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Tissues213-page11 - protein bodies P-protein plugs sieve...

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Plant Tissues - 11 Phloem Moves solutes, especially carbohydrate, throughout the plant Composed of sieve elements Sieve cells (found in gymnosperms and non-seed vascular plants) Narrow pores with uniform sieve areas in elongated cells. Sieve tube elements (found in angiosperms) Elongated cells form sieve tubes End walls have larger pores forming sieve plates (which are not found in sieve cells) Little cytoplasm at maturity, although sieve tubes retain a protoplast comprised of plasma membrane, smooth ER, mitochondria and plastids that line the wall. The nucleus, ribosomes, Golgi bodies, cytoskeleton and vacuole degenerate. Sieve tube pores of damaged phloem are lined with callose, a spiral-shaped glucose polysaccharide. Sieve tubes also have a protein (p-protein or phloem protein, sometimes called slime, that originates as p-
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Unformatted text preview: protein bodies. P-protein plugs sieve pores in damaged cells and is often conspicuous in prepared materials. P-protein and callose may function to protect the plant from loss of solutes in damaged phloem areas. Companion cells Parenchyma cells adjoining the sieve tube cells. Companion cells have many plasmodesmata connections to their adjacent sieve tubes. It is assumed that companion cells provide for the metabolic and energy needs of the sieve tube with which they are associated. Companion cells and sieve tube elements form from the same cell. Albuminous cells Parenchyma cells found in gymnosperms that are similar to companion cells. Phloem Fibers (sclerenchyma cells) Phloem parenchyma generally used for storage...
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