their axils grow out into other branches of unlimited growth. But the greater number of the axillary buds develop into dwarf branches that are leaflike in shape .
Butcher’s Broom is a xerophyte. It is a plant that grows naturally in dry situations, and the reduction of its leaves is a protective measure that prevents the plant from giving off too much water, because this giving off of water, or transpiration, takes place from foliage leaves.
The reduction of leaves, however, is attended with great disadvantages, because leaves also make the food for the plant. This difficulty is got over by the dwarf branches taking on the important work of carbon assimilation.
There is no doubt as to the branch nature of these structures. Not only have they developed from axillary buds, but each itself bears a scale-leaf on its central line. In the axil of this minute leaf a flower grows, followed later in the year by a bright red berry.
Such a dwarf branch as this, superficially resembling a leaf, is a dadode, or phylloclade.
The numerous needle-like outgrowths of the aerial shoots of the Asparagus are also phylloclades . They arise, crowded together, in the axils of small scale-leaves, and are the assimilating organs of the plant.
The xerophytic adaptation in the Asparagus is inherited from the native wild Asparagus that grows in sandy soils. The young edible shoots of the cultivated Asparagus grow from a horizontal rhizome. Buds that develop in the axils of the scale-leaves give rise to more slender branches and these to more slender branches still. The ultimate branches are the clusters of crowded needle-like phylloclades.
In the Gorse it is only by close examination that dwarf branches can be distinguished from leaves . The leaf is short, almost cylindrical, and ends in a sharp point. But the dwarf branch is also short, almost cylindrical, and ends in a sharp point. To distinguish them one from another their origin must be determined.
The leaf grows directly on the stem, and in the angle it makes with the stem there is either a bud or an axillary shoot.
The dwarf branch grows in the axil of a leaf and itself bears leaves.
The effect of this particular leaf-modification in the Gorse is to diminish transpiration. The loss of assimilating tissue is not so great in this case as in the Butcher’s Broom. Still, there is a definite reduction in leaf-surface and, to compensate, not only do the dwarf branches take part in food manufacture, but four prominent ridges, found on the branches of unlimited growth, give an additional area of assimilating tissue.
In the Pine the needles grow on dwarf branches, two needles on each dwarf branch . The only leaves borne by the branches of unlimited growth are small, rather woody, scales. In their axils the dwarf branches grow and bear both scale-leaves and foliage leaves. The foliage leaves are the pine needles. There is no regular periodic leaf-fall in the Pine. When the work of the leaves is over they drop from the tree, still attached to the dwarf branch that bore them. In this case leaf-fall is more than just leaf-fall. It is branch-fall as well.
The branches of unlimited growth of the Hawthorn bear foliage leaves with leafy, persistent stipules .
The axillary buds of these leaves may develop in four different ways : Some give rise to long branches, in every way resembling the axis that supports them.
Others develop into dwarf branches that bear the flower-clusters and, ultimately, haws.
From others, again, grow the protective short thorns. These are dwarf branches and frequently bear one or two leaves and axillary buds.
Lastly some of the axillary buds produce the long thorns which bear flower-clusters in the axils of their leaves .
It is not by any means the case that all thorns are dwarf branches. In the Gooseberry they are out- growths from the pulvinus, that is, the spreading base of the leaf- stalk. In the False Acacia they are stipules . In the Barberry they are modifications of the foliage leaves .
Thorns and prickles supposedly afford protection against grazing animals. It is a little dis- concerting to find sheep and even lambs tearing and eating Blackberry branches, while donkeys and thistles are proverbially connected.
On the other hand, grazing animals certainly avoid Gorse, which is rich in suitable food. When it is put through the chaff-cutter it makes excellent fodder to mix with hay. This is a common practice in some districts, especially those in which the grazing land is rough.
The bitter juices found in most parts of Ranunculaceous plants also ward off animal attacks. A field is golden-yellow with Buttercups because horses, cows, and sheep, alike avoid them.
Flocculent hairs, as well as preventing too great transpiration, protect the leaves on which they occur from being eaten.
Coltsfoot and Mullein are never grazed. In the rolling process in the animal’s mouth the hairs would come off, forming an unpleasant bolus, difficult to swallow.
The stinging hairs of Nettles are a very specialised form of protection, possibly against the delicate noses of grazing animals. The hair is a hollow cylinder containing formic acid, which is also the stinging principle in the bite of an ant. The hair rests on a cushion of cells in the epidermal region and ends in an expanded bulb .
The old rhyme says :
Tender-hearted stroke a nettle,
It will sting you for your pains.
Grasp it like a man of mettle
And it soft as silk remains.
The scientific explanation of this truism is as follows. The bulbous end separates from the hair upon the slightest contact. The separation is effected in such a way that a scooping edge, rather like a doctor’s hypodermic syringe, is left, and this pricks the skin. Through the puncture the formic acid enters. Mixing with the blood, it causes local blood-poisoning, which is attended with a certain amount of pain. When the Nettle is grasped roughly the hairs are broken lower down. The formic acid spills over the skin, but, as there is no puncture, it cannot penetrate.
It is interesting to note that plants that have no protection in themselves frequently grow among plants that have some weapon of defence.
It is difficult to pick the White Dead Nettle (which is not a Nettle at all) without getting stung, for almost invariably it grows among Stinging Nettles.
The beautiful fragile Stitchwort grows under hedges, as does the Hedge Parsley (Queen Anne’s Lace). This habitat makes them very difficult of approach.
Cuckoo Pint (Lords and Ladies) grows most commonly under a Hawthorn hedge. If the Hawthorn terminates abruptly to be followed by Beech, no Cuckoo Pint is found growing under the Beech hedge.