Sieve cell/plate/tube

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

Fig. so.—Vascular elements. A, annular tracheal tube B, spiral trachea tube C, reticulated tracheal tube D, pitted tracheal tube E, cross-section through plate of seive tube, and adjoining companion cell F, length-wise section of sieve tube G, portions of two companion cells. (A, B, C, D, Robbins E, F, and G, after Strasburger.)... 

A notable feature of the development and differentiation of the cell walls in phloem tissues is the formation of pores connecting the adjacent sieve-tubes to each other and to the companion cells. At an early stage of development, walls of the sieve tubes are marked by parts of endoplasmic reticulum on both sides where the pore is to be formed. As the sieve plate develops and the wall thickens, normal materials are deposited on the cell wall, except in the areas below the endoplasmic reticulum these areas grow, instead, by the... 

In the development of phloem, the plasma membranes ends of the cells are joined by elaborate plasmodesmata, which make contact through channels in the cell wall. Callose is deposited around these and the sieve tube is the much specialised chain of cells that results, with residual, functional cytoplasm still present, connected across the pores of the sieve plates. Connections to companion cells also remain though the lateral walls and these must serve to sustain the c5hoplasm of the sieve tubes. 

Inside forest trees under water stress, the plugging of sieve-plate pores with P-protein is an almost instantaneous reaction to pressure release in active sieve tubes. P-protein plugs, or slime plugs, are now thought of as the first line of defense against the loss of assimilates. Wound callose further strengthens the cell s defenses but at variable rates (44 cf.. Sect. 5.1.3.4). 

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