Nuclear crystalloids in sieve elements of species ofEchium(Boraginaceae)

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Abstract
Ultrastructural studies of differentiating phloem of the midvein in 15 species of Echium (Boraginaceae) have shown that in every species one or more crystalloids develop in sieve-element nuclei. These inclusions appear as soon as the cell begins to differentiate from a procambial derivative. Small at first, they enlarge later, particularly in length. The crystalloid may extend end-to-end in the correspondingly elongated nucleus. When the nucleus disintegrates in the maturing cell the crystalloid is released into the cell lumen where it remains intact or becomes fractured into large pieces. No dissociation into subunits was observed. The crystalloids are composed of tightly packed narrow rods (tubules) forming a two-dimensional lattice of squares in transections and a system of parallel striations in longitudinal sections. The similarities of these inclusions to the crystalline form of P-protein in Fabaceae raises the question of terminological delimitation of P-protein, a common component of angiospermous sieve-element protoplasts. The occurrence of nuclear crystalloids in another previously investigated genus in the Boraginaceae, Amsinckia, suggests that this inclusion should be explored in boraginaceous representatives for its possible value as a systematic character.