Stem

The stem is at the Plante S with Fleur S, the axis, generally air, which prolongs the root and carries the bud S and the Feuille S. the stem generally ramifies in Branche S and Rameau X forming the apparatus caulinaire.

It differs from the root by the presence of nodes where the buds axillaires and the sheets fit, by the absence of final cap and its anatomical structure. The transition between root and stem is done in the “Collet”. It can exist underground stems as there exist air roots.

By its mode of growth and ramification, the stem determines the port of the plant; it provides a function of support of the plant and a function of transport of the nutritive elements between the roots and the sheets.

Anatomical structure

One can observe with the Microscope the anatomy of the stems thanks to transverse sections fine of stem or floral Pédoncule S. These cuts can be coloured to make it possible to differentiate the cells and their components, generally thanks to double coloring Carmine-green of iodine (dye of Mirande).

This dye colors pink the cellulose structures (like the Parenchyme S, the Phloème (4) primary and secondary and the Collenchyme S) and in green the structures containing of the molecule connected with the Lipide S (subérifiés fabrics or lignified like the lignified parenchymas, Sclérenchyme S, the Xylème (2,3) primary and secondary) and into yellowish the Suber (also called cork).

The stem is generally a cylindrical body with axial symmetry, except in the case of the stems squares or triangular where symmetry is known as bilateral (ex: Allium triquetrum )

The stems can comprise only primary structures (case of the Monocotylédone S and Dicotylédone S young people) or also of the secondary structures (dicotylédones old).

Primary structure

A stem comprises 2 principal zones:
  • the bark (6) (or cortex or peripheral cylinder):

The bark is reduced in the stems while it is very developed in the central cylinder contrary to the roots where it is the opposite.

The bark consists of a fabric of primary coating of origin: the skin (7). This last is composed of only one sitted (layer) of Cellule S aligned side by side; one can notice stomata from time to time to allow the gaseous exchange between the external medium and the stem.

Under the skin is the cortical parenchyma, fabrics made up of cells larger and less better organized. It is the seat of the Photosynthèse and can be used as reserve with the plant. Often a parenchyma is found below lignified (it appears greener on the cut) which is used to support the plant, it is the Sclérenchyme (5).

  • the central Cylinder

The limit between the cortex and the central cylinder is delimited by the conducting beams. The latter are in another parenchyma called medullary parenchyma. The beams are composed of two types of vessels: the xylème (2,3) and the phloem (4). The xylème leads the crude sap made up of Eau, rock salt amino-acid and some of the roots towards the photosynthetic bodies where this sap takes care out of sugar. It is transformed then into elaborate sap which is led by the phloem towards the petitioning bodies in energy such as the buds, the roots, the tubers, the flowers, the fruits… In the stems, the xylème and the phloem are superimposed (the phloem is with the top of the xylème), contrary to the roots where they are laid out one beside the other. At the monocotylédones, these vessels many, are laid out on several circles, and of variable and smaller size. At the dicotylédones, the number of vascular beams is less (less than 8) and they all are on the same circle.

In the center of the stem the Moelle (1) is or medullary zone who contains parenchymas of reserve. At the Poacées (graminaceous), this zone is replaced by a central gap and the stem " then is called; chaume". That explains why the stem of corn is hollow when it is broken.

Secondary structure

Growth and ramification

Growth in length

The growth of the internodes is done under the effect of the Gibbérelline S.

Growth in thickness

The secondary growth (in thickness) is,

- Non-existent at the plants belonging to the class of the Liliopsida (Monocotylédones angiospermes) ex families: Poaceae (graminaceous), Arecaceae (palm trees), Cyperaceae (papyrus), Musaceae (banana),…

the plants belonging to this class do not have a Cambium and thus produce neither wood nor secondary roots (root pushing perpendicularly on another root). The only way for these plants with flowers of being able to increase their base and the rigidity of their stem, are to accumulate fabrics died around that Ci. (as at the palm trees)

- Important at the plants belonging to the class of the Magnoliopsida (Dicotylédones Angiosperme S)

ex families: Fagaceae (oaks, beeches, chestnuts), Cactaceae (cactus), Salicaceae (poplars, willows), Oleaceae (ashes, olive-trees, lilac), Rosaceae (rose trees, plum trees, apple trees, pear trees, cherry trees, ronciers, raspberry canes),…

the plants belonging to this class form a cord of cells little differentiated and not thickened within the wood-and-bast beam, between the phloem and the xylème called Procambium. The procambium, while dividing, sets up a cambium intrafasciculaire which will form secondary xylème towards the interior and of the secondary phloem towards outside. The meristematic activity of this cambium generates wood (xylème secondary) and the liber (secondary phloem).

Note: - At the Gymnosperme S (conifers), the growth in thickness is similar has that of the dicotylédones. - A different process of growth in thickness also exists at the tree ferns.

Modes of ramification

  • ramification monopodiale
  • dichotomic ramification sympodiale
  • ramification

Port of the plants

arborescent

buissonantes

Herbaceous

Lianas

Various types of stems

Air stems

  • plants Stemless S
  • cladodes
  • Graminaceous thatch of the S
  • feather-grass (example: the Palm tree S)
  • succulent stems
  • Stolon
  • radicant Stem
  • specialized branches
    • spines
    • gimlets

Underground stems

4. Various kinds of stem/Diagram One distinguishes three (3) standard from different stems: 1. Air stem - Plant Stemless - Cladodes - Thatch of the Graminaceous ones - Feather-grass (ex: Palm trees) - Succulent Stems (canes with sugar) - Stolon - Radicant Stem - Specialized Branches (Spines - Gimlets)

2. Underground stems - Rhizomes - Tubers - Bulbs

3. Watery stems

NB: The stem perhaps: Drawn up: the stem is sufficiently robust to develop with the vertical.

Rising: often relate to plants whose stock is long-lived and robust but whose air stems are hails and herbaceous.

Slept or crawling: the stems are spread out on the ground and do not go up or little. One also speaks about prostrate plants.

Voluble: surround a support to take support there. Climbing: fixes itself on a support by cramps which are adventitious roots or by gimlets, which are transformed sheets.

See too

vegetable Morphology | Sheet | Fleur | Plant | root

External bonds

  • See a detailed diagram of various types of stems.

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