Leaf . Part of the plant responsible for carrying out photosynthesis , as well as plant respiration and transpiration .They come in all shapes, sizes, and colors, and each one serves a unique purpose.

Concept

The leaf (from the Latin folium), is one of the most important parts of vascular plants, as it is the vegetative organ primarily specialized in carrying out photosynthesis. In general, leaves are laminar or acicular structures, which sprout laterally from the stems or branches, which have limited growth and contain mainly photosynthetic tissue, always located within reach of light . In addition, leaves are responsible for carrying out other functions such as transpiration and respiration . Secondarily, leaves can be modified to store water or for other purposes.

Functions

The leaf performs several functions in the plant, such as photosynthesis , respiration or transpiration. These functions can also be carried out by herbaceous stems and by the young portions of woody stems. In some plants that lack leaves, such as cacti , these functions are performed by the stems.

Photosynthesis

Main article: Photosynthesis .

Photosynthesis, also known as the chlorophyll function, consists of the synthesis or production of organic substances with nutritional properties (sugars) from atmospheric CO2 , which the leaves absorb through the stomata , and from the water supplied by the plant’s roots . This process requires energy consumption that is obtained from sunlight and oxygen is generated .

Photosynthesis is an essential process for life on the planet , since it allows the production of organic matter from inorganic matter and the energy provided by sunlight , and generates oxygen that is emitted into the atmosphere .

Photosynthesis is possible thanks to special pigments that plants contain, the most important of which is chlorophyll , responsible for the green color of plants.

Breathing

Main article: Cellular respiration .

Respiration is the process by which the energy necessary to develop biological activity is obtained through the metabolization of products (sugars) obtained through photosynthesis.

Respiration requires taking oxygen from the atmosphere and emitting CO2. During the day, respiration is masked by photosynthesis , but at night, the plant only breathes, releasing carbon dioxide and absorbing oxygen.

Respiration is carried out by all living cells of the plant, not just those of the leaves. But gas exchange takes place mainly through the stomata , which are much more abundant in the leaves than in other parts of the plant.

Perspiration

Transpiration is the loss of water through stomata in vascular plants. It is a fundamental process for raw sap to move from the soil to the leaf and to control the temperature of the plant.

Absorption

In addition to the essential functions mentioned above, plants are able to absorb some nutrients through their leaves and incorporate them into the vascular bundles. Tree species (olive, orange and other fruit trees) have the greatest absorption capacity

Parts of the leaf

Limbo

It is the wide part of the leaf, and it has two sides, the upper side is called the front, and the reverse side is called the underside. It is the most attractive part and what most people understand and identify as a leaf when this name is mentioned.

Within limbo we have to talk about:

• The upper surface: This is the upper part of the leaf. It is usually bright green in colour.
• The underside: is the part opposite the upper side. Its colour is normally darker and may have special hairs.

The petiole

The petiole is the filament that connects the leaf blade to the stem or branch. The conducting vessels run through it. There are some leaves that do not have a petiole. These leaves without a petiole are called sessile.

Types of leaves

The enormous variability of the leaves allows them to be classified into different types, among which we find:

By its rib

The venation or nerviation of the leaves varies depending on the species, although the most common are those with parallel or parallel-veined venation.

• Parallelline veins: when all the veins are parallel and extend longitudinally from the petiole along the leaf.
• Penninervia or pinnatinervia: when there is a central nerve, and all the others are born along its axis, like the barbs of a bird’s feather when they start from the rachis.
• Palmatinervias: when the petiole, at the junction with the leaf, branches into different nerves.
• Curvinervia: when several nerves that start from the petiole do not extend parallel, but describe a more or less smooth curve along the entire leaf to its apex.
• Palmate: when there is more than one main branched nerve coming out of the petiole, like the fingers of a hand. Radial: when the nerves come out from a common center in the form of rays.

By the shape of the edge

Leaves can also be classified by their edges. These may be smooth (entire); have indentations (serrated or slightly indented edges); have more or less pronounced indentations (lobed, scalloped, split edges), etc.

Through limbo

Depending on the composition of the leaves based on the characteristics and appearance of the blade, they are classified as simple or compound.

• Simple: the petiole does not branch, the blade being a single piece.
• Compound: they have the blade divided into small leaves (leaflets) which can be further subdivided. The difference between a true leaf and a leaflet (which can be large) is that in the axil of the former there is a bud, which the leaflets lack.

By the shape of the limbo

Depending on the shape of the leaf blade, the leaves can be classified as:

• Heart-shaped: when its shape resembles that of a heart. 
• Lanceolate: when it has a lance shape.
• Sagittate: when its shape resembles that of an arrow.
• Bilobed: when it is split or divided into two lobes.
• Elliptical: when it has the shape of an ellipse.
• Oval: when it is oval-shaped.
• Palmately compound: when a compound leaf has divisions or leaflets arranged like the fingers of a hand.
• Trifoliate compound: when a compound leaf has three divisions or leaflets

Due to its arrangement on the stem

• Alternates
• Opposites
• Connotes
• Whorled
• Basal them

By the petiole

Leaf fall

It is the defoliation process that occurs naturally in plants.

This defoliation process is called chorism; if the stimulus that causes it is internal, we have autochorism, the most frequent of all and of a periodic nature; thermochorism, chemochorism or traumatochorism, when thermal, chemical or traumatic factors are those that determine the fall of the leaves.

When the leaves fall, the transpiring surface is greatly reduced, so that the plant can withstand cold periods or periods of great drought in a cushioned state of life.

Leaf fall in deciduous plants is due to an interesting phenomenon of cellular reabsorption. The detachment of the leaf always occurs in a certain way, at a fixed point on the petiole, and leaves a scar, which is a true amputation. Only in a few cases, and primarily in monocotyledons, does the fall occur irregularly or by marcescence, as in the case of palms.

At the base of the petiole, a layer of cambium cells determines the formation of the scar pad; above this is the actual insulating layer where the tissue breaks down, followed in turn by a lignified layer. The scar pad, at the point of detachment, becomes lignified or suberified, or both phenomena occur simultaneously.

In some dicotyledons, the structure of the shedding mechanism is complicated by the formation of a second separating layer above the scar pad, which shows an evolution of these tissues, since they do not appear in relation to periodic seasonal factors, but as a consequence of traumatic or parasitic lesions, poisoning, etc. In these cases, it is a real expulsion of the leaves, as has been demonstrated experimentally by subjecting the plant to sudden increases in temperature or varying the concentration of anhydride in the surrounding air.

Before falling, the leaves get rid of most of the materials that can be useful to the plant: carbohydrates, protein substances, etc. The waste products of metabolism, such as excess salts, remain in the falling leaf, and in this way, the plant is exonerated from useless materials.

Utilities of leaves

• They are edible, those that serve as food for humans, such as lettuce , chard , cabbage , spinach and others.
• They are medicinal, those used for illnesses, such as eucalyptus , mallow, borage.
• They are industrial, those used for the production of products intended for commerce, such as tobacco , indigo, cocuiza, and others.