1911 Encyclopædia Britannica/Stem

Stem (O. Eng. staefn, stemn, cf. Du. stam, Ger. Stamm, &c., probably related to “staff”), in popular language the stalk of a plant, the trunk of a tree (for the technical use of the term in botany see below). There are many transferred uses of the word, such as for the slender structure which joins the foot or base of a vase or goblet to the bowl, a stock or branch of a family, or, in philology, a derivative from a root, the unchanged part in a series of inflected forms. The stem of a ship is the prow, properly a curved piece of timber or metal to which the two sides are attached at its foremost end. This was a Scandinavian use early adopted in English; the word meant simply post, and custom alone restricted it to the bows rather than to the stern; in Danish the distinction is made between fram stam and bak stam and also in German, Vorder-steven Hinter-steven.

In botany a stem may be defined as an axis bearing leaves. The stem with its leaves is known as the shoot. Structurally it differs from a root in having no development of cells forming a cap over the growing-point. Under the term caulome (stem structure) are included all those parts of a plant morphologically equivalent in bearing leaves. The stem generally ascends, seeking air and light, and has therefore been termed the ascending axis. Stems have usually considerable firmness and solidity, but sometimes they are weak, and either lie prostrate on the ground, thus becoming procumbent, or climb on plants and rocks by means of rootlets, like the ivy, being then called scandent, or twist round other plants in a spiral manner like Woodbine, when they are twining. Twining plants turn either from right Fig. 1.—Orchid with pseudobulbs, p. to left, as the French bean, convolvulus, dodder and gourd; or from left to right, as honeysuckle, twining polygonum, hop and black bryony (Tamus). In other cases climbing plants are supported by tendrils, as in vine, bryony, passion-flower, or by the tendril-like leaf-stalks, as in clematis and Tropaeolum. In warm climates twining plants (lianas) often form thick woody stems, while in temperate regions they are generally herbaceous. Some stems are developed more in diameter than in height, and present a peculiar shortened and thickened aspect, as Testudinaria or tortoise-plant, cyclamen, Melocactus, Echinocactus and other Cactaceae; while in many orchids (fig. 1) the stem assumes an oval or rounded form, and is called a pseudobulb.

Names are given to plants according to the nature and duration of their stems. Herbs, or herbaceous plants, have stems which die down annually. In some of them the whole plant perishes after flowering; in others, the lower part of the stem forming the crown of the root remains, bearing buds from which the stem arises next season. In biennial herbs the whole plant perishes after two years, while in perennial herbs the crown is capable of producing stems for many years, or new annual products are repeatedly added many times, if not indefinitely, to the old stems. The short permanent stem of herbaceous plants is covered partially or completely by the soil, so as to protect the buds. Plants producing permanent woody stems are called trees and shrubs. The latter produce branches from or near the ground; while the former have conspicuous trunks. Shrubby plants of small stature are called under-shrubs or bushes. The limits between these different kinds of stem are not always well defined; and there are some plants occupying an intermediate position between shrubs and trees, to which the name of arborescent shrubs is occasionally given.

The stem is not always conspicuous. Plants with a distinct stem are caulescent; those in which it is inconspicuous are acaulescent, as the primrose, cowslip and dandelion. A similar term is given in ordinary language to plants whose stems are buried in the soil, such as cyclamen or sowbread. Some plants are truly stemless, and consist only of expansions of cellular tissue representing stem and leaf, called a thallus, and hence are denominated Thallogens, or Thallophytes.

Fig. 2.—The Embryo of the Pea laid open.

c, c, The two fleshy cotyledons, or seed-lobes, which remain underground when the plant sprouts.

r, The young root or radicle.

t, The axis bearing the young shoot or plumule, g, which lies in a depression of the cotyledons, f.

