a) Elimination of other tissues:
Though both xylem and phloem are ideally located for translocation, the former is not suitable at least for downward translocation, because there is a rapid upward movement of water and inorganic salts in them. The cells of pith or ground tissue also may not be involved as sap concentration of transportable food in them is very less. Thus phloem is the only tissue suited for translocation.
b) Ringing or Girdling:
This involves the selective removal of a part of the stem tissue in the form of a ring or girdle. Two types of rings of tissues are removed; in one only the phloem is removed and in the other only the xylem is removed. In the riming experiment three plants are selected.
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The plants are kept in sunlight and allowed to photosynthesize. After some time it will be noticed that in plant B the portion of the stem immediately above the ring bulges, while no such thing is observed in A or C. This is explained on the basis that downward translocation taking place through phloem in is affected in B due to its (phloem) removal, while no such thing happened in A or C as the phloem was in tact. Further it is also clear that the removal of xylem has no effect on translocation as seen in A.
Similar ringing experiments may be conducted to show that the upward translocation also is conducted through phloem. In this experiment also three sets of plants are taken A- Xylem removed, B – Phloem removed and C- control. In the plants A and B shoots are defoliated above the rings. The cut surface of the stem is enclosed in a glass cylinder filled with water to keep the tissues moist. Plant A in which only xylem is removed show elongation of the stem indicating undisturbed movement of solutes thought phloem. Plant B shows no elongation indicating stoppage of solute flow as phloem has been removed.
Structure and distribution of phloem:
The phloem tissue is composed of sieve tubes, companion cells and phloem parenchyma. The seive tubes are elongated like the vessels of xylem and are joined end to end forming a continuous system throughout the plant. The perforated seive plates allow for cytomplsmic connections to help in conducting the solutes. The presence of a second group of phloem as seen in bicollateral vascular bundles (cucurbitaceous plants) found in thin stems with large leaves and large sized sieve tubes seen in large fruits is an aditional evidence for phloem being involved in translocation.
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Effect of blocking phloem:
Occasionally the sieve proes are blocked by callose deposits resulting in stoppage of translocation. Chemical analysis of phloem sap: Chemical analysis of the sap extracted from phloem reveals that there is a high concentration of soluble sugars. Since the carbohydrates are translocated in the form of sugars, it clearly indicates the phloem being the tissue for translocation. Further, the concentration of sugars seen in the sieve elements is very high compared to what necessary for the maintenance of normal osmotic relationship with the adjacent cells.