Detailed structure of phloem:
Phloem is a much more complex tissue than xylem. According to some plant physiologists, the exact structure of phloem is still a matter of conjecture (Bidwell 1979), because its fine structure is destroyed or altered during preparation for microscopic examination.
The phloem is mainly composed of sieve tubes and phloem parenchyma. In Angiosperms, the sieve elements are always accompanied by companion cells, while in gymnosperms aluminous cells replace companion cells.
Companion cells are living cells and are nucleated. The sieve elements however are enucleated, but unlike the vessel elements of xylem the sieve elements have protoplasm. They can be plasmolysed indicating that they are bound by a selectively permeable membrane, but they do not have a tonoplast. Much of the protoplasm present in sieve tubes is in the form of P- protein, a fibrillar protein. The exact structure and function of this P-protein is still to be unravelled completely (Bidwell 1979). According to Esau, in mature sieve tubes, the cytoplasm and vacuole become a single system called myctoplasm.
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Ribosomes and dictyosomes are absent. Mitochondira however arc present, but they differ from the normal ones in having less cristae. During the cause of maturation, the cell wall, specially the transverse 3 undergoes changes. The cross wall becomes porous, and through each pore runs a cytoplasmic fibril connecting the protoplast of another sieve tube. Occasionally the lateral walls also may be porous.
The end walls, where the pores are many in number are called sieve plates. Occasionally the sieve pores are plugged with proteinaceous masses called slime bodies. In some sieve tubes however, the pores are partially blocked by callose (carbohydrate) deposits. It is generally beleived that in an actively translocating sieve tube the pores are largely unplugged.
The phloem consists of phloem fibres, ray cells, sclerids etc; besides seive tubes and companion cells. These other cells however do not seem to play any role in translocation. Companion cells however are ontogenetically and physiologically related to the sieve elements.
The phloem mother cells cut off one or more companion cells before they mature into sieve tubes. Walls separating the two are thin and highly pitted. Death of companion cells renders the sieve tubes functionless. Since the sieve elements are enucleated it has been suggested that the companion cells have nuclear control over seive elements (Esau, 1939). According to Bieleski (1966), the companion cell and the sieve tube should be viewed as a single functional unit. 80.