NOVEMBER 2025

As the landscape is alight with fiery colour at present, it seems a good moment to examine why some plants change colour in the autumn, and lose their leaves, and indeed why others don’t. 

Firstly though why do plants have leaves, what do they do?

Plants unlike animals are able to synthesise their own food/energy stores, and the leaves are the main areas of energy harvesting. 

Through the process of Photosynthesis plants are able to combine molecules of WATER and CARBON DIOXIDE into SUGAR and OXYGEN using LIGHT energy. 

The oxygen is released as a waste product, and the sugars are converted into starch molecules, which are insoluble and readily stored by the plant for its use at a later date or for any other organisms that come grazing.

A typical leaf in section will have an Epidermis layer much like our own skin, but in plants it is highly transparent to allow light to pass through to the Palisade cells below. These cells are tightly packed and are full of Chloroplasts, the organelle that does the work of Photosynthesis. 

The Chloroplasts absorb most colours of light, but not green, which is why our brains interpret/see them as being green.

Leaves packed full of cells full of Chloroplasts in turn appear green to us. 

Below the Palisade cells are the spongy Mesophyll cells. These also contain Chloroplasts, but their main function is to act as the body and structure of the leaf ensuring that the leaf blade remains open and turned toward the light. They are much more loosely packed to allow the movement of water vapour out of the leaf through the Stomata, and oxygen and carbon dioxide back and forth as needed. When water is scarce the Stomata will close to prevent further water loss from the plant.

Over many hundreds of millions of years plants have evolved two broad strategies to cope with surviving cold environmental conditions, conditions that may damage their tissues through freezing and therefore impede their ability to grow. The danger posed to plants is that as the water present in softer tissues cools and turns to ice, it expands, rupturing the cell walls, and thereby killing the cells. Also dangerous to plants is the risk of drought, which might seem odd for winter. However plants can only use water in its liquid form, therefore if the ground is frozen and the only water is present as ice or snow then plants may have to wait many weeks before they can take in water. 

Some plants have developed to be deciduous, shedding their soft leaves in the winter, and others remain evergreen. Many evergreens have evolved in areas that never reach warm temperatures, think of the conifer trees of the encircling Taiga forest of the Arctic. 

To adapt to such conditions they grow needles that have a far smaller surface area than deciduous leaves and fewer Stomata to prevent water loss. The needles often have a smooth waxy coating to help shed excess water and snow from around the plant. The internal tissues of conifers are a fine lattice of cells enabling water and nutrients to be transferred both up and down the stems, but also sideways. This means that should cells and transport networks in one part become damaged there remain many routes to circumvent these areas. Finally they remain green, holding onto their Chloroplasts and so are always growing, often very slowly.

Where evergreens are very much the tortoises of the horticultural world, deciduous plants adopt the role of the hare. Growing in warmer climates with plenty of light and available water they unfurl their large leaves and readily photosynthesise, growing fast.

As cooler temperatures develop plants break down and remove valuable molecules from the leaves storing these in insulated stems or below ground where temperatures are warmer. The joining points of the leaves are then sealed with a corky layer, the stripped leaves eventually falling to the ground where they will be further broken down and their parts recycled by the plants for future use. The leaf litter both helps to insulate and enrich the growing environment of the roots.

The Chloroplasts are of great value to the plant, and so in the autumn these are extracted and disassembled into simpler molecules, stored over winter, and then reassembled in the spring.

Other pigments such as the Carotenoids (yellow coloured) and Xanthophylls (red coloured) absorb light energy, but plants tend to leave these. As the many green Chloroplasts are removed the other pigments in the leaves become visible making for wonderful autumn displays.