Botany Basics: Symmetry

Flowers are easily the most variable organs of flowering plants (hence the name), and botanists are interested in the mfor a variety of reasons, not least because of their value in systematics (my field). The great diversity of floral forms makes for lots of information as to what plants are closely related. A lot of work has been done in genetics on what determines the identities of the organs in a flower (why one the flowers of one species have both male and female reproductive parts while others have only one or the other, for example). More recent work has begun to focus on how the shapes of flowers are genetically determined, and more specifically, their symmetry. But what do botanists mean by 'symmetry'?

Symmetry is a mathematical concept. It means, roughly, immunity to transformation (a phrase I stole from this post on Good Math, Bad Math, which gives a good introduction to the concept). A flower is symmetrical if you can apply some kind of transformation to the shape which makes no difference: the flower looks exactly the same as it did before. 

Consider the  flowers pictured on the left. The Phalaenopsis orchid (top) has only one symmetry: if you divide it down the middle and take a mirror image about that line, the shape of the orchid is unchanged (we're ignoring transformations like rotation by 360°, which leave any shape unchanged). Such flowers are called zygomorphic (lit. 'yoke shaped'), bilateral or mono-symmetric ('with only one plane of symmetry'). The Iris (middle) by contrast has five planes of symmetry: three mirror symmetries down the middle of each petal, rotation by 120° and rotation by 240°. Plants which have more than around two planes of symmetry are called actinomorphic (lit. 'star-shaped') or poly-symmetric.  

The Arabidopsis flowers at the bottom are intermediate: the two petals on the sides are longer than the petals top and bottom. Such flowers are found only in the mustard family and are usually called cruciform (lit. 'cross-shaped'; the Arabidopsis family is sometimes called the Cruciferae). Some botanists also refer to them as 'bi-symmetric', which is inaccurate: they have three planes of symmetry (two mirror-image transformations and rotation by 180°). Botanists as a tribe are not known for our mathematical prowess.

Very occasionally, flowers have no symmetry at all: they are asymmetric. This condition is rare enough that I can't find a good example right at the moment.

The genetics of floral symmetry is something of a trendy area in molecular botany right now; and its where I do some of my own research. I'll be sure to post again on it!