BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. TRANSPORT IN PLANTS

13.8. Translocation of Organic Substances in the Phloem

13.8.2. Structure of Sieve Tubes

The Structure of the phloem as revealed by light Cell/15.html">Microscopy was described in Section 6.2.2. This tissue contains conducting tubes called sieve tubes, which are formed of Cells—known as sieve tube elements—end-to-end in a series. The elements are separated from each other by end walls called sieve plates, perforated with pores that allow fluid to flow freely from Cell to Cell¹.

Unlike xylem vessels, which are dead, hollow tubes through which solutions flow with little or no resistance, phloem sieve tubes are living cells, and the movement of solutions through them is impeded by the sieve plates and, to a lesser extent, by the Cytoplasm. Fig. 13.22 shows an electron micrograph of a mature sieve tube element, while Fig. 13.23 provides a diagram illustrating all the main structural details of sieve elements and their associated companion cells.

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Fig. 13.22. Electron micrograph of the contact region between mature sieve tube elements. CC — companion cell; P — sieve pore; Pl — plastid; P-protein — phloem protein.

During The Development of a sieve element from a meristematic cell, the Cell Nucleus degenerates, presenting us with the unusual example of a living cell lacking a nucleus; in this respect, it resembles a mammalian erythrocyte. At the same time, A number of other major changes occur, the results of which are shown schematically in Fig. 13.23. The Cell walls at both ends of the element are transformed into sieve plates. Here, the plasmodesmata connecting adjacent symplasts enlarge significantly, forming numerous sieve pores through which they pass. The surface view of a sieve plate is shown in Fig. 6.13. The ultimate outcome of all these transformations is The formation of a tubular structure lined with a thin, peripheral layer of living cytoplasm enclosed by a Plasmalemma. The central region of the sieve tube is occupied by what appears to be a single giant vacuole, which is nevertheless not separated from the cytoplasm by a tonoplast.

Fig. 13.23. Diagrammatic representation of a sieve tube element and a companion cell as seen in longitudinal section under an Electron microscope. If a sieve tube is damaged, for example by a grazing animal, additional callose is rapidly deposited, sealing the Pores in the sieve plate and thereby preventing the loss of valuable solutes from the phloem.

Each element is associated with one or more companion cells, which arise from the same parent cell via longitudinal division. Companion cells possess a very dense cytoplasm containing a nucleus, small vacuoles, and typical cell Organelles. Judging by the Abundance of Mitochondria and Ribosomes, companion cells are metabolically highly active (Fig. 13.23). Physiologically, they are intimately connected to the sieve elements and are essential for their survival: if the companion cells die, the sieve elements die with them.

In some plants, sieve elements produce large amounts of a fibrous protein called phloem protein (P-protein). Occasionally, it forms large deposits discernible under a Light Microscope. Although its function was once a subject of much debate, it is now generally agreed that it plays no direct role in translocation.

¹ This structure is typical only of flowering plants (angiosperms); lower vascular plants and gymnosperms lack well-defined sieve tubes and plates, but their phloem conducting elements—known as sieve cells—function on the same basic principle. — Transl.