A leaf is vital part of a plant, so much so, that after a seed germinates among the first parts to develop is a cotyledon, also known as the first leaf or seed leaf. And if a plant is without leaves for a long time, it begins to wither and die. This article takes a brief look at the leaf, its function and use.   

What is a leaf?

A leaf is an aerial (aboveground) part of a plant and the primary organ of photosynthesis in vascular plants. It is thin, flat, flexible, and typically green in colour. In general, a leaf has a leaf base which is where the leaf attaches to the stem, leaf blade (lamina) which is the green flat surface of a leaf, veins which transport water and food between the leaf and stem, and petiole which is the stalk that connects the leaf blade to the stem.  

How a plant makes a leaf

A seed contains the blueprint of the plant. This blueprint explains how the plant should be developed step by step. Besides the cotyledons other parts that are developed by a seed are the root and shoot apical meristem. The apical meristem is the source of cells needed to generate cells that will divide and differentiate to form complex organs, such as flowers, roots, stem, and leaves. The meristem can be divided into functional zones based on the development potentials, molecular markers, and rates of cell division. A leaf is developed from the cells found in the functional zones of the shoot apical meristems, which is situated at the tip of the stem. 

The morphology of a leaf

Leaf morphology is variable, meaning a leaf will look different depending on the type of plant (herb, vine, shrub, or tree) and the environment. In general, a leaf differs in terms of structures, arrangement, shapes, sizes, texture, margins, and venation. 

1. Structure

A leaf structure may be simple or compound. A simple leaf is a single leaf whose leaf blade is not divided. A compound leaf is a leaf that is divided into smaller leaflets that are joined to a single stem. Examples of compound leaves are pinnately compound (e.g. bipinnate, tripinnate, paripinnate, imparipinnate, etc.), and palmately compound (e.g. unifoliate, bifoliate, trifoliate, etc.). A leaf can also be double compound meaning it has been compound twice.   

2. Arrangement 

A leaf on the stem may be arranged as alternate (when a single leaf forms at each node alternately), opposite (when a pair of leaves are opposite each node), or whorl (when more than two leaves form at the node). 

3. Leaf shape

The shape of a leaf can be lanceolate (lance-shaped), linear (long and narrow), obovate (teardrop-shaped), ovate (egg-shaped), palmately lobed (divided by cleft), pinnately lobed (divided by cleft), reniform (kidney-shaped), and sagittate (arrowhead-shape). 

4. Leaf margins

Leaf margins refer to the edges of a leaf. These may be entire (smooth), crenulate (rounded teeth), dentate (square or rectangular teeth), serrate (teeth pointing toward the leaf apex), and lobed (curling). 

5. Leaf venation

The venation of a leaf refers to the arrangement of veins and veinlets in the leaf. These veins transport water and nutrients to and from the leaf. Their arrangement can be parallel (the veins run parallel to each other), pinnate (veins run from the midvein), palmate (veins diverge from a single point), and reticulate (the veins have a random and complex arrangement). 

The function of a leaf

The leaves have two major functions in a plant. The first and most important function is photosynthesis and the second is the production of oxygen. The leaf also defends the plant from being damaged by animals and stores food and water for later use.

Photosynthesis

The leaves of plants function similar to solar panels in that they use the sun to generate energy. Leaves produce this energy through a process called photosynthesis. 

Photosynthesis is a biological process that occurs in the presence of sunlight where the leaves’ stomata open up to absorb carbon dioxide from the atmosphere. The carbon dioxide is then combined with water transported from the roots to produce carbohydrates and oxygen. When the leaf stomata opens some of the water in the plant is lost through evaporation.  

Photosynthesis can be modelled in a chemical equation as follow: 

6CO2 + 6H2O  →  C6H12O6 + 6O2

The above equation means that six carbon dioxide molecules and six water molecules are converted by sunlight (indicated by the arrow) into a carbohydrate molecule and six oxygen molecules. The molecules before the arrow are called reactants, the arrow represents the chemical reaction, and the molecules after the arrow are the products.

The carbohydrates that are produced are transported to the different parts of the plant to be used as food (or energy) for growth and maintenance. 

The production of oxygen

Oxygen is colourless, odourless, and tasteless gas that makes up about 21% (or 1/5) of the volume of the atmosphere. This gas is necessary for life to exist on earth and it is produced as a by-product during photosynthesis. 

Defence 

The leaf defends the plant by developing spines (or thorns) to protect it from animals. An example is the leaf of an aloe. 

Storage

Depending on the environment, the leaf may be adapted to store water and nutrients for later use. For example, the leaves of succulent plants (such as an aloe) in the Karoo are adapted to store water. 

Definitions

1. Stomata are tiny pores on the surface of a leaf. These pores are so tiny, they can only be seen under a microscope.
2. Carbon dioxide is a greenhouse gas that humans breathe out and can be produced by burning inorganic or organic substances and from volcanic eruptions.

The uses of plants

Once harvested (removed from a plant), a leaf may be used to make food (spinach or kale), spice (bay leaves), basket weaving, natural dyes, and medicine. In traditional medicine, the leaves of the following plants are used to make medicine:

1. The crushed leaves of Abrus precatorius (umkhokha) are used to treat inflamed eyes.
2. The leaves of Acokanthera oblongifola (inhlungunyembe) are used to make poultice to treat snake bites. 
3. The powdered leaves of Adenia gummifera (impindamshaye) are used to treat biliousness.  
4. The leaf decoction of Albizia adianthifolia (usolo) is used to treat chicken pox.   
5. The leaf infusion of Aloe arborescens (inhlabane) is used to lower blood pressure.

In conclusion, the leaf plays a key role in the health and vitality of a plant. The type of plant (or its genetics) and the environment are the main factors that determine the type of leaf a plant has. For example: gymnosperms that are native to the arctic tundra will typically have reduced (needle-shaped) leaves to minimise water loss. 

Reference and further reading

• Byrne, M.E., 2012. Making leaves. Current Opinion in Plant Biology, 15(1), pp.24-30.
• Braybrook, S.A. and Kuhlemeier, C., 2010. How a plant builds leaves. The Plant Cell, 22(4), pp.1006-1018.
• Chapman, K.D., Dyer, J.M. and Mullen, R.T., 2013. Commentary: why don’t plant leaves get fat?. Plant Science, 207, pp.128-134.
• Corrigan, B.M., Van Wyk, B.E., Geldenhuys, C.J. and Jardine, J.M., 2011. Ethnobotanical plants used in the KwaNibela Peninsula, St Lucia, South Africa. South African Journal of Botany 77, 346–359.
• Dale, J.E., 1992. How do leaves grow?. BioScience, 42(6), pp.423-432.
• Gilges, W., 1953. Trees in South Africa 5,17 (Rhodes Livingstone. Occasional Papers 11).
• Smith, L.G. and Hake, S., 1992. The initiation and determination of leaves. The Plant Cell, 4(9), p.1017.
• Vasco, A., Moran, R.C. and Ambrose, B.A., 2013. The evolution, morphology, and development of fern leaves. Frontiers in plant science, 4, p.345.
• Watt, J.M. and Breyer-Brandwijk, M.G., 1962. Medicinal and poisonous plants of southern and eastern Africa, second edition. Livingstone, London.