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Atlas of Botany: Stem

Atlas of Botany: Stem

The gem

There apical bud it is formed by a meristematic apex wrapped by the young leaflets (leaf sketches) that it has produced, at the axella of which the sketches of the apices of the lateral branches may already be present (axillary or lateral gems). Meristematic areas susceptible to forming buds can persist or subsequently differentiate also in other positions, such as on the old portions of the branches, on the stem, on the roots and even on the leaves, in which case the relative buds are said to be adventitious. In the buds through which the plant survives the adverse season (hibernating buds) the outermost leaf sketches are transformed into protective organs (the bud), while the other innermost leaf sketches remain unchanged.

The stem: generalities and functions

The stem has the function of supporting leaves, flowers and fruits, of orienting them in space and of conducting water and nutrients. It can perform photosynthesis and have a reserve function (e.g. succulents) or defense (hawthorn spines). The stem connects to the roots approximately at the level of the soil surface, in an area called collar, while connecting to the leaves in enlarged areas called knots. The portions between the nodes are called internodes.
Depending on their consistency, the stems are divided into: herbaceous, when they are tender and green; woody, when protected by a layer of lignin (tree, shrub and bush).

The roots absorb water and dissolved mineral substances from the soil (blue arrows in the drawing) which go back along the stem in the woody vessels (ascending sap) to the leaves and shoots. Perspiration causes the loss of water from the leaves, creating a force that pulls the xylematic lymph (ascending) upwards. The leaves also exchange gas through the stomata, absorbing carbon dioxide which provides carbon for photosynthesis and eliminating excess oxygen. The sugars are produced in the leaves through photosynthesis and are transported in the phloem (green arrows in the drawing) in all parts of the plant where photosynthesis does not take place.

The stem (source Iprase Trentino)

The rise of the raw sap
The water present in the soil is transported to the leaves overcoming the force of gravity. First of all, it must be remembered that there are cohesion and adhesion forces that keep the water molecules together and prevent the very thin columns of ascending sap present in the woody vessels from breaking. Plants lose water through the leaves with perspiration, this generates negative pressure. which causes an aspiration of sap from the roots towards the leaves. The roots also contribute to the rise of the lymph through radical pressure. In addition, the woody vessels are very thin and act like capillary vessels helping the sap not to fall down.

Main areas of the stem

Four main areas are distinguished in the stem:
- Apical or growth zone
- Zone of differentiation and relaxation
- Primary structure area
- Area of ​​secondary structure

Apical (embryonic or growth) area
It is located at the apex of the stem, at the level of the apical bud. In most of the Gimnosperms and in all the Angiosperms there is an organization of the type “tunica-corpus". The tunic is the external part, formed by cells which, dividing, increase the surface area. The corpus is more internal and consists of a mass of cells that divide, leading to an increase in volume.

Zone of differentiation and relaxation
It is located just below the stone and is characterized by a meristematic fabric consisting of three layers, protoderma, procambio is fundamental meristem, intended to develop lepidermis, vascular tissue and parenchyma. The growth gradually decreases going towards the lower part, until it completely ceases.

Primary structure area
It is located about 2 cm from the tip and is made up of adult tissues. In cross section we distinguish: epidermis, cortex or cortical cylinder, central cylinder or stele.
Lepidermis it consists of a single layer of cells with the task of protecting the innermost tissues from injuries and counteracting the loss of water.
There bark it consists mainly of parenchymal tissue, its most superficial cells, which are reached by light, contain chloroplasts, in fact the young stems are green and carry out photosynthesis. In the cortex there are mechanical tissues: the collenchyme, in a more superficial position and the sclerenchyme, consisting mainly of fibers, in a deeper position. Both can completely surround the stem or form longitudinal bundles.
Frequently there is no clear border between the cortex and the central cylinder, which occupies most of the stem thickness and contains the vascular bundles. Sometimes there is a layer of cells (pericycle) that separates the cortex from the central cylinder.
The central cylinder o stele is the central and widest part of the stem, characterized by the presence of the conducting beams.

In Gimnosperme and in the Dicotyledons the conducting beams are arranged in a ring inside the pericycle. The marrow occupies the entire central part of the stele, inside the bundles, and can be partially reabsorbed.
Primary medullary rays are masses of parenchyma between the bundles. In these plants the central cylinder is almost always one eustele. Each bundle is made up of the book, facing the outside of the stem, and the wood facing the inside; between wood and book there is a meristem called intrafascial change.

