Sinks are places where sugar is being consumed or stored. Food moves through the phloem by a Pressure-Flow Mechanism. The pressure causes the sap to flow toward an area of lower pressure, the sink. In the sink, the sugar is removed from the phloem by another energy-requiring step and usually converted into starch or metabolized. Bulk flow and the movement of food from sources to sinks.
Nastic movements are plant movements independent of the direction of the stimulus.
The Darwins discovered that the tips of the plant curved first, and that the curve extended gradually down the stem. By covering the tips with foil, they prevented the plant from curving. They concluded that some factor was transmitted from the tip of the plant to the lower regions, causing the plant to bend. Phototropism in the coleoptile of a monocot. Image from Purves et al.