Many of us remember this from our childhood: putting food coloring into a glass of water and then placing a plant stem—often a white flower—in the glass. The plant actually changes color. It's a fun summer activity, and it's certainly effective, but why does it work? To understand this we first need to know what food coloring is: a chemical dye designed to dissolve in water, have no taste and be completely non-toxic. It's these properties that make it useful in baking—they can change the color of something, like icing, without affecting the taste or killing people who eat it. But it's also this property that allows it to color living organisms like plants.
How Plants Drink
Plants don't have mouths or throats with which to drink; they draw up liquid through their stems. Water already in the leaves, petals and stems evaporates out of the plant in a process called transpiration. Because this water leaves the plant, new water is pulled up the stem of the plant not unlike the way we drink water through a straw—the created vacuum pulls the water in the glass up the stem of the plant. This is how plants pull water out of the ground, and also how plants pull water out of a glass.
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The food coloring is dissolved into the water, meaning the food coloring is pulled up into the plant along with the water. Unlike the water, however, food coloring cannot evaporate. This means the food coloring can enter the plant but cannot leave it. Because the food coloring is non-toxic, this does not kill the plant, but it definitely does color the plant.