Magical Leaves
To understand the visible results of heat stress on our plants, we really have to understand a bit about how they are constructed. We all know what a leaf is and can identify many of the myriad of shapes and sizes they come in as belonging to specific plants. What's not evident to the naked eye is how they're made and how they work.
It's pretty incredible, really - those thin, often transparent leaves are crammed with working parts (wonderfully illustrated in this diagram of leaf structure) that permit them to exchange water and gasses, in the process of photosynthesis, to produce glucose, (a six-carbon sugar) the food that the plant burns to gain energy for metabolism.
The top of the leaf is covered with a transparent waxy cuticle, allowing light to enter for photosynthesis. This waxy cuticle stops transpiration from happening throughout the leaf, other than in the stomata, where it is supposed to take place. It also makes the leaf cells waterproof and protects them from water loss and invasion by fungal spores, insects and bacteria.
The upper epidermal cells are also transparent to allow light in to reach the chloroplasts (the structures which contain chlorophyll, where photosynthesis actually happens) for photosynthesis.
The lower epidermis has stomata, which allow gaseous exchange to occur. Gaseous exchange being the intake of carbon dioxide and the release of oxygen, which is essential for photosynthesis.
A leaf stoma consists of a pore and two guard cells in the epidermis of a stem or leaf. Stoma is the singular form of the word. The plural is stomata.
In very simplistic terms, a leaf functions this way:
Photosynthesis starts in the guard cells of a leaf in the presence of light. Glucose accumulates, drawing water into the guard cells by osmosis. When the guard cells become fully turgid they bend apart, opening the stomata so that gas diffusion is possible.
As the air temperature rises and humidity levels fall around the leaves, the guard cells become flaccid, closing the stomata and stopping the exchange of gas and water loss from the leaves.
When temperatures are cooler - usually at dusk and early in the morning - and humidity is higher the stomata open again and the cycle repeats so that photosynthesis can again accumulate stores of glucose.
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