In monocots the central cylinder is an atactostele. Closed collateral bundles are scattered in the medullary parenchyma, that is, without intrafascial change.
In each bundle the wood is always turned inwards and the book outwards of the stele; the larger bundles are towards the center and, towards the periphery, they become smaller and smaller; the bundles are sparser towards the center and denser towards the periphery. In the Graminacee, Monocotyledons which have the central hollow cylinder, the bundles are arranged on two concentric rings.

Central cylinder (photo www.sdasr.unict.it)

TO - Eustele
Bundles sorted according to a circle
Collateral open (with exchange)

B - Atactostele
Messy bundles, numerous
Collateral closed (without exchange)

In herbaceous plants (most of the Monocotyledons and some Dicotyledons), once the primary structure has differentiated, this is maintained throughout the life of the plant. In woody plants (many Dicotyledons and all Gimnosperms) instead, the area of ​​primary structure is followed by that of secondary structure, in which the most abundant fabric is wood, which is formed following the differentiation and activity of the cribrovascular change.

Secondary structure area
The secondary growth in thickness is due to the activity of lateral secondary meristems such as the cribro-woody change and the subero-fellodermal or phylogenic change.
The fellogeno (or subero-fellodermal change) is a lateral meristematic fabric that produces cork (fellema) towards the outside of the stem and felloderma towards the inside; these fabrics serve to protect the internal parts of the tree (book, gearbox and wood). As insulator, cork prevents exchanges with the outside and causes the death of all external tissues (which become rind or rhyme, commonly called bark).
Cork, phellogen and phelloderma form the periderm. as the diameter of the central cylinder increases, the layers are pushed outwards and the cells that are beyond the cork, isolated from this waterproof fabric, die after accumulating catabolic products and tannins. The set of dead tissues is called rhytidoma. This can crack and last long on the plant or be eliminated in the form of flakes. Some species such as cork oak (Quercus suber L.) produce cork in quantities exploitable by man.
The diametrical growth of the central cylinder is chaired by the cribro-vascular exchange. We can distinguish an intrafascicular change, of primary origin, interposed between primary xylem and phloem, and an interfascicular change, which originates as a secondary meristem starting from the parachymal cells of the medullary rays.
As a result of the increase in thickness, the fabrics external to the exchange are crushed around the periphery of the stem and are unable to follow the growth; consequently they would be torn.
The secondary phloem formations assume the wedge shape.
Thanks to the activity of residual meristems located in the medullary radii, new parenchymatic cells are able to fill the empty spaces between book wedges (dilation parenchyma).

Trunk and annual circles

In temperate regions, the activity of cribro-vascular exchange is not continuous throughout the year, but is limited to those periods in which the apical bud meristems are in active division and produce hormones that stimulate the division of promissory cells. In the spring, when the vegetative restart takes place, the change produces vases with a large lumen to transport the large amount of water necessary to activate the vital mechanisms. For this reason, spring wood is less dense than summer wood, which has vessels with a narrower lumen and a greater number of supporting fibers. The transition between spring and summer wood is gradual, while the separation between summer and spring wood from the following season is very evident. Thus rings are formed, or annual circles, each of which corresponds to the growth of the stem during a year. The thickness of each individual circle is affected by the seasonal trend.

As the number of circles increases, the most central part of the wood loses its functions except that of support. This now inactive part of wood is called the heart oduramen.
The outermost circles make up the sapwood and serve as a reserve; only the outermost layers are normally still involved in the transport function.
The duramen has thicker darker colors, due to the deposition of ethanine polyphenols which prevent it from spreading by fungi and bacteria.

The trunk (source www.sdasr.unict.it)

- Cork external with protection functions
- Floema o book in which sugars circulate, and other substances processed by the leaves
- Vascular change: determines the redness of the stem generating, layer by layer, the entire transport and support structure of the stem
- Physiologically active wood called sapwood, in it water and mineral substances rise towards the leaves (ascending sap)
- Non physiologically active wood called duramen, in it water and mineral substances rise towards the leaves (ascending sap)

Annual circles (source www.sdasr.unict.it)


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