The screen affects the plants growth because the growth tip is once the growth tip gets through the screen, it is bent under a neighbouring "square" and forced to grow horizontally rather than vertically. The plant responds by increasing the growth of the sidebranches vertically. If you're still interested I've got this pic someone drew explaining it nicely and can email it to you (dunno how to link images on this site + have no webspace). I didnt get it at first either, the concept is so simple but hard to explain in words