BOTANY WITH THE BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VII. PLANT TISSUES

Vascular Tissues

Sieve Cells

Elongated Cells (1.5–4.8 mm) with tapered ends and sieve areas on their lateral walls. They are characteristic of ferns and gymnosperms.

Sieve Tubes

These consist of a vertical series of living, enucleate member cells separated from one another by sieve plates. These plates contain openings—perforations—through which cytoplasmic strands, or plasmodesmata, pass. In sieve tubes, not only horizontal but also vertical sieve plates are formed, through which assimilates can move laterally. The sieve-tube members in angiosperms are accompanied by companion cells that possess a Nucleus.

Sieve tubes develop from meristematic Cells of the procambium (primary) or cambium (secondary). Both cells originate from the same mother Cell. During this process, the meristem cell divides longitudinally into two, one of which differentiates into a sieve-tube member, while the other becomes a companion cell. Subsequently, sieve plates form at the ends of the member cell. As the member cell matures, The Nucleus and the vacuolar membrane (tonoplast) break down, and The Cell fills with a mixture of cell sap and cytoplasmic matrix.

In herbaceous plants, sieve tubes die off along with the plant, whereas in woody plants, at the end of the growing season, the pores of the sieve plates become plugged with the carbohydrate callose, forming a callus that seals the sieve tube. In spring, under the action of Enzymes, the callose may dissolve, and the function of the sieve tube is restored. In dicotyledonous plants, sieve tubes function for 1–2 years, while in monocotyledons they can last up to 50 years.

Within plant Organs, vascular tissue forms vascular bundles comprising xylem with vessels and tracheids, and phloem with sieve tubes. Both xylem and phloem are complex Tissues composed of conducting, ground, and mechanical elements. Essential Components of the xylem include vessels and tracheids, along with xylem parenchyma cells and wood fibers. The phloem contains conducting tissue—sieve tubes with companion cells—and may also include phloem parenchyma and bast fibers.