SUCTIONAL RESPONSE AND ASCENT OF SAP 37 1 



In order to make the explanation easier, I have described 

 all these stop-cocks and their serial opening and closing 

 separately ; but this arrangement, apart from its clumsiness, 

 would involve a certain loss of time. In many of these 

 experiments, it must be remembered, it is necessary to know 

 the immediate effect produced. I have therefore simplified 

 the procedure by the use of a special key, K, by turning 

 which, in one direction or another, the requisite alternate 

 openings and closings are accomplished. 



Thus, on turning the key-handle to the left, A and a' are 

 opened, and B and b' closed. The balanced record is now 

 taken. As the handle is now being turned to the right, the 

 stop-cocks A and a' are first closed, arresting the index. Con- 

 tinued turning to the right opens B and B', by which the modi- 

 fying reagent is introduced into the plant-vessel. A sudden 

 turning of the key to the left now closes B and B', and opens 

 A and a', thus releasing the index and enabling the record 

 to be taken once more, under the changed conditions. The 

 whole process is thus made so rapid, that modified conditions 

 can be established, and the record renewed, within the short 

 interval of less than one minute. The photograph of the 

 completed apparatus is seen in fig. 157. Having now given 

 in detail all the experimental arrangements, I shall in the 

 next chapter describe the physiological modifications induced 

 by different agencies, as exhibited by the suctional response. 

 The periodic variation of ascent of sap may be recorded 

 continuously and automatically by photography. Another 

 method of recording transpiration will also be found described 

 on page 472. 



Summary 



This chapter gives a description of the Shoshungraph, by 

 means of which the rate of suction and its variations may be 

 indicated and recorded. The .sensitiveness of the apparatus 

 is very much increased by the Hydrostatic and Hydraulic 

 Methods of Balance. 



