1063. The cause of the motion of the sap has given rise to a great variety of opinions; but, as Dr. Lindley observes in his Elements of Botany (5th edit. 1847), 'all attempts at referring it to known agency have failed.' The real cause 'is vitality.' (p. 58.) 'Two kinds of motion have been observed in plants, partial and general.' The partial movement, which is also called rotation, 'is confined to the interior of cells and tubes, and appears to be universal,' at least, during the season of growth: the general motion of the sap is that by which it is transferred to all the different parts of the plant.