The main reason behind not survival of the part of plant bent or snapped from the plant is disconnection of vascular tissues providing food and water.
Vascular bundles are a group of tube-like tissues that flow through plants, transporting vital substances to different parts of the plant.
It transports nutrients from the roots to the leaves, and Phloem, which transports sugar from the leaves to the rest of the plant.
These "veins" can be broken when you bend or snap the stem, and they will no longer transfer nutrients to and from the upper part of the plant; even standing the stem back up will not reconnect these.
Thus, just as if you cut off circulation to your hand, the part that isn't getting nutrients will shrivel and die.
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