Last week’s post about xylem explained how it carries water from the roots to the rest of a plant. But there’s another transportation problem that plants have to solve. As you know, plants make food (sugars) through photosynthesis. (See my previous posts on photosynthesis.) This food factory requires both chlorophyll and sunlight, and can only take place in the green parts of a plant. Usually this means the leaves, although cacti and other xeric species (such as this Palo Verde, above right) often have chlorophyll in their stems.