Bryant wrote and contributed the photograph for today’s entry. It’s my fault it’s late in being posted, but I’ve been catching up since going on a collecting foray late last week. He writes:
Continuing with the series on colour, I thought I would dip into the more structural side of things. In particular, I want to focus on blue colouration in foliage. Today’s photo is of a compact blue-needled selection of Picea pungens, taken in the E.H. Lohbrunner Alpine Garden here at UBC. Picea pungens is a high-altitude species, which grows at elevations between 1,750-3,000 meters in the southern Rocky Mountains. I chose this species because I was intrigued by David Lee’s fascination with blue foliage as described in his book, Nature’s Palette. Lee’s focus is mainly on iridescent blues found in tropical species such as Selaginella willdenowii.
The more subtle blue hues that are found in Picea pungens (commonly the Colorado blue spruce) are not produced by modified anthocyanin pigmentation like the blues found in many flowers. Species like the blue spruce produce a thin film of epicuticular surface waxes on their needles. These deposits diffract light at short wavelengths, which we perceive as a pale blue. This scattering of radiation is a physical phenomenon known as Tyndall scattering–the same reason why the sky and ocean are blue. The surface waxes are thought to reduce the absorption of photosynthetically-active radiation, reduce transpiration, influence gas exchange and lower leaf temperature (see: Physiological Effects of Surface Waxes). These results caused by the diffraction of light by epicuticular surface waxes can be advantageous or disadvantageous depending on the biogeoclimatic location of the individual.
In the case of the Selaginella examined by Lee, the multiple layers of convexly-shaped epidermal cells are what cause the diffraction of a more iridescent blue colour on the leaves. Selaginella willdenowii is a shade-dwelling plant, and the blue iridescence is only found on leaves that are rarely exposed to sunlight. Lee was curious as to why the fern would evolve structures that diffract much of the scarce light that is available to them. After thirty years of pondering this question, Lee’s explanation is that the iridescent shade leaves deflect short wave radiation and are thus able to absorb more long wave radiation. This is advantageous in the shaded understory of tropical rain forests, because long wavelength radiation is more available than short wavelength under the canopy.