According to mass flow hypothesis, the sugars in solution can move freely through the sieve tubes from the leaves to all non-photosynthetic parts of the plants. This movement is because of the difference in the concentration gradient at the place of manufacture of sugar.
Objection of Hypothesis: One objection against the mass flow hypothesis is that the phloem often transports substances in the opposite direction too i.e. dup the stem to provide nourishment and energy to the leaves and shoots for their growth and also to the developing seeds and fruits.
But this objection does not hold good if we recall the structures of the phloem tissue. The phloem tissue consists of many sieve tubes, not just one, and the fluids can move in opposite direction in two different but closely situated sieve tubes. Here too, the movement of fluids in opposite direction would also depend on opposing pressure gradients in the two sieve tubes.