Glenn was examining yesterday’s collection of buckeye leaves when he found something a little interesting. To the left is a photo of a normal bract on the branch of a buckeye. In buckeyes, the leaf is palmately compound, with 5-7 leaflets radiating from a central petiole. In painted buckeye both the first leaf and the young inflorescence emerge from the bud. The bracts are a pair of leaflike organs that cover and protect the bud that contains the developing shoot. When spring arrives, the bracts open up and the new leaves expand. Bracts, like leaves, are terminal organs - they don’t develop into anything else or act as a source of new growing points. Once their role in protecting the new shoot is over, they senesce and fall off.

You can see how the bracts function in the photo below. They’re the curly reddish pair attached to the old twig of last year, and though the new leaves are now greatly expanded you can see how they fit within the enclosure made by those bracts.

But look closely at the tips of the bract at center.

There is a tiny, but perfect palmately compound leaf emerging from the tip of the bract. (This is also true for the other bract in the pair.) Looks like the bract itself is a bit abnormal, more swollen, thicker, and curlier than the normal bract in the first photo. Glenn suggests that the bract or a patch of cells on the bract have been redefined to act as a petiole. The petiole is the stemlike structure that connects the stem to the blade of a leaf and it will also enclose all the plumbing, the xylem and phloem, that nourishes the leaf blade. Here the bract, or a part of it, seems to be acting as a petiolar source, thereby producing leaves, like a petiole normally does. This is not necessarily a genetically transmittable mutation; we’ll have to go back to look at the plant that produced this branch to see if all the bracts look like this. It is a developmental abnormality, but it could have been produced by environment. Temperature, disease, and other insults can cause one organ to develop as another. Or it might be a somatic mutation in a cell that led to the formation of this pair of bracts, but which mutation has not made it into the germ line that produces pollen or egg. Or, and this would be the jackpot, it might be a mutation that is also included in the gametes and then it would be genetically transmissible.

Mutations that produce changes in body parts fall into the group of homeotic mutations. Homeotic mutations cause identity changes in organs, since body shape is defined by homeotic genes. A part that would normally develop into one organ instead develops into another, so you get the peculiar classic examples of legs growing out of the head of a fly, instead of the proper antennae.

Since mutations in homeotic genes redefine body parts, they’re avidly studied. Homeotic genes were first discovered and mapped out thoroughly in animals, and in particular the fruitfly Drosophila, with its enormous collection of spectacular mutations. But the same strategy of using homeotic genes to make a body and to define body parts is also employed in plants, though by a set of genes that is not homologous to that in animals.

And, in plants as well as in animals homeotic mutations have been isolated to map out and understand how the plant body is formed. Mutations in one homeotic gene may convert a petal into a sepal, or a sepal into a leaf. And so a host of such floral mutations in the model mustard plant Arabidopsis thaliana has been exploited, as in fruitflies and other animals, to unravel the molecular genetics of flowering, called the ABC model of floral development.

In plants, homeotic mutations have been unwittingly used as sources for crop and floral varieties. Brocolli and cauliflower, both forms of the species Brassica oleracea, are homeotic mutants that cause inflorescences to arrest and proliferate into chubby edible structures.

Or take cultivated roses, for instance. Cultivated roses have multitudes of petals, but the family Rosaceae is characterized by five petals and many stamens, the male flower parts. Where did all the extra petals come from? The extra petals in the cultivated rose derive from conversion of stamens into petals, and if you look at the innermost petals in the center of a rose you’ll probably see some stamens that seem to be halfway in between being a petal and a stamen.

As for this particular buckeye abnormality, even if it is a transmissible mutation it doesn’t look to be a very successful experiment. The tiny leaves are already rather parched, and seem to have stopped developing as the supporting tissue of the bract senesces and prepares to fall off.