In most vascular plants, the leaf is the main photosynthetic organ, although green stems also perform photosynthesis. They are keys not only to plant life but to all terrestrial life. The primary role of leaves is to collect sunlight and make food by photosynthesis. There is great diversity in the leaves of plants and it’s not surprising that there is no single best way to collect solar energy other than leaves for photosynthesis.
Leaves of non flowering plants
Leaves may vary in size, shape, and their arrangement on stems. Nonflowering vascular plants have three basic types of leaves: microphylls (“tiny leaves”), fronds and needles.
Microphylls are the tiny leaves of club mosses. The first leaves to evolve were microphylls.
Fronds are the finely divided leaves of ferns. Fern fronds grow by unfurling their ” fiddleheads”.
Needles are the very thin, pointed leaves of conifers. They have thick cuticle to reduce water loss and lots of chlorophyll for maximum absorption of sunlight.
Leaves of flowering plants
Flowering vascular plants also have diverse leaves. However, the leaves of all flowering plants have two basic parts in common .It consist of a lamina or flattened blade and a stalk, the petiole, which joins the leaf to the stem at a node. The lamina is flattened with distinctive upper and lower surfaces. Some leaves have outgrowths called stipules where the petiole attaches to the stem. Grasses and many other monocots lack petioles; instead, the base of the leaf forms a sheath that envelops the stem.
Generally, the form and arrangement of leaves maximizes light exposure while conserving water, reducing wind resistance, or benefiting the plant in some other way in its particular habitat. The flowering plants leaves vary depending on the arrangement of leaves on stem and division of blade. They can be as follows:
- In alternate arrangement, one leaf, branch, or flower part attaches at each point or node on the stem, and leaves alternate direction, to a greater or lesser degree, along the stem.
- Leaves arranged in whorls encircle upright stems at intervals. They collect sunlight from all directions.
- Leaves arranged in basal rosettes take advantage of warm temperatures near the ground.
- Leaves arranged in alternate or opposing pairs collect light from above. They are typically found on plants with a single, upright stem.
- The blades of simple leaves are not divided. This provides the maximum surface area for collecting sunlight.
- The blades of compound leaves are divided into many smaller leaflets. This reduces wind resistance and water loss.
Leaves and photosynthesis
A leaf consists of several different kinds of specialized tissues that work together to make food by photosynthesis. The major tissues are mesophyll, veins and epidermis.
Mesophyll makes up most of the leaf’s interior. This is where photosynthesis occurs. Mesophyll consists mainly of parenchymal cells with chloroplasts. The air spaces in spongy layer allows carbon dioxide to diffuse through the leaf, and increase the surface area. Veins are made primarily of xylem and phloem. They transport water and minerals to the cells of leaves and carry away dissolved sugar. The epidermis of the leaf consists of a single layer of tightly-packed dermal cells. They secrete waxy cuticle to prevent evaporation of water from the leaf. The epidermis has tiny pores called stomata (singular, stoma) that control transpiration and gas exchange with the air.