DIRECTIVE MOVEMENT OF SAP 57 



condition. The result of watering the roots is a renewal 

 of suctional activity by which water is supplied to the 

 pulvinus, causing its expansion and the erectile movement 

 of the leaf. The record (fig. 26) exhibits this in a clear 

 manner. Water was supplied to the root at the point 

 marked by the vertical arrow, and the erectile movement 

 occurred after 10 seconds, the delay being due to the time 

 taken for the ascending water to reach the pulvinus. In 

 order to ascertain the effect of withdrawal of water, a 

 5 per cent. KNO3 solution was rapidly applied to the root 

 at the horizontal arrow. The effect of the consequent 

 withdrawal of water was the fall of the leaf, which occurred 

 in the course of 40 seconds. 



The increase of turgor by more rapid supply of water 

 finds thus two parallel expressions, namely, an erectile 

 movement of the leaf of Mimosa and an enhanced rate 

 of growth in a growing organ. Withdrawal of water, 

 resulting in a diminution of turgor, causes, conversely, a 

 fall of the leaf and a retardation of the rate of growth. 



Change of Turgor under Directive Movement 



OF Sap 



The movement of growth, as well as the erectile 

 movement of a pulvinated organ, is generally ascribed to 

 increase of turgor. But this cannot take place without 

 an adequate supply of sap. The law which governs the 

 directive movement of sap is that it follows the stimulation 

 gradient from the stimulated to the unstimulated region.'^ 

 The turgor is diminished at the point from which the 

 sap is expelled, and becomes increased where the sap is 

 accumulated. 



The effects of quicker or slower rate of ascent of sap 

 on movement of pulvinated and of growing organs are 

 summarised under A and B. 



^ The Motor Mechanism of Plants (192S), p. 357. 



