[[en/index|English]] · [[zh/index|中文]] · [[en/phytolith_植硅体/wheat-phytolith|EN]] · [[zh/phytolith_植硅体/wheat-phytolith|中文]] # What do the phytoliths on wheat chaff look like? ## Images from the Hodson 1988 paper - Hodson, M. J., & Sangster, A. G. (05/1988). Silica deposition in the inflorescence bracts of wheat (Triticum aestivum). I. Scanning electron microscopy and light microscopy. Canadian Journal of Botany, 66(5), 829–838. https://doi.org/10.1139/b88-121 #### FIGS. 1–12. Scanning electron micrographs of intact glume and lemma tissues. - Fig.1. The awn of the glume near its base. Two ribs show numerous prickles (pr) and papillae (p); between the ribs stomatal rows (st) are evident. - Fig.2. Prickles (pr) and papillae (p) of the glume awn. - Fig.3. Broken prickle from a glume awn. Note the concentric rings of wall material and empty lumen. - Fig.4. Outer surface of the glume (central region) illustrating a rib (r) and numerous papillae. - Fig.5. Papillae on outer surface of glume near the margin of the bract. Note layer of wax covering all but papillae. - Fig.6. Area of outer surface of the glume near the margin of the bract, showing a row of stomata (st) and several papillae. Note heavy deposition of wax on this surface. - Fig.7. Margin of outer surface of the glume showing many stout macrohairs. - Fig.8. Stout macrohairs (tn) from near the margin of the outer surface of the glume. Note wax (w) only extends a short way up the trichome. - Fig.9. Single prickle (pr) from near the margin of the outer surface of the glume. Some wax is present over the whole hair. - Fig.10. The awn of the lemma near its tip. Note prickles (pr) and a row of stomata (st). - Fig.11. Higher magnification of material shown in Fig.10. Note stoma (st) and prickles (pr). - Fig.12. Two prickles from the awn of the lemma. ![[Hodson1988SEM_A.jpg]] #### FIGS.13–24. Scanning electron micrographs of intact lemma and palea tissues (Figs. 13–22) and ashed lemma tissues (Figs. 23, 24). - Fig.13. Outer surface of the lemma (central region) at low magnification. Note numerous papillae and isolated macrohairs (arrowheads). - Fig.14. Outer surface of the lemma at the margin showing stout macrohairs, prickles (pr), and papillae (p). Note that the surface is covered with wax. - Fig.15. Outer surface of the lemma in a central area showing less wax. Several papillae (p) and a stoma (st) are visible, as are the five depressions (arrowheads), which possibly represent silica cells (hats). - Fig.16. Inner surface of lemma (central region) showing numerous prickles (pr) and papillae (p). - Fig.17. Inner surface of lemma (central region) showing prickle hairs (ph), which grade into macrohairs. Also visible are two stomata (st) and long cells (lc) with crenulate walls. - Fig.18. Outer surface of palea showing the apical region at low magnification. The inner surface (is), which is densely covered with hairs, is folded over onto the outer surface (os), which has few trichomes. - Fig.19. Inner surface of the palea at margin (folded back) showing large macrohairs at the extreme margin, papillae (p), and a dense covering of smaller macrohairs (m). Part of the outer surface adpressed to the next floret is almost devoid of hairs (*). - Fig.20. Outer surface of the palea at the margin, showing one large macrohair, many smaller macrohairs, prickles (pr), and papillae (p). - Fig.21. Inner surface of the palea near apical region, showing a dense covering of trichomes. - Fig.22. Inner surface of the palea (central region), showing numerous papillae (p) and occasional macrohairs. - Fig.23. Ashed awn of lemma. Fragment showing three prickles (pr), a stoma (st), and epidermal cells (e), all of which have survived ashing intact. - Fig.24. Ashed awn of lemma. Single prickle showing empty lumen and stout wall. ![[Hodson1988SEM_B.jpg]] #### FIGS.25–36. Scanning electron micrographs of ashed bracts. - Fig.25. Lemma awn showing sheet of epidermal cells, viewed from inside, with pits in the walls. - Fig.26. Lemma showing papillae (p) and silicified long cells (lc). - Fig.27. Silicified lemma material containing papillae (p) and what appear to be trichome bases (arrowheads). - Fig.28. Lemma. Higher magnification of trichome bases shown in Fig.27. - Fig.29. Lemma. Dendriform opal phytoliths (d) with round short-cell phytoliths (sc) (hats) between them. - Fig.30. Lemma. Higher magnification of Fig.29, showing dendriform opal phytoliths and round short-cell phytoliths (hats). - Fig.31. Palea, showing a macrohair (m), a silicified long cell (lc), and several trichome or papilla bases (arrowheads). - Fig.32. Palea. Base of papilla. Note a ring of holes (pits) in the periphery. - Fig.33. Palea. Papilla illustrating ornamentation of the periphery of the base. - Fig.34. Palea. Single dendriform silica body (opal phytolith). - Fig.35. Palea. The