Break into leaf

The sheet is the body specialized in the Photosynthèse at the higher plants (Spermaphyte S, Ptéridophyte S and Bryophyte S). It is inserted on the Tige S of the plants into the level of the nodes. With the armpit of the sheet a Bourgeon axillaire is.

To achieve its role, a sheet is generally made of a blade punt and fine, the limb , which enables him to expose to the light a maximum of surface. It is the palisade Parenchyme, a particular type of fabrics of the sheet, which will carry out photosynthesis, thanks to its cells containing the Chloroplaste S and to give to the sheet its green color. It is also the seat of the Respiration and the Transpiration. The sheets can specialize, in particular to store nutritive elements and water.

The sheet presents a large variety of form, size, color, texture or of ornamentation in the vegetable kingdom. These characteristics of the sheet are often characteristic of a vegetable , or at least of a kind.

Description

The figure 1 watch various parts of the sheet:

a limb plan (A) traversed of veins (b),
with often a Petiole (c) which attaches the sheet to the stem,
sometimes widened in sheath (d). This one can embrace the stem as at the Graminée S. the petiole can miss, the sheet is then known as sessile . It can sometimes be winged, or provided at its base with stipulate more or less developed.

To the difference of the remainder of the vegetative apparatus of the plant ( root and Stem), the sheet has a bilateral and nonaxial symmetry in general.

Criteria of description

detailed Article: foliar Form (terminology)

Four criteria make it possible to describe the sheet:

The composition of the sheet

The sheet is made up of Pectine, Cellulose and Lignine. These components are large chemical molecules “imprisoning” of many biogenic salts such as Calcium, Potassium, Sodium, Magnésium, Soufre, Phosphore. During the decomposition of the sheets in Humus, these elements are slackened in the ground and contribute to its improvement.

The shape of the sheet

The sheet is known as simple if the limb is whole, or made up if it is cut out in several small sheets or Foliole S. According to the provision of the leaflets on the main axis of the sheet or rachis , said that the sheet is:

pennate , if the leaflets are laid out as the barbs of a feather;

imparipennée if their number is odd (there is a final leaflet);
paripinnate in the contrary case;

trifoliolée if there are three leaflets (Trèfle);
webbed or digitée if they are laid out like the fingers of the hand. However the palms, sheets of the Palmier, which are in fact of the whole sheets, but torn, have sometimes a form Pennée in the shape of feather (Dattier), or Costapalmée (Sabal Palmetto) if the structure is intermediate between the 2 other forms (either the general aspect of a range but with marked an enough central axis);
pedalled if each leaflet is inserted on the close leaflet (Hellébore);

The sheet can be doubly made up;

bipennée if the leaflets are made up of foliolules ;
tripennée if the leaflets are themselves made up.

The division of the limb

The limb can be plain (like fig. 1,2) cut out (fig. 4). In this last case (fig.4), one can specify if the sheet is:

lobed (A), when the lobes are not very deep and reach approximately 1/8 of the width of the sheet, pennatilobate (b), when the lobes are laid out out of comb, palmatilobate when the lobes have a webbed provision.
pennatifid or palmatifid when divisions reach 1/4 of the width of the sheet and in any case less than half of this one;
pennatipartite (c), or palmatipartite when the cutting out exceeds the medium of the sheet;
pennatiséquée (synonymous with compound pennate) or palmatisect (made up webbed) if divisions reach the rachis;
laciniée if the limb is divided into narrow thin straps (watery plants).

The edge of the limb

The edge of the limb (fig.3) can be plain , corrugated , sinué , sawn (A), serrulé (finely sawn), cogged (b) or crenelated (c).

The shape of the sheet

The shape of the sheet (fig. 2): in heart (A), oval (b) or lancéolée , or webbed , in the shape of palm. the forms are very diversified: sickle-shaped (in form of forgery), auriculée (with auricles at the base of the limb: Laiteron), sagittée (in the shape of arrow), peltate (when the petiole fits in the middle of the limb: Nasturtium), décurrente when the limb is prolonged on the stem by wings, perfoliate when the auricles are welded giving the impression which the stem crosses the limb, cone when two opposite sheets are welded by the limb (Chèvrefeuille)…

Provision on the stem

Moreover, the distribution of the sheets on the stem is also a very variable character, whose study is the Phyllotaxie. One can say sheets which they are:

alternate , i.e. isolated and laid out alternatively on both sides from the stem;

opposite , when they are laid out by two, on the same level, one opposite the other on a Tige or a Rameau. ;

verticillate , if they are joined together, by three or more, in circle around the stem, successive stages.

Foliar polymorphism

The shape of the sheets can vary on the same plant. It is the case at the Lierre climbing where the fertile sheets of branches are different from the sheets of the sterile branches (true polymorphism).

Polymorphism can also result from the influence of the medium: at the Sagittarius with sheets out of arrow the immersed sheets are ribboned, the sheets nageantes are cordiformes, the sagittées air sheets.

The youthful Cotyledon S and sheets which succeed to them immediately can also be different from those of the adulthood.

