Sources are places where sugars are being produced. 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.
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. Image from W.H. Freeman and Sinauer Associates, used by permission. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.