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Sieve tube arrangement in variant phloem. (A) Transverse section of Amphilophium crucigerum, with sieve tube elements diffusely arranged (arrows). (B) Transverse section of Mansoa difficilis, with sieve tube elements radially arranged (arrows). (C) Transverse section of Cuspidaria convoluta, with sieve tubes tangentially arranged (arrows). (D) Ancestral character state reconstruction of sieve tube element arrangement. Scale bars = 100 µm.
