In botany, the stem axis denotes one of the three basic organs of the plants with corm (outdated: cormophytes). It connects the other two basic organs, root and leaf , which are used for nutrition, in both directions. The stem axis carries the canopy and moves it as favorably as possible to its environmental conditions (see plant movement ). It is a body which in the course of shore leave has developed the plant. It is used for stabilization, storage and as a transport organ for water , nutrients and assimilates . Depending on the form, the stem axis is also called shaft, stalk , stem or trunk .
Hypocotyl and epicotyl
The hypocotyl lies between the roots and the cotyledons . This section of the shoot is formed first during germination. The epicotyl lies between the cotyledons and the base of the first following leaf .
Nodus and internodes
The branch axle is often somewhat thickened at the attachment points of the leaves, so they call this place the Node ( node ). The section between two nodes is accordingly called internode . In the seedling plant, these internodes are initially still compressed, so that the leaves on the nodes sit close together. The elongation of the stem axis takes place through an elongation growth of the internodes ( intercalary growth ).
Short shoot / long shoot
Many plant species have two different types of shoots. A shoot axis with fully extended internodes is called a long shoot, whereas a shoot that remains compressed is called a short shoot. The two terms are correlative, which means that this statement can only be made if the respective plant has both types.
The intercalary tissue, which is mainly located at the bases of the internodes, stops working in the case of a long shoot.
In many deciduous trees (e.g. all fruit trees) the short shoots bear the flowers and thus the fruits. Therefore they are also called fruit wood . In the larches and pines , the needle leaves also sit on short shoots.
In some spore plants , such as some mosses and ferns , the original dichotomous branching still occurs , in which the apex cells of one shoot split into two forked shoots. In the case of seed plants , on the other hand, branches of the stem axis arise almost exclusively from the sprouting of the side buds . Exceptions are only a few, mostly strongly succulent plants such as the genus Mammillaria (Cactaceae). As a rule, the branches of the axes occur axillary through embryonic tissue in the leaf axes ( leaf axillary meristems between leaf and axis). The different branching patterns can be traced back to two basic types, the monopodial and the sympodial branching.
The monopodium is a branch with a continuous axis. The previous year's shoot section is continued annually by the same, acroton- funded top meristem and side buds and side shoots are suppressed (e.g. in spruce ).
A sympodium is a type of branching in which the further growth of the shoot is not continued from the main axis, but from subterminal side axes. The terminal bud dies and the side buds sprout. (e.g. with beeches and linden trees ).
If the further growth is taken over by two roughly equally strong lateral axes, one speaks of a dichasium (e.g. lilac ). A monochasium is when only one side axis takes over further growth (e.g. linden tree ). This almost always aligns itself in the same direction as the peaked main axis, then soon exhausts itself and is again peaked by another side axis. Such a monochasium is composed of various successively linked side axes and at first glance is usually hardly distinguishable from a branch with a continuous main axis. This creates an apparent axis . A monochasium can be recognized by the arrangement of the leaves. Since side axes always arise from the axilla of a leaf , in a monochasium the leaves on the pseudo-axis appear to be opposite the inflorescences (e.g. grapevine ). With a continuous main axis, however, the inflorescences would be found in the axils of the leaves.
If mainly the buds of the upper shoot region sprout, this is an acrotonic growth , which leads to a tree-shaped growth. If the side shoots arise through the buds of the lower shoot region, this is a basitone growth and the result is a bush-shaped growth.
The vegetation cone (also called "apex") is the tip of the shoot on which the growth in length takes place. The vegetation cone is divided into different development zones:
The initial cell zone / formation zone is the extreme tip of the cone at which new cells arise. This zone is only about 50 micrometers long. In the seed plants this tissue is the apical meristem , while in horsetail and ferns it is a three-edged apical cell. In some highly developed gymnosperms and in angiosperms, the apical meristem is divided into two areas: tunic and corpus. The corpus is a central complex of tissue that is surrounded by the cell layers of the tunic like a jacket.
Behind the initial cell zone / formation zone lies the 50 to 80 µm long determination zone . Here a decision is made about the differentiation of each cell, but the final differentiation follows in the following differentiation zone / extension zone. In the determination zone, the vegetation cone is already divided into a central tissue complex (corpus) and a tunic enveloping it. A meristem remains between the body and the tunic. In addition to elongation growth, the primary thickness growth also takes place in the elongation zone.
The zone of determination is followed by the zone of differentiation, in which the cells completely differentiate. The preliminary stages of the vascular bundles are formed here by the residual meristem, which differentiates into a procambium in this zone . It forms a proto phloem on the outside and a proto xylem on the inside. The corpus differentiates itself into the parenchymal marrow and the tunic into the epidermis and cortex . The tunic also creates the leaf systems.
