The Physiology of Leaves

The powerhouse for plant growth had been laid down at the very beginning of plant evolution, with the process of photosynthesis, using the energy of the Sun to produce sugars from available minerals. This process is dependent on the green pigment chlorophyll, present in all leaves, even when masked in coloured leaved cultivars of plants.

As with the Bark, the leaves are covered in the Epidermis, which is peppered with Stomata, the pores through which the tree breathes.

Photosynthesis

s the process by which chlorophyll containing plants and some bacteria, capture energy as light and convert it to chemical energy.

The energy produced is used to drive a series of chemical reactions, which produce simple sugars, which provide the food for your trees and indeed all life. The plant must have a supply of carbon dioxide, light and water for photosynthesis to occur.

Photosynthesis takes place inside chloroplasts, which are found in leaf cells of plants.

Oxygen is a by-product of photosynthesis, almost all oxygen in the atmosphere has been produced by photosynthesis.

Chlorophyll

Chlorophyll is the green pigment common to all photosynthetic cells, it absorbs all wavelengths of visible light with the exception of green, which it reflects. This reflected green light is detected by our eyes, giving leaves their colour.

Daylight may be considered to be made up of the three primary colours Red, Green and Blue, although blue is by far the largest component and the best suited to photosynthesis.

The Red/Blue elements of light are absorbed by the leaf, passing into the palisade cells in the leaf and fuelling Photosynthesis in the chloroplasts. The green light is reflected.

Normal (tungsten) household lights emit a lot of red light and little blue, this means that, although to our eyes a room is well lit, the plant is getting little blue light and is unable to photosynthesise. Normal fluorescent lights are if anything worse as the following diagrams illustrate.

Tungsten filament lamps emit a continuous spectrum peaking in the red area and falling away in the blue, the colour most suited to Photosynthesis, reducing the plants ability to turn light into energy.

Fluorescent lamps emit a different spectrum, with peaks at varying wavelengths depending on the type of lamp. Some tubes, particularly those made for the Aquarist trade will give a good approximation of daylight.
The impact of unsuitable lighting on trees kept indoors cannot be understated. If you must keep bonsai indoors please talk to your local garden centre about appropriate lighting.

Although the two graphs above are crude representations of actual Spectral Emission Diagrams they are as accurate as my memory allows, having spent five years studying photography.

The leaf is a complex structure, having a number of specialist functions. In this section we will look at how leaves work.

The epidermis is the outer layer of the tree, being present on both the bark and leaves. It provides a waxy layer, reducing the amount of water the tree loses through evaporation. In the leaves this layer is about one cell thick, allowing maximum sunlight to get into the leaf, powering the process of photosynthesis.

The palisade cells are directly under the skin (epidermis) of the leaf, they contain the chloroplasts, which are the site of photosynthesis. Being close to the surface of the leaf they are able to obtain as much sunlight as possible.

Chloroplasts are photosynthetic organelles found in the leaf cells of higher plants.

Chloroplasts contain the green pigment chlorophyll along with the enzymes and other products needed for photosynthesis. However the green chlorophyll may be masked by other pigments such as phycoerythrin, or phycocyanin, giving red or blue colours to the leaves.

Even red leaved cultivars of species have chlorophyll, although it may not be that obvious

As with all other living parts of the tree, the leaves contain the tubes of the xylem, bringing water and chemicals to the chloroplasts in the palisade cells, to convert to sugars. They also contain the phloem, the tubes that distribute the sugar rich sap around the tree, they are however carried in bundles rather than the layered rings of the trunk/branches.

Carbon dioxide is essential to the process of photosynthesis and this gets into the leaves through the Stomata. These holes are opened and closed when needed by the guard cells.

The guard cells will open or close the stomata to allow Carbon dioxide in and when used allow Oxygen out, oxygen being a waste product of photosynthesis. They will also close if the tree becomes dry, stopping water evaporating out through them.

In conifers the stomata are often closely grouped along the underside of the needles, creating light coloured bands.

A maple leaf (in autumn colour), showing the veins. Those veins contain the same vascular tissues, the xylem and phloem that are present in the trunk and roots.

A Normal and an Etiolated shoot.

A lack of light will cause growth to become thin and pale. This is called 'etiolated growth' and such growth has a greatly reduced potential for photosynthesis and limits the plants ability to produce food.

Bonsai kept indoors, in inadequately lit places, will be prone to this.

Leaf fall

Leaf drop in deciduous trees is controlled by a process called Abscission, which is also responsible for the shedding of flowers and fruit.

Should your bonsai suffer a trauma, such as drying out, Abscission can be triggered as a self-defence mechanism.

As with most processes in plants, abscission is brought about by the release of hormones, Absissic acid, and Ethylene, a type of hydrocarbon that can act as a plant hormone. Production of these is usually triggered by the decreasing amount of light and lowering of temperature as the year ends. These hormones modifiy cells at the base of the leaf petiole, creating two layers of cells.

In the upper layer, the cells breakdown, allowing the leaf to separate.

At the base of each leaf you will find a dormant bud, waiting to produce next years shoot.

Allen. C. Roffey 16:45 03/02/2003 Monday, August 6, 2018 8:37