The first rudiment of the young shoot of the embryo appears from the seed after the radicle (young root) has protruded. It is termed the plumule (fig. 2), and differs from the radicle in the absence of a root-cap and in its tendency to ascend. The apical growing portion constitutes the terminal bud of the plant, and by its development the stem increases in height; projections appear at regular intervals, which are the rudimentary leaves, and in addition there is a provision for the reduction of lateral buds, which develop into lateral shoots more or less resembling the parent stem, and by these the branching of the plant is determined (fig. 3). These buds are found in the axil of previously-formed leavers; or, in other words, in the angle formed between the stem and leaf. They are hence called axillary. They are produced like the leaves from the outer portion of the stem (exogenous), and at first consist entirely of celluar tissue, but in the progress ofgrowth vascular bundles are formed in them continuous with those of the stem, and ultimately branches are produced, which in every respect resemble the axis whence the buds first sprang. In the Lycopods branching takes place by forking of the growing-point, the main axis being thus replaced by two equivalent axes (fig. 4); in most cases the new axes develop unequally, the weaker becoming pushed aside and appearing later as a lateral branch of the stronger. The place of origin of the leaf is called a node; the intervals between nodes are called internodes. The stem, although it has a tendency to rise upwards when first developed, in many instances becomes prostrate, and either lies along the ground partially covered by the soil, or runs completely underneath its surface, giving off roots from one side and buds from the other. Some stems are therefore subterranean, and are distinguished from roots, by the provision made for regular leaf-buds.

(From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.)

Fig. 3.—Apex of a shoot of a phanerogamic plant.

v, Extreme apex, so-called vegetative cone.

f, Leaf rudiment.

g, Rudiment of an axillary bud.

Growth in length of the stem is due to elongation of the internodes; the zone of most rapid growth is at some distance below the apex; below this the rate of growth gradually diminishes until the portion is reached where growth in length no longer takes place. In some cases, as in the stems of grasses, growth in length persists for a longer time in a small region at the base of the internodes; this is known as intercalary growth. In the dwarf or short shoots, such as those of the larch, the internodes do not elongate and the leaves remain close together. Lateral buds give rise to branches, from which others, called branchlets or twigs, arise. The terminal bud, after producing leaves, sometimes dies at the end of one season, and the whole plant, as in annuals, perishes; or part of the axis is persistent, and remains for two or more years, each of the leaves before its decay producing a bud in its axil. This bud continues the growth in spring. In ordinary trees, in which there is provision made for the formation of numerous lateral buds, any injury done to a few branches is easily repaired; but in palms, which only form terminal buds, and have no provision for a lateral formation of them, an injury inflicted on the terminal bud is more likely to have a prejudicial effect on the future plant. In the trees of temperate and cold climates the buds which are developed during one season lie dormant during the winter, ready to burst out under the genial warmth of spring. They are generally protected by external modified leaves in the form of scales, which frequently exhibit a firmer and coarser texture than the leaves themselves. They serve a temporary purpose, and usually fall off sooner or later, after the leaves are expanded. The bud is often protected by a coating of resinous matter, as in the horse-chestnut and balsam poplar, or by a thick downy covering, as in the willow. Linnaeus called leaf-buds hibernacula, or the winter quarters of the young branch. In some plants, as in the plane, the buds destined to live through the winter are so completely surrounded by the base of the petiole as not to be visible until the leaf has fallen off. These are said to be intrapetiolar.

(From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.) Fig. 4.—Longitudinal section of a bifurcating shoot (p) of Lycopodium alpinum, showing equal development of the rudimentary shoots, p′ p″. b, Leaf rudiments; c, Cortex; f, Vascular strands.
Fig. 5.—Leaf-bud of Sycamore (Acer Pseudo-platanus) covered with scales.		Fig. 6.—Transverse section of the same leaf-bud.

In the bud of a common tree, as the sycamore (fig. 5), there is seen the cicatrix or scar left by the leaf of the previous year c, then the scales e, e, arranged in alternate pairs and overlying each other in what is called an imbricated manner. On making a transverse section of the bud (fig. 6), the overlying scales e, e, e, e, are distinctly seen surrounding the leaves f, which are plaited or folded round the axis or growing-point. In plants of warm climates the buds are often formed by the ordinary leaves without any protecting appendages; such buds are called naked. A bud may be removed in a young state from one plant and grafted upon another by the process of budding, so as to continue to form its different parts; and it may even be made to grow in the soil, in some instances, immediately after removal. In some trees of warm climates, as papawtree, palms and tree-ferns, growth by terminal buds is well seen. In these plants the elongation of the stem is generally regular and uniform, so that the age of the plant may be estimated by its height; as there is no great increase in the leaf area owing to absence of branching, there is no need for a great increase in the diameter of the stem.

