1. 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 upwards 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.
2. Ringing or Girdling:
This involves selective removal of a part of 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. After some time it is seen that translocation is affected only where phloem is removed.
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1. Structure and distribution of phloem:
The phloem tissue is composed of seive 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 cytomplasmic 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.
2. Effect of blocking phloem:
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Occassionally the seive proes are blocked by callose deposits resulting in stoppage of translocation.