Glen L CreasyAn example of the net direction of flow of phloem (plant's sugar transport system).
This pine tree has grown big enough for the step to block further expansion and phloem development, but because net sugar movement is from needles to roots, the result is a massing of cells storing sugars because they can't get past the physical block of the step.
The tree survives because enough sugars are transported back and forth to the roots through the rest of the trunk.