dendriform silica body of Fig.34 at higher magnification showing detail of branched arms (arrows). - Fig.36. Palea. Single macrohair with bulbous base, and the base of another trichome that has broken (arrow). ![[Hodson1988SEM_C.jpg]] #### FIGS.37–48. Light micrographs of ashed bracts. - Fig.37. Glume near the base of the awn. Low-magnification micrograph showing prickles (pr), papillae (p), a group of silicified long cells (lc), several rows of stomata (st) and numerous silicified short cells (arrowheads). - Fig.38. Glume. Silicified long cells (lc). Note also papillae (p), which are just out of the plane of focus and which are either at the ends of the long cells or at the sides, but never directly above. - Fig.39. Awn of lemma showing general silicification of outer cell walls. - Fig.40. Awn of lemma illustrating silicification of the walls of two prickles. - Fig.41. Lemma illustrating two veins (v) and numerous papillae (p) and silicified short cells (sc). - Fig.42. Lemma, showing silicification of outer walls of long cells (lc) and many papillae (p). - Fig.43. Lemma. Dendriform and hat opals resulting from the infilling of long cells (lc) and silica cells (sc), respectively. - Fig.44. Low-magnification micrograph of the palea, showing marginal large macrohairs (lm), papillae (p), and many small macrohairs. - Fig.45. Palea macrohairs illustrating the bulbous nature of their basal attachment. - Fig.46. Large marginal macrohairs from the palea. The hairs have thin walls and large empty lumina. - Fig.47. Palea. Macrohairs (m), papillae (p), and stomata. The papillae show a series of holes around their edges. One papilla (arrow) is in side view. - Fig.48. Palea. Papillae in surface view showing sculptured margins (pits?). ![[Hodson1988SEM_D.jpg]] # The Hodson 1989 paper - Hodson, M. J., & Sangster, A. G. (02/1989). Silica deposition in the inflorescence bracts of wheat (Triticum aestivum). II. X-ray microanalysis and backscattered electron imaging. Canadian Journal of Botany, 67(2), 281–287. https://doi.org/10.1139/b89-041 What a treasure for me. - FIGS.1–6. Figures 1, 3, and 5 are secondary electron images, while Figs. 2, 4, and 6 are the corresponding silicon X-ray distribution images. - Figs. 1, 2. Tip of the glume awn showing that silicon is concentrated in the tips of prickles (pr). Figs. 3, 4. The awn of the lemma near its base. Tips of prickles and papilla (p) are major sites of silicon accumulation. - Figs. 5, 6. Inner surface of the lemma. Silica deposition occurs mainly in the tips of prickles and in papillae. ![[Hodson1989SEMandXraySilicaA.jpg]] #### Energy dispersive X-ray microanalysis > Figures 1–6 compare representative examples of silicon X-ray microanalysis results from the wheat bracts, with corresponding secondary electron images. Hodson and Sangster (1988) showed that while the tip regions of the lemma and glume awns were dominated by prickles, the basal regions had prickles, papillae, and stomatal rows. X-ray microanalysis indicated that silicon was present in all of the outer epidermal walls of the awns, but was concentrated in the tips of prickles (Figs. 1–4) and in papillae (Figs. 3, 4). It has been shown previously that the wheat inflorescence bracts have trichomes on both outer and inner surfaces (Hodson and Sangster 1988). Microanalysis indicated that silica deposition was generally heavier on the outer surfaces, but some silicon was deposited on inner surfaces, particularly in trichomes. Silicon was concentrated in papillae and in the tips of prickles from the inner surface of the lemma (Figs. 5, 6). - FIGS.7–12. Figures 7 and 10 are secondary electron images, Figs. 8 and 11 are corresponding backscattered electron images, and Figs. 9 and 12 are corresponding silicon X-ray distribution images. - Figs. 7, 8, 9. Inner surface of glume just below awn. Many long macrohairs (m) and papillae (p) are present in this region, and both trichome types contain silicon. - Figs. 10, 11, 12. Outer surface of the glume, showing many papillae and depressions marking positions of short cells (arrowheads). Both papillae and short cells are major sites of silicon accumulation. ![[Hodson1989SEMandXraySilicaB.jpg]] - FIGS.13–18. Figures 13 and 16 are secondary electron images, Figs. 14 and 17 are corresponding backscattered electron images, and Figs. 15 and 18 are corresponding silicon X-ray distribution images. - Figs. 13, 14, 15. Inner surface of the lemma showing many prickles, the tips of which are silicified, and a stoma (st). - Figs. 16, 17, 18. Outer surface of the palea. Macrohairs and prickles are silicified, with silicon concentrated in hair tips. ![[Hodson1989SEMandXraySilicaC.jpg]] #### A comparison of secondary electron imaging, energy dispersive X-ray microanalysis, and BEI > Figures 7–18 compare secondary electron, backscattered electron and silicon