Although provision is made for the regular formation of buds, there are often great irregularities in consequence of many being abortive or remaining in a dormant state. Such buds are called latent, and are capable of being developed in cases where the terminal bud, or any of the branches, have been injured or destroyed. In some instances, as in firs, the latent buds follow a regular system of alternation; and in plants with opposite leaves it frequently happens that the bud in the axil of one of the leaves only is developed, and the different buds so produced are situated alternately on opposite sides of the stem. Occasionally, after a partial development as branches, buds are arrested and form knots or nodules. The so-called embryo buds or woody nodules in the bark of the beech, elm, olive and other trees are of this nature. They are partially developed buds, in which the woody matter is pressed upon by the surrounding tissue, and thus acquires a very hard and firm texture. When a section is made, they present woody circles arranged around a central pith, and traversed by medullary rays. The nodules sometimes form knots on the surface of the stem, at other times they appear as large excrescences, and in some cases twigs and leaves are produced by them.

When the terminal bud is injured or arrested in its growth the elongation of the main axis stops, and the lateral branches often acquire increased activity. By continually cutting off the terminal buds a woody plant is made to assume a bushy appearance, and thus pollard trees are produced. Pruning has the effect of checking the growth of terminal shoots, and of causing lateral ones to push forth. The peculiar bird-nest appearance often presented by the branches of the common birch depends on an arrestment in the terminal buds, a shortening of the internodes, and a consequent clustering or fasciculation of the twigs. In some plants there is a natural arrestment of the main axis after a certain time, giving rise to peculiar shortened stems. Thus the crown of the root is a stem of this nature, forming buds and roots. Such is also the case in the stem of cyclamen, Testudinaria, and in the tuber of the potato. The production of lateral in place of terminal buds sometimes gives the stem a remarkable zigzag aspect.

The mode in which branches come off from the stems gives rise to various forms of trees, as pyramidal, spreading or weeping—the angles being more or less acute or obtuse. In the Italian poplar and cypress the branches are erect, forming acute angles with the upper part of the stem; in the oak and cedar they are spreading or patent, forming nearly a right angle; in the weeping ash and elm they come off at an obtuse angle; while in the weeping willow and birch they are pendulous from their flexibility. The comparative length of the upper and under branches also gives rise to differences in the contour of trees, as seen in the conical form of spruce, and the umbrella-like form of the Italian or Stone pine (Pinus Pinea). The branching of some trees is peculiar. In the Amazon district many Myristicaceae and Monimiaceae have whorled branches coming off in fives. This is also seen in the Chili pine.

Branches are sometimes long and slender, and run along the ground, producing buds with roots and leaves at their extremity. This is seen in the runner (flagellum) of the strawberry. In the house-leek (Sempervivum) there is a similar prostrate branch of a shorter and thicker nature, known as an offset, producing a bud at its extremity capable of independent existence. In many instances the branch decays, and the young plant assumes a separate existence. Gardeners propagate plants by the process of layering, which consists in bending a twig, fixing the central part of it into the ground, and, after the production of roots, cutting off its connexion with the parent. A stolon differs from these in being a branch which curves towards the ground, and, on reaching a moist spot, takes root and forms an upright stem, and ultimately a separate plant. This is a sort of natural layering, and the plant producing such branches is called stoloniferous. In the rose and mint a subterranean branch arises from the stem, which runs horizontally to a certain extent, and ultimately sends up an aerial stem, which becomes an independent plant. Such branches are denominated suckers, and the gardener divides the connexion between the sucker and the parent stem, in order to propagate these plants. In the case of asparagus and other plants which have a perennial stem below ground, subterranean buds are annually produced which appear above ground as shoots or branches covered with scales at first, and ultimately with true leaves. These branches are Fig. 7.—Branch of the Sloe (Prunus spinosa) producing spines or thorns, which are abortive branches, as shown by their bearing leaves. herbaceous and perish annually, while the true stem remains below ground ready to send up fresh shoots next season. In bananas and plantains the apparent aerial stem is a shoot sent up by an underground stem, and perishes after ripening fruit. Branches are sometimes arrested in their development, and, in place of forming leaves, become transformed into spines or thorns, as in the hawthorn. Plants which have spines in a wild state, as the apple and pear, often lose them when cultivated, in consequence of their being changed into branches; in some cases, as in the sloe (Prunus spinosa) (fig. 7), a branch bears leaves at its lower portion, and terminates in a spine. In some climbing plants some of the shoots are transformed into tendrils, which help the plant to climb by twining about a support, as in passion-flower and vine; or, as in Ampelopsis Veitchii, by forming adhesive disks at the tips of their branchlets which enable them to cling to flat supports. In some cases branches become flat and leaf-like, taking the place in the plant economy of the leaves, which are reduced to small scales or spines, as in butcher's broom; branches showing this modification are termed cladodes or phylloclades (fig. 8). In Cactaceae (e.g. Opuntia, prickly pear, fig. 9) and fleshy euphorbias, where the leaves are reduced to spines, the fleshy stems become green and perform the functions of leaves; they also serve as water reservoirs for the plants, which are natives of very dry countries.

