Biology » Plant and Animal Tissues » The Leaf As An Organ

The Leaf as an Organ

The Leaf as an Organ

Recall the fact that an organ is a collection of tissues that are joined together to perform a common function. A group of organs work together to form an organ system. Organs exist in all higher biological organisms, they are not restricted to animals, but can also be identified in plants. For example, the leaf is an organ in a plant, as is the root, stem, flowers and fruits. In this lesson and the next few, the leaf is used as an example of an organ.

The leaf structure will be discussed using a cross-section of a dicotyledonous leaf. Structure needs to be related to functions, such as transport, gaseous exchange and photosynthesis. Link this section with the plant tissues already taught, the cell organelles and the movement of molecules across membranes into, through and out of the leaf.


You have learnt about individual tissues found in plants and animals. We will now look at how tissues join together to form organs. An organ is a collection of tissues joined together as a structural unit in order to perform a common function. In a another tutorial, we will look at the various organs found in animals. In this lesson and the next few, we will discuss how a plant leaf is an example of an organ. We will present its structure with respect to its functions in photosynthesis, gaseous exchange and transport.

Leaves are typically found in vascular plants, which have lignified tissues (xylem) that enable them to conduct water. Leaves are usually flat and thin to allow for maximum gaseous exchange and capture of light. The organisation of the leaf has evolved to allow maximum exposure of chloroplasts to light, and to absorb carbon dioxide. Leaves have stomata, pores found in the leaf epidermis, which allow the plant to regulate the exchange of carbon dioxide, oxygen and water vapour with the atmosphere.

The shape and structure of leaves varies considerably from one plant to another. This depends on the climate, available light intensity, presence of grazing animals, nutrients and competition from other plants. Leaves are either dorsiventral or isobilateral. Dorsiventral leaves have both surfaces differing from each other in appearance and structure. Isobilateral leaves have both surfaces looking the same. Leaves can also store food and water and are modified to perform these functions.

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