If you know a gardener who is a math teacher or even just a little bit interested in geometry, an ideal gift would be Keith Critchlow's book The Hidden Geometry of Flowers.
Actually, any gardener would enjoy this wonderful exploration of patterns manifest in the world of flowers and the discussion of how humans relate to them. The photographs, diagrams, information, and insights provide a treat for the mind and spirit of the reader.
For example, the discussion of spiraling describes how the process is more obvious in some flowers than in others. For instance, the unfolding flower of St. John's wort has more spiraling than a water lily, and roses unfold with a great deal of spiraling. The book includes many lovely photographs, and there's one striking photo of the spiral genesis of a single white arum lily.
The author also delves into the way flower faces affect viewers. Flowers, he explains, face three different ways: upwardly toward the sun, horizontally toward each other, or downward toward the ground. The face primarily attracts insects for pollination and reproduction. However, humans also respond to the symmetry, proportion, harmony of form, color, and number of petals in flower faces.
In buds, we see the earliest form of spirals, previewing the future unfolding.