Analysing photonic structures in plants

Abstract

The outer layers of a range of plant tissues, including flower petals, leaves and fruits, exhibit an intriguing variation of microscopic structures. Some of these structures include ordered periodic multilayers and diffraction gratings that give rise to interesting optical appearances. The colour arising from such structures is generally brighter than pigment-based colour. Here, we describe the main types of photonic structures found in plants and discuss the experimental approaches that can be used to analyse them. These experimental approaches allow identification of the physical mechanisms producing structural colours with a high degree of confidence.

Keywords: iridescence; multilayer interference; plant cuticle; spectroscopy; structural colour in plants.

Figure 1.

Macro- and microscatterometry techniques. (a) Goniometre set-up. A collimated beam of light macroscopically illuminates the sample at an angle of incidence θ_i that can be varied from 0° (normal incidence) to about 90°. The scattered light in transmission and reflection is collected for different angles θ_d. (b) Conoscopic imaging principle. Light from the sample (illuminated in reflection or transmission) is collected by lens L₁ with high NA. With the sample in the focal plane P₁ of L₁, the light rays (represented in different colours) are focused on the back focal plane P₂. The second lens L₂ is mounted in telescopic configuration with L₁ to create an image of the sample in plane P₃. The lens L₃ is again mounted in telescopic configuration with L₂ to form an image of the back focal plane of L₁, P₂ in P₄. For simplicity, all lenses are drawn with the same focal length, and the distances between lenses correspond to the double of their focal length.

Figure 2.

Diffraction gratings. The distance has to be a multiple of the wavelength of light to satisfy the constructive interference condition. (Online version in colour.)

Figure 3.

Striking iridescence of the tulip Queen of the night. (a) (i) Photograph of the flower adapted from [40]. The dark violet coloration is due to the pigment, whereas the blue appearance of the petal edge contains a contribution from the grating interference. (ii) Cryo-SEM image of the petal epidermis. The cells of the tulip epidermis are flat and uniaxially elongated; the cell dimension is approx. 80 × 20 µm², while the distance between striation lines is ≈1 µm. (b) Optical spectrum of the epidermal layer obtained by the set-up as shown in figure 1a in reflection configuration for θ_i = 30°. The intensity is plotted on a violet-to-red colour-scale and the collection angle 0° corresponds to the specular reflection direction (sin(θ_d) = 0). (c) Optical transmission microscopy image of the petal epidermis. (d) Diffraction pattern of the epidermis obtained with the set-up of figure 1b in transmission, adapted from [40].

Figure 4.

Multilayer interference mechanism. (a) Incident beam is reflected at the interface between layers of different materials, represented with different colours. (b) Zoomed image of the light reflection–refraction at the interface between layers. (Online version in colour.)

Figure 5.

(a) Photograph of a juvenile Selaginella willdenowii leave. (b) TEM transverse section image of the outer cell wall and cuticle of the upper epidermis of a juvenile blue leaf. (a,b) Reproduced with permission, 2010 The Royal Society [10]. (c) Photograph of a juvenile (blue) and adult (green) leaf of Danaea nodosa. (d) TEM transverse section image of the outer cell wall of a juvenile leave. (c,d) Reproduced with permission from [16].

Figure 6.

Schematic of a helicoidal stack. Cellulose microfibrils are oriented parallel to each other forming a plane. The planes are superposed with a small rotation angle. The distance p between layers of fibrils with the same orientation determines the reflected wavelength, while the rotation direction determines the circular polarization of the reflected light. (Online version in colour.)

Figure 7.

(a) Photograph of a Pollia condensata fruit. The colour of the fruit arises from the interference of light and not from pigment. (b) Spectra from two different cells (continuous and dotted lines, respectively) for the two polarization channels (red and blue colour, respectively). The double-peak structure arises from the helicoidal stacks in the epicarp cells. The varying peak positions are indicative of varying values of the stack pitch p. (c) Transverse section TEM image of the multilayered cell wall that gives rise to the blue colour. (d,e) Optical microscopy images of the fruit in epi-illumination in circularly left and right polarization channels, respectively. (Reproduced with permission from [11].)

Figure 8.

(a) Photograph of Delarbrea michieana fruits. The whole fruit is about 1.7 cm long. (b) TEM transverse section image of the fruit, where an iridosome responsible for the fruit coloration is visible at the right-hand side of the image. (Reproduced with permission from [16].)