X-ray distribution images of four different areas on the bracts. Backscattered electron images rely far less on specimen topography than do secondary electron images. Areas containing elements of high atomic number give a greater signal than areas containing elements of lower atomic number, and thus appear differently contrasted within the same BEI micrograph. Areas of greater silica deposition give more signal and are brighter than surrounding areas of less deposition in a given micrograph. This may not always be the case, however, if other elements of higher atomic number are present in the sample. We also detected Na, S, P, K, and Ca in our samples using X-ray microanalysis; however, the levels were much lower than for silicon, and would be unlikely to interfere with the BEI results for silicon. > Macrohairs of the glume and lemma are restricted to isolated areas, particularly on the inner surface just below the awns (Hodson and Sangster 1988). Figures 7–9 show the inner surface of the glume below the awn. Figure 9 is the X-ray map of this area, which is seen in Fig. 7 as a secondary electron image. Comparing Figs. 7 and 9, we see that silicon is present in both papillae and macrohairs, the greatest concentrations being in the tips of the latter. The same area seen as a backscattered electron image also shows the macrohairs and papillae giving greater signal than the other areas of the bract (Fig. 8). On the outer surface of the glume, the most conspicuous features are the papillae and the depressions marking the positions of short cells (Figs. 10–12). Figure 12 clearly indicates that both of these structures are silicified. The backscattered electron image (Fig. 11) shows that a greater signal emanates from the papillae than from other cell types. In the case of the lemma, the inner surface is characterised by papillae, prickles, and stomata (Figs. 5, 13). Figures 13–15 suggest that silica is confined mainly to the tips of prickles (in areas where papillae were present these were also silicified, Figs. 5, 6). BEI gave some suggestion of increased signal from prickle tips (Fig. 14). The palea macrohairs are silicified as indicated in Figs. 16–18. Again silicon is concentrated in the tips of macrohairs (Fig. 18). In Fig. 17, the contrast of the backscattered electron image has been greatly accentuated, and the macrohair tips are very bright. The same macrohairs appear somewhat larger in Fig. 16 because of different detector systems and signals received. #### Silica deposition in the glume, lemma, and palea: a summary > The entire surface of the glume and lemma awns was found to be silicified, with greatest concentrations in prickles and papillae (Figs. 1–4). Depending on the area, stomata and silicified papillae, prickles and short cells were observed on the outer surface of the glume (Figs. 10–12). Lower concentrations of silicon were found on the inner surface, associated with prickles, macrohairs, and papillae (Figs. 7–9). The outer surface of the lemma was covered with silicified papillae and short cells (data not presented). On the inner surface, silicon was present at lower levels in both papillae and prickles (Figs. 5, 6, 13–15). Large areas of the palea are pressed between the caryopsis and the next floret, and both surfaces have very few trichomes here (Hodson and Sangster 1988). At the apex and margins, macrohairs are abundant, and Figs. 16–18 indicate that these trichomes are silicified. #### Authors' discussion of literature > The only previous detailed study of silica distribution in wheat has concerned the epicarp hairs of the caryopsis (Bennett and Parry 1981; Parry et al. 1984; Sangster and Hodson 1986). Silicon was present along the whole length of these hairs, but was more concentrated towards the hair tip. This pattern appears to be very similar to that observed in the macrohairs of the inflorescence bracts (Figs. 9, 18). There have been a number of studies concerning silica deposition in the mature inflorescence bracts of the Gramineae using an electron probe or scanning electron microscopy and energy dispersive analysis: Avena sativa inflorescence bracts (Kaufman et al. 1972); the lemma, palea, and awns of Hordeum sativum (Hayward and Parry 1973); the inflorescence bracts of Oryza sativa (Soni and Parry 1973); the paleas of Dichanthelium, Panicum, and Echinochloa species (Clark and Gould 1975); the lemmas and awns of four Zizania species (Terrell and Wergin 1981); the inflorescence bracts of Setaria italica (Parry and Hodson 1982); the lemmas and paleas of four Phalaris species (Sangster et al. 1983); and the spikelets of 13 Leersia species (Terrell et al. 1983). A wide range of genera have been investigated, but two common features emerge. First, as in wheat, silica is predominantly deposited on the outer surfaces of the bracts. Second, trichomes