It is a complex tissue that conducts organic soloutes from the leaves to the other parts of the plant body. The term phloem was coined by K. Nageli. It is also known as bast or leptome.
The phloem is composed of following types of elements or cells:1. Sieve Elements 2. Companion cells 3. Phloem fibres and 4. Phloem Parenchyma.
Among these only phloem fibres are dead cells and the others are living cells.
Sieve Elements:
The chief conducting elements of phloem are collectively called as sieve elements. The most characteristic features of sieve elements are the sieve areas in the walls and the absence of nucleus in their protoplast. They can be classified into two types namely sieve cells and sieve tube members or elements. They differ in the degree of differentiation in their sieve areas and in the distribution of these areas on their walls.
A. Sieve cells:
The sieve cells are elongated, slender, broad at the middle and narrow, tapering ends. They have unspecialized sieve areas i.e., the sieve areas are not organized into sieve plates. The sieve areas are wall areas with pores or perforations through which the protoplasts of vertically or laterally adjoining sieve elements are interconnected. The sieve area takes its name from its resemblance to a sieve.
The sieve cells are arranged in vertical rows and are slightly overlapping. Gymnosperms and pteridophytes have sieve cells
Sieve Tube Elements or members:
Sieve tube element are sieve elements in which sieve areas are highly specialized and are localized in the form of sieve plates. The wall parts bearing more highly differentiated sieve areas, that is, areas with comparatively large pores, are called sieve plates. Sieve plates usually occur on the end walls.
If a sieve plate consists of single pore or perforation, it is called simple sieve plate (e.g., Cucurbita). If there are many perforations, it is called compound sieve plate (e.g., Vitis).
Sieve plates provide a more complete interconnection between sieve tube members than do the less differentiated lateral sieve areas in sieve cells.
A sieve tube is a series of sieve tube members connected end to end by means of sieve plates. Angiosperms have sieve tube members and sieve tubes.
Protoplast:
A common property of the protoplast of the mature and active sieve element is the lack of nucleus. The important property of the sieve-element protoplast of dicotyledons is the presence of viscous substance which stains readily with cytoplasmic stains,. This substance, which was called slime, is now recognized as proteinaceous and has been renamed as P-protein.
In mature stage, the slime is disposed in the vacuolar sap. The slime readily aggregates when the phloem is injured or killed state and becomes displaced towards the sieve areas. The slime accumulation on a sieve area is called slime plug.
The plugging prevents continued exudation from sieve tubes.
Callose:
The cytoplasmic strands in the sieve area remain encased in cylinder of substance called callose. Callose is a carbohydrate and stains blue with stains like Anilin blue and Resorcin blue. In old sieve tube elements massive accumulation of called is noticed forming callose plugs that block the sieve pores.
Companion Cells:
The specialized type of parenchyma cells closely associated with sieve-tube elements are called as companion cells. The sieve tube members and the companion cells are related ontogenetically as they develop from the same meristematic cells.
Such a meristematic cell divides longitudinally, one of the resulting cells, usually the largest, specializes to form sieve tube member and other develop into the companion cell. One or more companion cells may be associated with one sieve element and they may occur on one or more sides of the sieve element.
The companion cells are living having abundant granular cytoplasm and prominent elongated nucleus which is retained throughout the life of the cell.
The companion cells have functional association with sieve elements regulating the translocation. They are closely connected with sieve elements by plasmodesmata and their longevity depends on that of the contiguous sieve elements. Companion cells and phloem parenchyma play an important part in the maintenance of a pressure gradient in sieve tube.
Companion cells occur only in angiosperms. The sieve cells of the gymnosperms and pterodphytes have no companion cells.
In gymnosperms instead of companion cells albuminous cells are associated with the sieve cells. Ontogenetically these cells develop from the different precursor. These cells are apparently connected physiologically and morphologically to the sieve cells,
They possess albumin, a proteinaceous substance. They stain intensely with cytoplasmic stains. Albuminous cell can be distinguished from the other parenchyma cells of the phloem by the presence of connections with the sieve cells and by usual absence of starch.
Phloem Parenchyma:
The phloem contains variable numbers of parenchyma cell other than companion cells. These are concerned with man of the activities characteristics of living parenchyma cells, such as, storage of starch, fat and other organic food materials and accumulation of tannins and resins.
