MODIFICATION OF SUCTIONAL RESPONSE 387 



arrested, and this was followed almost immediately by a 

 slight movement in the negative direction, showing that, by 

 some spasmodic contraction, water was being expelled from 

 the tissue. This phase was succeeded by an almost com- 

 plete arrest of suction, there being now only the feeblest 

 ascensional movement (fig. 165). Within a short period 

 after this, on washing off the poisonous reagent, it was 

 found that the arrest had been temporary only, suction being 

 renewed at the rate of eleven, instead of the normal fifteen, 

 cubic mm. per minute. 



I applied the poison once more, and allowed it to act for 

 thirty-six hours. The arrest was then found to be permanent 

 — that is to say, the substitution of fresh water induced no 

 revival of response, the plant being killed throughout. 



Strasburger, as we have seen, in his experiments on the 

 effect of poisonous reagents on plants, found that the reagent 

 is carried to the top of the tallest tree ; from this fact it was 

 inferred that since all the cells in the path of the poisonous 

 solution must necessarily be killed by its action, therefore 

 the activity of living cells was not the essential factor in the 

 ascent of sap. But I have proved that the ascent of sap 

 is brought about, not by any localised group of cells in a 

 particular region, but by cells which extend throughout the 

 length of the plant. Even after some of these have died, 

 therefore, by the access of poison, those above are still active, 

 and will continue to exhibit suction till they in their turn are 

 finally killed. It will thus be evident that the movement 

 of ascent cannot be completely abolished till the poison 

 has effectively reached the very top. As all the living cells 

 are actively concerned in the work of suction, this con- 

 veyance of poison to the top of the plant is what was 

 to be expected. Only after such conveyance, indeed, could 

 permanent arrest possibly take place, and, in fact, Strasburger 

 himself mentions that the movement of water did come to a 

 stop when the poison reached the top of the tree. 



