plants have a vascular system (analogous to ours) made of Xylem, cells which bring nutrients from the roots to the leaves, and Phloem, cells that move sugar from the leaves to the rest of the plant. when you bend or snap the stem, these "veins" can be broken, and will no longer transfer nutrients to and from the upper part of the plant, even standing the stem back up will not reconnect these. and, like cutting off circulation to your hand, the part not receiving nutrients will soon shrivel and die.
