404 PLANT RESPONSE 



Summary 



The ascent of sap is fundamentally due to excitatory 

 reaction, its uni-directioned flow being brought about by the 

 passage, from point to point, of the co-ordinated excitatory 

 reaction, propelling water forward. 



This rhythmic excitation is initiated in the intact plant 

 at its root, by stimulus of contact with soil, the friction of 

 the growing organ against rough surfaces, the excessive tur- 

 gidity caused by the absorption of water, and possibly by 

 the chemical stimulus of substances present in the soil. In 

 the case of cut branches placed in water, the excessive 

 turgidity at the cut end initiates rhythmic activity. Again, 

 if stimulus be applied at the top instead of at the root, the 

 direction of water-conduction may be reversed, along with 

 the reversal of propagation of excitation. 



The following facts show the intimate relation between 

 the conduction of stimulus and conduction of water : 



(a) The movement of water takes place preferentially 

 through the fibro-vascular elements, these being also the 

 better conductors of excitation. 



(d) The conduction of excitation along a plant is greater 

 than across. The same is true of its power of transport of 

 water. 



(c) Though conduction of excitation may take place 

 either upwards or downwards, yet there is a preferential 

 direction for such conduction. The same is true of the trans- 

 port of water. 



The same movement of water produced by the co- 

 ordinated rhythmic activity of cells throughout the plant 

 appears as either suctional or pressure movement, according 

 to the point of view. 



When the removal of water from the plant is in any way 

 arrested, a positive pressure is produced, owing to the exces- 

 sive accumulation of water. When, on the other hand, the 



