Who has not been amazed at the beauty of a rose ? Many societies have symbolized love and friendship with this flower. It was already considered a symbol of beauty by Babylonians, Egyptians and Greeks, and today it is the best-selling flower by nurserymen and florists.
It has inspired poets and composers. References to roses are found in literature, cinema and even in politics, and not a few traditions include the rose in their corpus. Its fragrant aroma, whose main component is geraniol, is used in perfumery and has great evocative power.
There are roses of many colors : red, pink, orange, yellow, white, there are those of a purple so dark that they almost seem black… We even have the long-awaited blue roses! Some varieties even have several colors in the same flower.
One of the most striking features of roses is, of course, their petals. A garden rose can have dozens of petals, in some varieties more than a hundred .
The roses that we find in the florist are domestic varieties, in many cases, hybrid forms of species of the Rosa genus, of the Rosaceae family —sometimes botanists do not complicate their lives—. But one of the peculiarities of the Rosaceae family is that all its species have flowers with five petals. Five, not tens, not hundreds. only five. The only known exception is the species Rosa sericea , which has four.
What they do have in abundance are stamens.
The answer lies in evolution
Originally, the different floral parts are leaves, whether they are sepals, petals, stamens or carpels, they are. Leaves that have lost their photosynthetic capacity and have acquired other functions. The sepals protect the budding flower, the stamens and carpels perform the reproductive function, and in roses, the petals attract pollinating insects. Each concentric layer of flower pieces is called a “whorl.”
In some flowers that retain primitive features, such as the Magnolia nitida , the pieces are not arranged concentrically, but in a spiral. But the real peculiarity of this case is in the transition between the petals and the stamens. The pieces of the first turns of the spiral are clearly petals, and the last ones, clearly stamens, but the intermediate ones, which are called petaloid stamens, are a kind of stamens whose filament has the shape of a short and narrow petal .
It is not the only case in which evolution shows intermediate pieces between two forms that are usually different ; For example, lilies and irises, belonging to the genus Lilium , also highly valued by florists, do not have separate sepals and petals , but both whorls each have three pieces, called tepals , which are a sort of intermediate version between the two.
The stamens that transform into petals
Evolution, then, gives us an answer to the mystery of the origin of the large number of rose petals. That enormous amount of stamens of wild roses are what are transformed into petals in garden roses. It is a trait determined by the expression of certain genes.
Of course, this happens from time to time, spontaneously, and has happened many times independently in the recent evolutionary history of roses. And when it has happened, the human being has preferred those with the largest number of petals.
In this way, thanks to artificial selection, the trait has been inherited and enhanced more and more. And in the same way that we have selected the orangest carrots or the sheep with the most wool, we have gone from having wild roses with five small petals, to thousands of varieties of domestic roses with between 10 and more than 100 petals.
REFERENCES:
Bendahmane, M., Dubois, A., et al. 2013. Genetics and genomics of flower initiation and development in roses. Journal of Experimental Botany, 64(4), 847-857. DOI: 10.1093/jxb/ers387
Dubois, A., Raymond, O., et al. 2010. Tinkering with the C-Function: A Molecular Frame for the Selection of Double Flowers in Cultivated Roses. PLoS ONE , 5(2), e9288. DOI: 10.1371/journal.pone.0009288
Pérez Morales, C. 1999. Morphology of spermatophytes. Ed. Celarayn.
Tan, J., Wang, J., et al. 2017. Genetic relationships and evolution of old Chinese garden roses based on SSRs and chromosome diversity. Scientific Reports, 7(1), 15437. DOI: 10.1038/s41598-017-15815-6
