Transmission Electron Microscope; Margaritaria nobilis fruit anatomy; Anatomy of Pericarp; Microscopy of Pericarp; Hyperspectral Microscopy from Structural colour from helicoidal cell-wall architecture in fruits of <i>Margaritaria nobilis</i>

Imaging the helicoidal structure with high resolution with TEM can be challenging. Here in Figure S1 we show how the same fixation procedure can fail to reveal the helicoidal structure of the cell wall of Margaritaria nobilis fruits when as stained fresh; Anatomy of Margaritaria nobilis fruit. (a,b,c) Transverse section of a fresh fruit shown at different magnifications. (d) Transverse section of fruit stained with toluidine blue. The pericarp is the light blue outermost layer. (e,f) EM transverse sections of the cell wall of a single pericarp cell, with multilayered structure visible in (e), and Bouligand arch pattern, the fingerprint of helicoidal cell-wall architecture, visible at higher magnification in (f). Scale bars: 1mm in (a) and (b), 0.5 mm in (c), 200 µm in (d), and 4 µm in (e) and 0.5 µm in (f); Scheme of the fruit cross section, showing the pericarp with the multilayered cells and the seed in wet (a) and dry (b) state. The change of the appearance can be understood in terms of scattering. When the fruit is wet the seeds adhere to the pericarp and the light that is not reflected by the helicoidal cells in the pericarp layer is absorbed by dark-pigmented cell in the the seed coat, making the cell appear blue macroscopically. On the other hand, when the fruit is dry the seed are well separated by the pericarp and the presence of a second interface also partially scatters all the wavelength of the visible light and give the whitish appearance pearlescent appearance; Optical Micrograph picture obtained using a 5x magnification objective in epi-illumination without any polarisation filter, of the pericarp layer alone on a glass slide.; Raw data on the iridescence on the single cell level of Margaritaria nobilis fruit obtained with hyperspectral microscopy as shown in Figure 4(a). Set of micrograph pictures obtained with a 20x magnification objective (NA=0.45) in epi-illumination with the liquid crystal filter in real colour. Processed image as reported in Figure 4 (a) where the averaged intensity is normalised with respect the spectrum reported Figure 4 (b).