Investigation of surface structures in animals and plants that give rise to colour

We study the optical properties of plants and relate their photonics properties to their surface texture.

Life of plants strongly relies on light. Plants develop and adapt their morphology and anatomy depending on the light condition in which they grow and consequently exploit light for different aspects and for many porpoises. In the case of flowers, the shape and the anatomy of the different petal’s tissues necessarily affect their optical appearance and the amount of light coupled to the tissue itself. As an example, flowers develop photonic structures that manipulate light on the wavelength scale to increase the chance of pollination by creating structural colour or other peculiar effects like extreme glossiness. 
In order to fully understand the optical properties of flowers and relate them to their anatomy a complete spectroscopic characterization is required. Such investigation allows to disentangle the contribution of the different tissue of the plants contributing to the optical response.

Structural Colour of Pollia condensata fruit

Pollia condensata fruits reveal an unique example of multilayer-based strong iridescent colouration in plants. The colour is caused by Bragg-reflection of helicoidally stacked cellulose microfibrils, which form multilayers in the cell walls of the epicarp. The bright blue colour of this fruit is more intense than that of many previously described biological materials. Uniquely in nature, the reflected colour differs from cell to cell, as the layer thicknesses in the multilayer stack vary, giving the fruit a striking pixelated or ’pointillist’ appearance.

Pollia Condensata Fruit

Because the multilayers form with both helicoidicities, optical characterisation reveals that the reflected light from every epidermal cell can be polarised either circularly left or right, never previously observed in the same tissue.

Reflection Spectra

Figure: Polarised reflection of Pollia fruit. (a) Left Handed and (a) Right Handed optical micrographs of the same area of the fruit under epi-illumination. The insets show a zoom of the central areas, with white lines delimiting the cells. (c) The same area of the fruit surface was also imaged between crossed polarisers. All three images were obtained with a ×10 objective. (d) Schematic representation of light reflection from a curved multilayer, representing the ellipsoidal shape of the epicarp cells. Only light reflected from the central part of the cell is reflected into the numerical aperture of the objective (NA = 0.3). This results in a colour stripe in the centre of the cell, as seen in a,b. (e) Spectra from two different cells (continuous and dotted lines, respectively) for the two polarisation channels (red and blue colour, respectively).


Structural Colour in Flowers

Flowers can produce iridescencent colours via diffraction gratings. In many flower species, the presence of surface striations in the epidermal layer gives rise to an angular colour variation. The Tulipa "Queen of the Night" is an example of iridescencent species. also theGriellum humifusum shows regular striation on the petal surface.

Tulip Striations

Extreme glossiness of Buttercup

The bright and glossy appearance of the flowers of Ranunculus repens is directly correlated with the layered anatomy of the petal. The highly directional reflected light arises from the partially transparent, pigment-bearing epidermal layer, while a more diffused yellow colour is the result of scattering from the lower starch layer. This directionality of the light reflections causes the unusually intense gloss of the buttercup flower and the strong yellow reflection evident when holding the flower under the chin.

Buttercup optics


The mirror crack’d: both pigment and structure contribute to the glossy blue appearance of the mirror orchid, Ophrys speculum


The most striking visual cue of the Ophrys is the highly reflective blue speculum region at the centre of the labellum. The brightness of the colour, and its apparent angular dependence is the product of a combination pigment and structure. The blue colour is the result of pigmentation, but the final appearance of the labellum is modified by the combination of this pigment with the specular reflection arising from the ultrastructure of the cell wall and cuticle.

Bee Orchid

Adolphe Merkle Institute - Chemin des Verdiers 4 - CH-1700 Fribourg - Phone +41 26 300 9254