Fig. 8.—Twig of Butcher's Broom (Ruscus aculeatus) slightly enlarged, showing cladodes, c.

(From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.)

Fig. 9.—Opuntia monacanthia, showing flower and fruit. The leaves are reduced to thorns.

Buds sometimes become extra-axillary in consequence of the non-appearance or abortion of one or more leaves, or on account of the adhesion of the young branch to the parent stem. In place of one bud there are occasionally several accessory ones produced in the axil, giving origin to numerous branches. By the union of several such buds branches are produced having a thickened or flattened appearance, as is seen in the fir, ash and other trees. In some cases, however, these fasciated branches are owing to the abnormal development of a single bud.

Fig. 10.—Rhizome of Polygonatum multiflorum (Solomon's Seal), forming buds and adventitious roots.

a, bud which will form the aerial shoot next season; b, c, d, e, scars of successive aerial shoots; w, root.

Fig. 11.—Corm of Meadow Saffron or Autumn Crocus (Colchicum autumnale).

a,	Old corm shrivelling.
b,	Young corm produced laterally from the old one.

(After Sachs.)

Fig. 12.—Corms of Colchicum autumnale in autumn when the plant is in flower.

k,	Oldest corm.
h, h,	Brown scales covering it.
w,	Its roots.
st,	Its withered flowering stem.
k′,	Younger corm produced from k.
w′,	Roots from k′, which grows at expense of k.
s, s′, s″,	Sheathing leaves.
l′, l″,	Foliage leaves.
b, b′,	Flowers.
k″,	Young corm produced from k′ in autumn, which in succeeding autumn will produce flowers.

