Usually, the surface of petals is not as smooth as what we see with the naked eye. Petal epidermal cells will form many protrusions, and the surface of cells will be covered with waxy components, thus forming a hydrophobic interface to prevent water from filling these air-filled spaces-because water in nature is one of the main ways for pathogens to invade plants. If you have a plant photographer, you will definitely remember the water droplets on the petals deeply: these water droplets form a crystal clear ball and are adsorbed on the petals, and a little shaking will not let the water droplets slip. This is actually a physical phenomenon: because the petals are not easily penetrated by water, the water droplets on the petals shrink into a sphere due to surface tension, which is the so-called "petal effect". It is worth noting that this "petal effect" is similar to the so-called "lotus leaf effect" in which water drops roll on the surface of lotus leaves, but the effect is different: in the petal effect, the contact position between water drops and petals has a certain adhesion, so the water drops will not roll at will, while in the lotus leaf effect, this adhesion is also very low, so the water drops are easy to slide down. Lotus leaves rely on this effect to let the rain wash away the dust, thus "getting rid of mud and not getting stained".

But in some cases, water can break through this "hydrophobic barrier" of plants: for example, due to species differences, the waxy layer of petals itself is thin; Or petals aging, epidermal cell gap increased; Or soak in water for a long time. In these cases, water will invade the intercellular spaces and vacuoles in the petals. Because there is no waxy layer on the cell surface inside the leaf, and the cellulose, hemicellulose, pectin and other components of the plant cell wall are hydrophilic, water will quickly fill and infiltrate the entire petal internal space, thus eliminating the originally widespread gas-liquid interface and further losing the condition of extensive diffuse reflection. Because the refractive index of cell sap is close to that of water, the transparent part increases and the petals become transparent. This is the same as squeezing white petals to make them transparent: squeezing destroys the gas-containing spaces, and the cell fluid released by cell squeezing also fills these gas-containing spaces, which also eliminates the total reflection interface.

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