are usually major sites for silica deposition. Okay, so Hodson here did not say the difference between wheat and rice. #### Inner vs. outer — bract surfaces of wheat > We noted previously (Hodson and Sangster 1988) that the presence of trichomes on the inner surface of the inflorescence bracts was little studied. Where this had been investigated, only wheat and Secale cereale 黑麦 (Blackman and Parry 1968) showed this feature. As far as we are aware, the present work is the first report, substantiated by X-ray microanalysis, of silica deposition in trichomes on the inner surface of grass inflorescence bracts. I read this before. Did not pay attention. Now another big clue to me. > Trichomes, especially if silicified, have been recognized previously as conferring considerable ecological and physiological benefit (see for example, Sangster and Hodson, 1986; Hodson and Sangster, 1988) by functioning as a deterrent to both insect and pathogen penetration of inflorescences, as well as to grazing by herbivores in grassland biomes. They also reduce evapotranspiration. > In a negative sense, silicified trichomes of the inner surface of wheat bracts may fragment to form a major component of a fine dust generated during grain handling. On areas of inner surface of the wheat glume, lemma, and palea not contacting adjacent structures, epidermal differentiation may yield abundant long silicified macrohairs (see for example, Figs. 7–9) representing considerable potential for airborne fragments hazardous to human health if lodged in broncho-pulmonary passages. > Biomineralization of outer bract surfaces, by contrast, occurs more typically in the walls of less elongate cell types, for example, papillae and short prickles (Figs. 10–12; 16–18), or else in cell lumens. These also contribute hazardous siliceous dust particles, but frequently they constitute a coarser size category, such as the spinose forms resulting from the infilling of epidermal long cell lumens (Hodson and Sangster 1988). #### Backscattered electron images (BEI) not as useful > Grass inflorescence bracts are generally more silicified than leaves, and we consider that BEI is less applicable to this type of material. In addition, highly silicified areas do not always appear as brighter areas on backscattered images. > ... for inflorescence bracts, energy dispersive microanalysis is the superior technique. > Silica deposits have been divided into three broad categories: wall, lumen, or extracellular (Sangster and Parry 1981). The first two of these types are dominant in the wheat inflorescence bracts. #### Much less silica in inner epidermis of Japonica paddy rice > Percival (1921) reported that both surfaces of the inflorescence bracts of Triticum species were hairy, and we confirmed this observation for T. aestivum. However, inner surfaces have received only scant attention, many investigators concentrating entirely on the outer surfaces. Much less silica is deposited in the inner epidermis than in the outer epidermis of the lemma of Japonica paddy rice (Kaufman et al. 1981). #### Triticum as the only grass genus with inner surface differentiation? > Where inner surfaces have been studied, little differentiation has been noted: rye (Blackman and Parry 1968); rice (Soni and Parry 1973); Setaria italica (Parry and Hodson 1982); Phalaris species (Sangster et al. 1983a, 1983b). Papillae, prickles, or macrohairs were sparse or not observed on this surface. It seems unlikely that Triticum is the only grass genus with such highly differentiated inner surfaces, and this requires further investigation. The degree of differentiation of the inner surfaces of inflorescence bracts may be controlled by compression between the caryopsis and the neighbouring floret. Our observation of little epidermal differentiation in such areas of the palea of T. aestivum provides support for this hypothesis. Wow ... ... ! #### Wheat dust and health > Two earlier papers have specifically identified wheat dust as a potential health hazard (Baker 1961; Simmonds et al. 1970). There is little doubt that many of the phytoliths observed by Baker (1961) in wheat dust originated from the inflorescence bracts. In this respect, the palea is possibly the most significant wheat bract in terms of producing long siliceous fibres from macrohairs (Figs. 20, 21, 44, 45). The awns are also contributors, although the trichomes tend to be shorter and stouter (Figs. 2, 12, 23, 40). Moreover, dendriform opals (lumen deposits) from epidermal long cells (Figs. 29, 30, 34, 38, 43) figure prominently in Baker's (1961) illustrations of airborne wheat dust components. > BAKER, G. 1961. Opal phytoliths and adventitious mineral particles in wheat dust. Mineral investigations technical paper no. 4. Commonwealth Scientific and Industrial Research Organization, Melbourne, Australia.