Specializations of the sheet

The sheets generally ensure the chlorophyllian function, but this one is also, at least assured partially, by the stem which is generally green, and whose Parenchyme comprises Chloroplaste S. an good example is the Genêt with brushes, at which all the stem is known as photosynthetic.

A whole variety of specialization is found:

at the thorny plants, the sheets are often transformed into spines , resulting from the modification of the leaflets, or of the Stipule S, or simply hairs. Like the xerophilous plants, it is about a mechanism of defense de against the dryness, or of defense against the chattering of the animals Herbivore S;
at the plants carnivores, they take very specialized forms, in ballot box at the Nepenthes, traps some at the Dionée S which have a limb in two parts provided with pivots and able to fold up one on the other to imprison insects;
at the fatty or succulent plants, the sheets are often transformed into body of reserve;
at the climbing plants, the sheets or the leaflets is transformed into gimlets enabling them to cling to their support. Sometimes it is the Pétiole which fulfills this function (Clématite);
at the watery plants, the sheets can be transformed into floats (Jacinthe of water);
at the plants Xerophilous S (adapted to the Dryness), the sheets can be reduced in scales or Aiguille S (Conifère S). The plant decreases its leaf aera in order to limit the evapotranspiration. Thus the Holm oak can have several shapes of sheets: in favorable medium, where the moisture of the air is not limiting, it will have sheets with almost oval limb, while in dry medium, the sheets for the majority will be cogged.

Anatomy

The petiole

detailed Article: Petiole

A petiole (of Latin petiolus: small foot) indicates the stalk of a sheet, connecting the limb to the stem. The petiole with the structure interns of a stem.

When that it is widened until replacing the sheet in its function, one speaks about Phyllode. A sheet without petiole, or with very short petiole, is known as sessile.

Some petioles have functions specific to a plant:

at the sensitive one, it allows the movement of the sheets;
at water sweet chestnut, it allows the floating of the plant.

The limb

detailed Article: foliar Limb

The limb of a sheet is consisted of the following fabrics:

the protective skin which recovers the upper and lower surface; it is generally made up by a single base of cells which generally do not comprise chloroplasts, sometimes covered by an external protective coating, the cuticule. Certain cells of the skin can be transformed into hairs.

On the lower skin one finds the Stomate S. They are kinds of pores, formed by two cells in the shape of kidneys, which leave between them a variable opening, the ostiole ;

the Mésophylle or foliar parenchyma, which comprises two layers:

under the higher skin, a palisade parenchyma , formed fabric of several lines of lengthened perpendicular to the surface of the limb and tightened cells enter they, without gaps.
between this one and the skin a lower lacunary parenchyma , larger cells sparing between them a network of gaps, which communicates with the stomata, and ensures the gaseous exchange with outside.

Skin

The skin is the layer of external cells of the sheets. This layer generally transparent (these cells do not have chloroplasts) and is covered by a Cuticule of waxy aspect making it possible to limit the water losses at the time of too strong heats. At the plants of the dry climates this cuticule is thus thicker. The cuticule is sometimes finer on the skin lower than on the higher skin.

The lower skin is covered with pores called Stomate S. Those allow the Oxygène and the Carbon dioxide to return and leave the sheets. The steam is also evacuated by the stomata during perspiration. To preserve water, the stomata can be closed during the night.

Hairs recover the skin of many species of plants.

The mésophylle

Most of the interior of a sheet, i.e. between the lower and higher skin, is composed of a Parenchyme called Mésophylle. This fabric plays a very important part in the Photosynthèse. The mésophylle is composed of two parts:

towards the higher face: the palisade parenchyma composed of vertical, lengthened and tightened cells; the cells which constitute it is rich in Chloroplastes, it is in this parenchyma that is held photosynthesis;
towards the lower face: the lacunary parenchyma (or spongy). The cells have a rounder form and are less tight. The gaps between the cells are used to store gases exchanged between the sheet and the atmosphere.

Veins

The veins of a sheet are the prolongations of the petiole in the foliar limb. One distinguishes the principal vein and the secondary veins on the basis of the first. It is on the level of the veins, being detached by their relief curvature from the remainder from the limb, that are located the main part of conducting fabrics (xylème and phloem) of sap, organized in beams.

The provision of the veins (or nervation) varies according to the species or the families. One can distinguish three great types of nervation:

sheets with pennate nervation (penninerves), in which a principal vein, prolonging the petiole, divides the limb in two virtually identical parts according to the axis of symmetry and which the secondary veins are detached according to an alternate or opposite provision.
sheets with webbed nervation (palminerved) where several veins, of odd number, are detached from the petiole at the contact point with the limb (example: the vineleaf).
sheets with parallel nervation (parallelinervate), whose veins are parallel, without anastomoses between them. It is the case of the majority of graminaceous (Poaceae), whose sheets are generally without petiole.

Phenology

The sheets can be persistent or null and void according to the species, the climatic conditions and the seasons. For the nonpersistent plants, the appearance of the sheets is called “foliation”.

See too

External bonds

See a detailed diagram of the structure of a sheet.

Simple: Leaf

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