What direction do plants grow towards in the dark? (Tropisms)Watch
There's no light so phototropism is out of the question. Hydrotropism concerns only the roots.
The only tropism exhibited by the shoot is geotropism, specifically negative geotropism. Auxin will stimulate the cells to elongate and grow upwards to counteract gravity. Since there's no light to stimulate a positive phototropic effect, they simply just keep growing up straight.