The parenchyma cells of primary phloem are elongated and are oriented, lie the sieve elements. In secondary phloem, parenchyma occur in two systems, the axial and the ray system. In axial parenchyma the cells are arranged parallel to the long axis of sieve elements in vertical rows. In ray parenchyma the cells radiate from the centre to the periphery i.e., they are arranged horizontally and form the phloem rays.
Phloem fibres:
Sclerenchyma cells occurring in the phloem are known as phloem fibres. These are dead elements. They are also known as bast fibres. They occur both in primary and secondary phloem.
They are elongated, spindle shaped with sharp tapering ends and have narrow lumen. Only simple pits are found on the walls of the phloem fibres. The wall may be lignified or non-lignified. The Cannabis (hemp) fibres are lignified, whereas fibres of Linum(flax) are of cellulose and without lignin.
They provide mechanical strength. Because of the strength of strands of phloem fibres, they have been used in the manufacture of cords, ropes, mats and cloth. They are known as bast fibres.
A common property of the protoplast of the mature and active sieve element is the lack of nucleus. The important property of the sieve-element protoplast of dicotyledons is the presence of viscous substance which stains readily with cytoplasmic stains,. This substance, which was called slime, is now recognized as proteinaceous and has been renamed as P-protein.
In mature stage, the slime is disposed in the vacuolar sap. The slime readily aggregates when the phloem is injured or killed state and becomes displaced towards the sieve areas. The slime accumulation on a sieve area is called slime plug.
The plugging prevents continued exudation from sieve tubes.
Callose:
The cytoplasmic strands in the sieve area remain encased in cylinder of substance called callose. Callose is a carbohydrate and stains blue with stains like Anilin blue and Resorcin blue. In old sieve tube elements massive accumulation of called is noticed forming callose plugs that block the sieve pores.
Companion Cells:
The specialized type of parenchyma cells closely associated with sieve-tube elements are called as companion cells. The sieve tube members and the companion cells are related ontogenetically as they develop from the same meristematic cells.
Such a meristematic cell divides longitudinally, one of the resulting cells, usually the largest, specializes to form sieve tube member and other develop into the companion cell. One or more companion cells may be associated with one sieve element and they may occur on one or more sides of the sieve element.
The companion cells are living having abundant granular cytoplasm and prominent elongated nucleus which is retained throughout the life of the cell.
The companion cells have functional association with sieve elements regulating the translocation. They are closely connected with sieve elements by plasmodesmata and their longevity depends on that of the contiguous sieve elements. Companion cells and phloem parenchyma play an important part in the maintenance of a pressure gradient in sieve tube.
Companion cells occur only in angiosperms. The sieve cells of the gymnosperms and pterodphytes have no companion cells.
In gymnosperms instead of companion cells albuminous cells are associated with the sieve cells. Ontogenetically these cells develop from the different precursor. These cells are apparently connected physiologically and morphologically to the sieve cells,
They possess albumin, a proteinaceous substance. They stain intensely with cytoplasmic stains. Albuminous cell can be distinguished from the other parenchyma cells of the phloem by the presence of connections with the sieve cells and by usual absence of starch.
Phloem Parenchyma:
The phloem contains variable numbers of parenchyma cell other than companion cells. These are concerned with man of the activities characteristics of living parenchyma cells, such as, storage of starch, fat and other organic food materials and accumulation of tannins and resins.
The parenchyma cells of primary phloem are elongated and are oriented, lie the sieve elements. In secondary phloem, parenchyma occur in two systems, the axial and the ray system. In axial parenchyma the cells are arranged parallel to the long axis of sieve elements in vertical rows. In ray parenchyma the cells radiate from the centre to the periphery i.e., they are arranged horizontally and form the phloem rays.
Phloem fibres:
Sclerenchyma cells occurring in the phloem are known as phloem fibres. These are dead elements. They are also known as bast fibres. They occur both in primary and secondary phloem.
They are elongated, spindle shaped with sharp tapering ends and have narrow lumen. Only simple pits are found on the walls of the phloem fibres. The wall may be lignified or non-lignified. The Cannabis (hemp) fibres are lignified, whereas fibres of Linum(flax) are of cellulose and without lignin.
They provide mechanical strength. Because of the strength of strands of phloem fibres, they have been used in the manufacture of cords, ropes, mats and cloth. They are known as bast fibres.
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