The granitic flatrock ecosystem imposes harsh conditions on the resident plant communities in the heat of summer. Plant foliage must tolerate intense sunlight from the open sky, and reflected light and heat from the rock surface. Thin, narrow foliage is the commonest adaptation seen among the conventional, non-succulent plants on the flatrocks.
Linear or narrowly lanceolate leaves restrict the amount of surface area exposed to the sun. The low surface area also lowers moisture loss during periods of intense heat or drought. In many areas of the flatrocks, narrow leaves are not just the best design, they are the only design.
One such area of the Mitchell Mill State Natural Area is dominated by 3 species of annual plants from 3 separate plant families. And yet their leaves are nearly identical. During the spring they can be very difficult to tell apart in their early growth stages. Pictured above and immediately below, Diodia teres (Buttonweed) forms a thick groundcover in many areas of the Mitchell Mill flatrocks.
As Helianthus porteri (Porter’s Sunflower) grows and matures, it is nearly invisible in the Buttonweed groundcover. Only when it reaches a size that takes it above and out of the groundcover, is it easily distinguishable.
Trichostema setaceum (Narrow-leaved Bluecurls) tends to like a little partial shade at Mitchell Mill. The Buttonweed is thinner there, making the young bluecurls a little easier to distinguish as they grow. The flowers are typical of bluecurls – it is the foliage that gives away its identity.
While the Buttonweed and the Narrow-leaved Bluecurls can be found in other environments, the Porter’s Sunflowers are found only in the granitic flatrock ecosystem.