Views: 8

The leaf as an organ 

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 later chapters we will look at the various organs found in animals. In this section, 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.

Leaf structure 

The leaf is a collection of tissues which include:

1. The epidermiswhich covers the upper and lower surfaces.
2. The mesophyllinside the leaf which is rich in chloroplasts.
3. The veinscontains the vascular tissue (where xylem and phloem are present).

Epidermis

Epidermal cells form the outer layer covering a leaf, separating internal tissues from the external environment.

Epidermis tissue has several functions:

1. protection against water loss via stomata and a waxy cuticle
2. regulation of gaseous exchange
3. secretion of metabolic compounds

Mesophyll cells

The mesophyll is located between the upper and lower layers of the leaf epidermis, and is mostly made up of parenchyma (ground tissue) or chlorenchyma tissue. The mesophyll is the primary location for photosynthesis and is divided into two layers, the upper palisade layer and the spongy mesophyll layer.

The upper palisade layer lies beneath the upper epidermis and consists of vertically elongated cells that are tightly packed together to maximise the number of cells exposed to sunlight. In addition, these cells contain many chloroplasts, thus maximising their photosynthetic ability. The palisade layer thickness depends on the extent of exposure to the sun. Leaves that are exposed to the sun have a thicker palisade layer. Those that are typically found in the shade have a thinner palisade layer. Beneath the upper palisade layer is the spongy mesophyll. The cells in the spongy mesophyll are slightly rounder and less densely packed and have air spaces to allow for gaseous exchange.Figure below show the leaf and tissue structure of a dicot plant.

                                     The Leaf structure.

Vascular tissue is made up of the xylem and phloem vessels you learnt about earlier in this chapter. Xylem transports water and minerals to the leaf. Phloem transports dissolved sucrose made in the leaf out of its site of synthesis to the rest of the leaf. Most leaves have a bundle sheath around the xylem and phloem, consisting of sclerenchyma or collenchyma, for extra support.