The typical form of stems is rounded. They are sometimes compressed or flattened laterally (fig. 9), while at other times they are angular. Various terms are applied to the forms of stems, as cylindrical or terete, quadrangular or square, jointed or articulated, &c. The following are some of the more important modifications of stems: The crown of the root is a shortened stem, often partially underground, which remains in some plants after the leaves, branches and flower-stalks have withered. In this case the internodes are very short, and the nodes are crowded together, so that the plant appears to be stemless. It is seen in perennial plants, the leaves of which die down to the ground annually. A rhizome or root-stock (fig. 10) is a horizontal stem usually sending out numerous roots and leaf-buds from its upper surface. It occurs in ferns, iris, Hedychium, Acorus or sweet flag, ginger, waterlily, many species of Carex, rushes, anemone, &c. The leaves are reduced to scales and by their presence, and the absence of a root-cap, a rhizome can be distinguished from a root. A rhizome such as occurs in Solomon's seal (fig. 10) is not a single stem, i.e. the product of a single bud, but is composed of portions of successive axes, the aerial parts of which have died off, leaving their scars (fig. 10, b, c, d, e). Rhizomes are well seen in British ferns. A rhizome sometimes assumes an erect form, as in Scabiosa succisa, in which the so-called praemorse root is in reality a rhizome, with the lower end decaying. The erect rhizome of Cicuta virosa (water-hemlock) shows hollow internodes separated by partitions. In the coral-root orchid Corallorhiza, which grows in soil rich in humus, no roots are developed, the coral-like branching rhizome acting as the absorbing organ (fig. 13). A pseudobulb (fig. 1) is an enlarged bulbous-like aerial stem, common in epiphytic orchids; it is covered with a thick epidermis and acts as a water-store for the plant, which from its growth on branches of trees and in similar positions is often unable to get sufficient water for its immediate needs. A sobole is a creeping underground stem, sending roots from one part and leaf-buds from another, as in couch-grass Carex arenaria, and Scirpus lacustris. It is often called a creeping root but is really a rhizome with narrow elongated internodes. A tuber is a thickened stem or branch produced by the approximation of the nodes and the swelling of the internodes, as in the potato. The eyes of the potato are leaf-buds. Tubers are sometimes aerial, occupying the place of branches. The ordinary herbaceous stem of the potato, when cut into slips and planted, sends off branches from its base, which assume the form of tubers. Tubers frequently store up a quantity of starch, as in Maranta arundinacea, whence arrowroot is derived. Another form of thickened underground stem is the corm, as seen in the autumn crocus (Colchicum, fig. 11), gladiolus, &c. Structurally it is composed of a solid more or less rounded axis covered by a layer of thin membranous scales (fig. 12, h, h). A corm is only of one year's duration, giving off buds annually in the form of young corms. In autumn the young corm gives origin to leaves, the lower of which (s, s′, s″) form sheaths round the corm and flower stalk, the upper (l′, l″) remaining very small; and in the axil of the uppermost leaves the flowering-stem develops and bears the flowers (b, b′). Meanwhile in the axil of one of the middle leaves on the corm, a bud—the rudiment of a new corm—appears (k″). The flowering-stem dies down, and the young corm k′ from which it arose enlarges greatly during the winter at the expense of its parent corm (k), which thus becomes shrivelled. In spring the leaves produced on it (l′, l″), which were merely rudiments in autumn, appear above ground as conspicuous large leaves. At the end of spring these leaves die down, the bases of the lower ones alone remaining, and, constituting thin brown scales around the corm (as at h). Meanwhile, the young, bud-corm.(h″) in the axil of the middle leaf grows rapidly at the expense of its parent corm (k′), but it does not attain a great size. In autumn it produces new leaves, which remain small, but from the axil of the two upper the flowering stem rises up and bears flowers; whilst in the axil of one of its middle leaves a new bud-corm appears, which will the following autumn produce young leaves, flowering stem, and a new bud-corm, and thus the cycle goes on. The buds or new corms formed from the old corms may be produced either laterally, as in Colchicum autumnale, or terminally, as in crocus and gladiolus. The bulb is another form of underground stem or bud. The axis in this case is much shortened, and the internodes are hardly developed. The bases of the leaves rising from the stem are quite close together, and become succulent and enclose the axis. In the lily the thick and narrow scales are arranged separately in rows, and the bulb is called scaly; while in the leek, onion, squill and tulip the scales are broad, and enclose each other in a concentric manner, the outer ones being thin and membranous, and the bulb is tunicated. In the axils of these fleshy scales new lateral shoots arise, forming new bulbs. The lateral buds or cloves sometimes remain attached to the axis, and produce flowering stems, so that apparently the same bulb continues to flower for many years, as in the hyacinth and tulip; at other times the young bulbs are detached, and form separate plants. In the axil of the leaves of Lilium bulbiferum, Dentaria bulbifera, and some other plants, small conical or rounded bodies are produced, called bulbils or bulblets (fig. 14, b). They resemble bulbs in their aspect, and consist of a small number of thickened scales enclosing a growing-point. These scales are frequently united closely together, so as to form a solid mass. Bulbils are therefore transformed leaf-buds, which are easily detached, and are capable of producing young plants when placed in favourable circumstances. The scales in bulbs vary in number. In Gagea there is only one scale; in the tulip and Fritillaria imperialis they vary from two to five; while in lilies and hyacinths there are a great number of scales. In the tulip, a bud is formed in the axil of an outer scale, and this gives rise to a new flowering axis, and a new bulb, at the side of which the former bulb is attached in a withered state.

(From Strasburger's Lehrbuch der Botanik by permission of Gustav Fischer.)

Fig. 13.—Rhizome of Corallorhiza innata, (Nat. size.)

a,	Floral shoot.
b,	Rudiments of new rhizome branches.

Fig. 14.—Stem of Bulbiferous Lily (Lilium bulbiferum) showing bulbils b, produced in the axils of the leaves.

Adventitious shoots are those, which arise elsewhere than in the normal predetermined place, as from old stems, or roots. Such Fig. 15.—Leaf of Bryophyllum calycinum, producing buds along the margin, at the extremities of the primary veins. shoots are frequent on the roots of elm, poplar, plum and other fruit-trees. Occasionally adventitious buds are produced on the edges of leaves, as in Bryophyllum calycinum (fig. 15), Malaxis paludosa, and various species of Asplenium, and on the surface of leaves, as in Ornithogalum thyrsoideum. These are capable of forming independent plants. Similar buds are also made to appear on the leaves of Begonia, Gesnera, Gloxinia and Achimenes, by wounding various parts of them, and placing them in moist soil; this is the method often pursued by gardeners in their propagation. The ipecacuanha plant has been propagated by means of leaves inserted in the soil. In this case the lower end of the leaf becomes thickened like a corm, and from it roots are produced, and ultimately a bud and young plant.