• SUCTIONAL RESPONSE AND ASCENT OF SAP 361 



There then remains the question as to whether living 

 cells by some unknown physiological activity might not be 

 instrumental in effecting this transport of water. But the 

 experiments of Hartig, Bohm, and Strasburger have been 

 held to contradict such a possibility. Thus, Strasburger 

 set the cut ends of trees in tubs of poison, such as copper 

 sulphate solution. The poison ascended to the leaves, a 

 distance, in the tallest trees, of twenty-one metres. Now, if 

 such violent protoplasmic poisons ascend the trunk, it is clear 

 that they must kill all the cells lying in their path. That 

 the living cells of the stems could not be necessary to the 

 rise of sap was taken to be a necessary inference from 

 this experiment. Strasburger also killed portions of the 

 stems of living trees by heat, and yet the upper living and 

 leafy portion was found to remain turgid for a few days. 

 Another well-known experiment which was held to negative 

 the theory of protoplasmic activity, was that in which 

 boiling water was poured on the roots, when the plant con- 

 tinued to transpire, in spite of the roots having been killed. 



From such considerations, Strasburger was led to con- 

 clude that ' the supposition that the living elements in any 

 way co-operate in the ascent of the transpiration-current is 

 absolutely precluded.' l 



I shall nevertheless show that the ascent 6f sap is funda- 

 mentally due to the physiological activity of living cells, 

 and that the experiments described above in no way negative 

 this, being, on the contrary, capable of a different, and very 

 satisfactory, explanation. Many difficulties connected with 

 the problem of the ascent of sap will be found to disappear, 

 when the physiological activity of living tissues is once 

 clearly established as the essential factor. But a vague 

 assumption of protoplasmic activity will not be sufficient for 

 the elucidation of the phenomenon. It will be necessary 

 to show further how this excitatory activity is initiated, 

 and by what means a definite-directioned flow is imparted to 

 the sap. 



1 Strasburger, Text-book of Botany ■, English translation, 1903, p. 188. 



