A quick biology review:
Guard cells are really neat. They are specialized cells in the epidermis of leaves and other parts of the plants that control gas exchange with the environment.They are produced in pairs and have a central opening called a stomatal pore. When a pair of guard cells are swollen (turgid) with water, the stomatal pore opens allowing carbon dioxide to flow in and oxygen (produced through photosynthesis) to flow out. While this gas exchange is essential to the plant, water is also lost through the stomatal pore; thus the plant has to balance the amount of CO2 absorbed from the environment with the amount of water lost through the pores.
Mary Parker kindly contributed the following image of stomata from the lower surface of an ash leaf and mentioned that "in a fresh sample as in the image, the plastids would contain greenish chlorophyll, but after gut transit, this would be lost."
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