This article is an overview of leaf form and function and a look at variation in leaves
Many indoor tropical plants are prized for their fantastic blooms, as the scent and appearance of a flower can brighten up an entire room for days or even weeks. However, when the floral fireworks are over you’re going to have to spend most of the year looking at its leaves.
Foliage is around for a lot longer than flowers, so is an important consideration with any plant. My first attraction to any plant is the foliage as there is something about a leaf that fascinates me.
This is a brief look at why leaves are integral in a plants’ survival…
Leaves are the solar panels that plants use to power photosynthesis, using the sun’s energy to create food. They are also the primary sites for gas exchange so are the lungs of the plant, and they perform many specialised functions too.
To survive, a plant needs to let CO2 (found in the air) in. Leaves therefore have the difficult task of allowing air in whilst having to conserve water. This calls for a highly specialised structure – the Leaf.
The Upper Epidermis, which is usually the side of the leaf exposed to the sun, is made up of large flat cells. These are often waxy and almost completely transparent. This means that the upper epidermis can let light in, but retains water.
Just below the upper epidermis is the leaf mesophyll layer. This consists of several layers of photosynthetic material, with air spaces in the lower layers to allow gas exchange.
On the underside of the leaf, or the ‘Lower Epidermis’ are the majority of stomata. These are specialised cells that allow air in and out to the mesophyll layer. Water loss, or transpiration, occurs during gas exchange, so plants have engineered a clever way of controlling it.
Rates of water loss depend greatly on the temperature at the leaf surface and relative humidity. On dry, warm, windy days the relative atmospheric humidity will be low and high levels of water loss will occur through the stomata. This water is replaced across adjacent cells by gradient, ultimately being pulled from the leaf veins and then from the vascular system, roots and soil. This water movement throughout the plant is fuelled by pressure gradients between the plant and the atmosphere which are sufficient to drive water from the roots to the canopy of the largest trees. If water is not available, then the plant will wither and ultimately die, so water must be conserved.
The largest areas of water loss are the gas exchange sites in the leaves, so this is where the plant uses this highly specialised system.
Depending upon the water needs of the plant, the guard cells that are found around the stoma, change shape. The shape change causes the stoma to open or close, allowing gas exchange only when it is absolutely necessary. Whilst it is impossible to completely prevent water loss, this active management controls it sufficiently.
Generally, stomata open more in the daytime, allowing gases in to support photosynthesis. In the dark of night when photosynthesis ceases, stomata close, restricting gas exchange and therefore, water loss.
Variation in Form
Leaves are in direct contact with the environment and as such are heavily impacted on by environmental conditions. Over millions of generations, natural selection has shaped the form of the leaf to ensure it is as fit as possible in a given environment.
For example, Gunnera manicata leaves can span over 8 feet. They are huge solar panels catching every last drop of sunlight, crowding out other plants whilst the roots sit in damp, rich soil pumping water and essential nutrients into the plant. They have sharp spines and are poisonous, which is an adaptation to deter herbivores from eating them.
Looking at the other end of leaf size scale, tiny spines found on many cacti are adapted leaves. They have evolved into incredibly effective physical deterrents, whilst taking on a form that conserves water extremely effectively. The lack of ’real’ leaves effectively removes the ability of the plant to transpire through its leaves. This means that it does not lose water through the leaf surface, conserving its reserves extremely effectively.
You can tell a lot about a plant by its leaves.