(Original post by jamesmact)
It's the same as in xylem, but don't say water is too cohesive because there isn't water transported in the phloem. So after the sucrose is actively loaded into the phloem sieve tubes, the concentration increases, causing mass flow to occur. This is where the sucrose molecules will naturally move from one end of the tube to the other, DOWN the concentration gradient, causing the sucrose to move from the source to the sink. The bi directional flow doesn't happen at the same time obviously because you cannot have two gradients occurring simultaneously. The reason it can move both ways is because the majority of the plant is capable of making sucrose, but the whole plant also needs sucrose for energy. It's called the mass flow hypothesis
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Thanks for the reply!
Firstly, I wouldn't agree with you on that one - whilst transporting water is not the primary purpose of phloem, the sucrose is still carried in solution and so water is always being moved around through phloem sieve-tubes. I understand your concentration gradient part, but that still does not explain the bidirectionality of sap movement through phloem. I have found a Wikipedia article that says there actually is no evidence for the two-way flow:
"The objection leveled against the pressure flow mechanism is that it does not explain the phenomenon of bidirectional movement i.e. movement of different substances in opponent directions at the same time. The phenomenon of bidirectional movement can be demonstrated by applying two different substances at the same time to two different points of phloem of a stem and following their longitudinal movement along the stem. The bidirectional movement occurs in a single sieve tube or not. If the mechanism of translocation operates according to pressure flow hypothesis, bidirectional movement in a single sieve tube is not possible. Experiments to demonstrate bidirectional movement in a single sieve tube are technically very difficult to perform. Some experiments indicate that bidirectional movement may occur in a single sieve tube, whereas others do not."