The stem axis carries the leaves and flowers. It grows towards the light and, like bushes and trees, can become hard and woody. Conductive paths, the vascular bundles, run inside the stem axis. They consist of many very fine tubes and connect the roots with the above-ground parts of the plant. The vascular bundles transport water, sugar and minerals from the roots to the leaves and flowers. The water keeps the plants tight.
After the cells have differentiated, the following types of tissue are found:
The epidermis is the upper or lower according to cuticle, the outermost layer of the primary stem axis. As with the leaf, it can have stomata and a cuticle . The layer below is initially the primary bark. In contrast to the epidermis, it mostly contains chloroplasts. If a secondary growth in thickness sets in, the primary cortex is usually quickly replaced by a secondary closure tissue, the bark , because the cortex cannot follow the dilation growth . As a substitute for the stomata, the bark usually contains characteristic lenticels for gas exchange. The bark is replaced by cork cambia that lie outside the cambium and are constantly being re-created when they tear through the growth in thickness. This creates a bark structure that is characteristic of the individual species.
This tissue mostly consists of elongated cells with thickened walls. A distinction is made between sclerenchyma and collenchyma . Sklerenchyma consists of dead cells and usually occurs as a layer around a vascular bundle . Sklerenchyma cells form thickened secondary cell walls, these are often reinforced by lignin . The cells die as a result of the deposits. They are divided into two groups:
- Isodiametric cells (stone cells, e.g. in the fruit of pears )
- Prosenchymatic cells (sclerenchymal fibers)
Collenchyma, on the other hand, is still growing and stretchable, not lignified, firming tissue made from living cells. The living cells of the collenchyma are usually rich in chloroplasts , the edges or individual walls are reinforced by cellulose or pectin deposits .
There are three different types of collenchyma:
- Corner / edge collenchyma (cell wall thickening in the cell corners; not thickened on the central lamella)
- Plate collenchyma (thickening of the tangential cell walls)
- Gap collenchyma
The basic tissues consist mainly of parenchyma and the medulla in the middle of the shoot. The pulp is mainly used to store substances, but in some plants it can be torn, creating a medullary cavity.
The tissues used for transport are grouped into strands, the vascular bundles . Vascular bundles are responsible for the long-distance transport of water, dissolved substances and organic substances (mainly sugar) in the shoot , in the leaf and in the roots of higher plants ( vascular plants ). Vascular bundles consist of the xylem , i.e. the wooden part with cell elements for water transport ( e.g. trachea and tracheids ) and the phloem , i.e. the bast part, for the transport of the assimilates with sieve cells , sieve tubes and escort cells .
There are different types of vascular bundles: simple vascular bundles only consist of a sieve or wooden part. Composite vascular bundles have a sieve and a wooden part. In the case of concentric vascular bundles, the sieve part lies around the wooden part (or vice versa). The most common type is the so-called collateral vascular bundle, in which the sieve part is on the outside and the wooden part is on the inside. In the case of open vascular bundles (occurs in dicotyledonous plants) there is also a cambium between the xylem and phloem. In roots the vascular bundles are combined to form a radial vascular bundle system, where the wooden part is arranged like the spokes of a wheel - the bast part lies between the spokes.
The horizontal growth in plant growth in thickness called. A primary and a secondary growth in thickness can be distinguished. The primary growth in thickness is solely due to the growth of the cells already formed in the young shoot by the apical meristem (formation tissue ), while in the secondary growth in thickness from the cambium , which lies between the phloem and xylem , additional cells are separated on both sides , which grow in width . The cork cambium produced in the phloem also contributes to the secondary growth in thickness; this is particularly noticeable B. in the cork oak .
Metamorphoses of the stem axis
Like the leaf and root , the stem axis is also often modified through metamorphoses , either to fulfill its original function adapted to certain environmental conditions or to take on other functions at all.
Stolons (runners, creeping shoots) are used for vegetative reproduction. They are elongated side shoots that creep above or below the ground and extend from the base of the stem, from the leaf rosette or from the root neck. From axillary buds formed at nodes, young plants emerge, which are initially taken care of by the mother plant until they have developed their own roots and leaves. Then the stolons die. Examples of stolon-forming plants are strawberries ( Fragaria ), lilies such as Lilium lankongense and house lice such as Sempervivum tectorum . There are smooth transitions between stolons and rhizomes, with stolons more aboveground and rhizomes more underground.
Rhizomes are used for vegetative reproduction and for storing reserve materials (e.g. starch and inulin ). They have a root-like shape, but can be distinguished from real roots by the presence of nodes and (scaly or thread-like) reduced leaves. If the above-ground parts of the shoot in herbaceous plants die at the end of a vegetation period , they can regenerate from the rhizomes at the beginning of the new vegetation period. Herbaceous plants with rhizomes are therefore often geophytes . Examples of plants that form rhizomes are wood anemone ( Anemone nemorosa ), lily of the valley ( Convallaria majalis ), ginger ( Zingiber officinale ) and grasses such as beach grass ( Ammophila ).
