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. 2019 Dec 4;126(1):39–59. doi: 10.1093/aob/mcz190

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© The Author(s) 2019. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

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Fig. 4. — Differentiation of the two outermost seed coat layers, oi2 and oi1, during stage 17 of C. hirsuta fruit development. Bright field and transmission electron micrographs of HPF-FS-processed samples showing cells at the seed margin (left column) and at the flat seed surfaces (middle and right columns). (A–C) Early stage 17a. Undifferentiated oi2 cells. Polar deposition of cell wall material occurs on the inner periclinal wall of oi1 cells, especially at the seed margin (arrowhead, A). Amyloplasts are present in oi1 and oi2 (C). (D–F) Mid stage 17a. Mucilage secretion is first visible in oi2 cells at the seed margin (arrowhead, D) and is preceded by the migration of the nucleus towards the centre of the outer periclinal cell wall (arrowhead, F). The inner periclinal wall of oi1 cells continues to thicken. In oi1 cells at the seed margin, thickening extends halfway up the anticlinal walls (D). Amyloplasts are present in oi1 and oi2 (F). (G–I) Late stage 17a. Mucilage secretion is visible in all oi2 cells. Asymmetric cell wall thickening continues in oi1 cells. Amyloplasts are present in oi1 and oi2 (I). (J–L) Early stage 17b. Mucilage accumulation in oi2 cells extends towards the inner periclinal cell wall, constricting the cytoplasm into a columnar shape (columella, arrowhead, J). Asymmetric cell wall thickening continues in oi1 cells. Amyloplasts are present in oi1 and oi2 (L). (M–O) Mid stage 17b. Mucilage extends from the outer to the inner periclinal cell wall in oi2 cells, surrounding the columella (arrowheads). No organelles are present in oi2 cells. Amyloplasts are still present in oi1 cells (O). The inner periclinal cell wall in oi1 cells is now very thick. (P–R) Late (exploded) stage 17b. The seed coat is desiccated. The oi2 layer mainly consists of mucilage, the oi1 layer of periclinal and anticlinal cell wall thickenings and the ii1 layer of phenolic compounds. Abbreviations: a, amyloplast; al, aleurone-like layer; co, columella; ii, inner integument; mu, mucilage; n, nucleus; oi, outer integument. Scale bars = 20 µm (A, D, G, J, M, P) and 10 µm (B, C, E, F, H, I, K, L, N, O, Q, R).