They push through cracks in the asphalt and germinate between the cobblestones of towns; unless they're blooming in the wastelands of suburbs, in the shadow of towers or in backyards. Fragile-looking but pugnacious – pardon the anthropomorphism – they plot their own way across unlikely terrain in the middle of cities. "These little yellow flowers, high on legs, standing between the concrete of the entrance wall and the cobblestones of the street...," wrote French author and poet Christian Bobin, in Le Murmure ("The Whisper").

Taking off to the lush fields? Plants don't have that option. So, to spread their corollas in the heart of the metropolis, they follow their compass. There are numerous challenges, including how to grow in poor-quality soils, endure urban heat islands and cope with the density of surrounding buildings, which limit the amount of sunlight they receive. Ribwort plantain, red clover, black medic, orchard grass: Behind these picturesque names lie four herbaceous plants that adorn urban green spaces in spring and summer with a discreet sowing of white, pink, yellow and orange flowers.

A National Centre for Scientific Research-Strasbourg University team measured their morphological traits (leaf thickness, plant height and seed weight) in 60 wastelands, meadows and lawns in the city of Strasbourg. How were these traits associated with urban heat islands, soil degradation, building density and mowing frequency? In a second stage, the researchers harvested the seeds of each species on site and cultivated them under similar conditions at the Strasbourg Botanical Garden, after which they measured the same morphological traits.

The question was: For each species, are there morphological differences between the sites and the garden? If not, this is proof of evolutionary adaptation: Natural selection has retained individuals with hereditary traits that make them more tolerant of the constraints associated with urbanization. On the other hand, if the traits observed on the sites disappear in the garden, it's because the plants have adapted to urban conditions through a rapid, reversible and non-heritable response; with, in return, often reduced growth performance. This is plasticity. Clearly, each individual has modified his or her morphology by modulating the activity level of certain genes.

The results of their study were published in the Journal of Ecology in October: In the majority of cases, these plants responded to urbanization with plasticity. On the most mown sites, for example, ribwort plantain produced thinner leaves; and red clover, smaller individual plants. These changes were not genetically transmissible, since they were no longer observed in the garden-grown offspring of these plants. In the face of repeated mowing, on the other hand, neither orchard grass nor black medic showed sufficient plasticity: Their populations declined.

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