Evidences
There are different evidences that support the hypothesis. Firstly, there is an exudation of solution from the phloem when the stem is cut or punctured by the mouthparts of an aphid, a classical experiment demonstrating the translocation function of phloem, indicating that the phloem sap is under pressure. Secondly, concentration gradients of organic solutes are proved to be present between the sink and the source. Thirdly, when viruses or growth chemicals are applied to a well-illuminated (actively photosynthesising) leaf, they are translocated downwards to the roots. Yet, when applied to shaded leaves, such downward translocation of chemicals does not occur, hence showing that diffusion is not a possible process involved in translocation.
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