17^ • PLANT RESPONSE 



time absorbed some quantity of hot water, which cannot 

 immediately be displaced by the water of ordinary tempera- 

 ture which is applied at the roots. The rate of suction, 

 therefore, could not at once revert to the normal, but must be 

 expected for some short period to show a slight enhancement. 

 The curve shows that on the return to 23° C. the rate fell 

 from fifty-eight to fourteen instead of to the original eight 

 cubic mm. per minute. 



(2) Method of Hydrostatic Balance : {a) Action of 

 cold. — For this experiment I took a Croton stem cut at both 

 ends, the lower end being placed in the plant-vessel of the 

 apparatus. There was now found to be a considerable 

 movement of water upwards. This movement was arrested 

 by suitable hydrostatic pressure, the upper end of the stem 

 being connected with an india-rubber tubing filled with 

 water and ending in a funnel (fig. 160). To prevent evapora- 

 tion, the surface of the water in the funnel was covered 

 with a thin film of oil. Rather a high hydrostatic pressure 

 was required to produce a balance. When the pressure of a 

 column of 75 cm. was applied, there was still a movement of 

 water upwards in the tissue, at so great a rate as ten cubic 

 mm. per minute. 



In this experiment, then, the root having been cut off, 

 there is still a considerable propulsion of water upwards 

 through the stem. It is thus clear that root-pressure is not 

 the essential factor in the ascent of sap. As the leaves had 

 also been cut off, and the cut end of the stem covered by 

 water sealed with a film of oil, evaporation from the leaves, 

 and the osmotic action thereby produced, are also seen to be 

 excluded. These, like root-pressure, therefore, cannot consti- 

 tute the essential factors in the process of suction. But since, 

 on the contrary, any small length of the stem is competent 

 to show this water-movement, the required activity must 

 reside in the tissue of the stem. 



A balance was finally obtained, by the pressure of a 

 water-column of 105 cm. This equilibrium is not to be re- 

 garded as merely the result of equality between the upward 