Sprout tubers also serve to store reserve substances and partly also for vegetative reproduction . They can be built above ground or half to completely underground. Examples of plants that form sprouts are:
- above ground: kohlrabi ( Brassica oleracea var. gongylodes );
- semi-underground: celeriac ( Apium graveolens var. rapaceum );
- underground: potato ( Solanum tuberosum ). At the beginning of a new vegetation period , the individual potato plants regenerate from the tuber's lateral axes, known as "eyes".
Beets are also used to store reserve substances. Some beets are formed exclusively from the root, but others also partly from the hypocotyl as part of the stem axis. Examples of beet-forming plants are radishes ( Raphanus ) and beetroot ( Beta vulgaris ssp. Vulgaris var. Conditiva ).
Water reservoir (succulent)
The stem of succulent plants serves as a water reservoir to bridge a dry dormancy. The plants have a fleshy appearance due to the system of water-storing tissue. Many stem succulent plants approach the spherical shape, as this means the largest possible volume with the smallest possible surface and thus the lowest possible loss of water through evaporation. Often the leaves are greatly reduced, transformed into thorns or missing completely, so that photosynthesis takes place in the cortex cells of the stem axis. This often happens according to the CAM mechanism . Examples of plants with stem succulents are cacti (Cactaceae), Didiereaceae , Fouquieriaceae and many spurge species ( Euphorbia ).
Leaf replacement, flax shoots
The photosynthesis that takes place in the bark cells of the stem axis serves as a leaf replacement for plants with severely reduced or missing leaves. This is often the case with succulent plants, but not necessarily associated with succulence. In addition to green cylindrical or more or less angular rungs, green flat rungs are also formed. Platycladia , long shoots widened over a large area (main shoots), resemble normal shoots and are only flattened. Phyllocladia , flat, widened short shoots (secondary shoots), however, often look deceptively similar to pinnate leaves. Examples of platycladiene-forming plants are Homalocladium platycladium and many cacti of the genera Disocactus , Schlumbergera and Opuntia . Examples of phyllocladic plants are asparagus ( Asparagus ), Phyllocladus and butcher's broom ( Ruscus ).
Shoot tendrils are used by some climbing plants to anchor them to surfaces such as rocks and accompanying vegetation. The touch-sensitive tendrils carry out search movements and then wholly or partially wind around the found object ( plant movement ). Examples of plants that form shoots are passion flowers ( Passiflora ) and grapevines ( Vitis ).
The clamping bars of the spreading climbers are also used for anchoring to the ground. Often the rungs at the nodes are angled so that a clearly zigzag-shaped growth emerges, which enables it to get caught on rocks and accompanying vegetation. Often thorns or spikes are also formed with which the rungs can hook and fix. Examples of plants that produce climbers are blackberries ( Rubus fruticosus agg.), Roses ( Rosa ), winter jasmine ( Jasminum nudiflorum ) and spiced vanilla ( Vanilla planifolia ).
Spikes serve to ward off herbivores and, in the case of spreading climbing plants, to improve the climbing strategy by hooking them on. The thorns are formed from the pointed, woody ends of the side rungs. Examples of plants that produce sprout are sloe ( Prunus spinosa ), hawthorn ( Crataegus ) and bougainvillea .
Sprout born haustories
Haustoria are used by parasitic plants to remove nutrients and water from their hosts . In most parasites, the haustoria represent transformed roots, in some, however, transformed shoots. Examples of plants that form sprout-born haustoria are the types of silk ( Cuscuta ).
The side rungs that share a common support sheet are called sprouts or accessory rungs . They arise through fractionation of the axillary meristem and, depending on the arrangement, can be subdivided into serial sprouts (arranged one above the other) and collateral sprouts (arranged next to one another).
Ornamental strawberry rooting stolon
Ginger rhizome ( Zingiber officinale )
Sprouting potato tuber
Sprossrübe of garden radish ( Raphanus sativus )
Phylloclades of the piercing ends mice mandrel ( Ruscus aculeatus )
Sprout thorns of the common hawthorn ( Crataegus monogyna )
- U. Lüttge, G. Kluge, G. Bauer: Botany. A basic textbook. 1st edition. 1. corrected reprint. VCH, Weinheim 1989, ISBN 3-527-26119-2 .
- N. Campbell et al .: Biology. 1st edition. 1. corrected reprint. Spectrum, Heidelberg 1997, ISBN 3-8274-0032-5 .
- P. Sitte, EW Weiler, JW Kadereit, A. Bresinsky,; C. Körner: Strasburger . Textbook of botany for universities. 34th edition. Spectrum, Heidelberg 1999, ISBN 3-8274-0779-6 .
- U. Kull: Outline of General Botany . 2nd edition, reprint. (January 2, 2006). ISBN 978-3-510-65218-1 .
- U. Lüttge, M. Kluge, G. Bauer: Botany. 4th edition. Wiley, 2002, p. 386.