Discuss theories proposed for mechanism of translocation of food in phloem with proper diagram.

 Theories proposed for mechanism of translocation of food in phloem are -

1. Cytoplasmic Streaming Hypothesis:

It was put forward by de Vries (1885) and later developed by Curtis (1929-35). The hypothesis considers the transport to take place by combination of two forces, diffusion and cytoplasmic streaming. Cytoplasmic streaming carries organic sol­utes or food materials from one end to the other end of a sieve tube cell from where they diffuse according to their concentration gradients to an adjacent sieve tube cell through the sieve plate.

The hypothesis can explain bidirectional movement of substances in the same sieve element. The important drawbacks of ___ _________ _________ _______ _____ __________ _____ ______ _____ ________ ______ ________.
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