ABSORPTION OF FOOD BY PLANT 361 



sections of the stem at different heights are then made and 

 placed in the field of a microscope. There is now nothing 

 distinguishing to be seen, as the solution sucked up by the 

 stem was colourless. Dilute solution of potash, however, will 

 act on this as a developer. The particular tissues, therefore, 

 through which the solution has been conducted, on now 

 being subjected to the action of this agent, become of a rich 

 crimson colour. By this means it is easy to see, as already 

 determined by various observers, that it is the younger, or 

 ' sap-wood/ which is concerned in the work of the rapid 

 conduction of water, the older, or ' heart- wood,' being 

 ineffective for this purpose. If, however, the wood taking 

 part in the ascent of sap had been dead, and acted as a 

 passive agent merely, it is difficult to understand the reason 

 of this selective action of the younger, and presumably more 

 living, woody tissues. 



It occurred to me, finally, that as electrical response is an 

 indubitable concomitant of the excitatory reaction of living 

 tissues, the question as to whether sap-wood was alive or 

 dead could be subjected to a decisive test. For this purpose 

 I took various strips of sap-wood from different woody plants. 

 The cortical tissue was in each case carefully removed, and 

 the specimens were placed in water, allowing them a period 

 of rest. The first experiment was to observe whether local 

 stimulation by the Rotary Mechanical Stimulator did or did 

 not evoke electrical response. I found from this, that mecha- 

 nical excitation of the sap-wood induced considerable excita- 

 tory response of galvanometric negativity. I then subjected 

 the same tissue to the action of boiling water for a length of 

 time, and again tested its electrical reaction by the same 

 method. The wood was found to be very resistant to the 

 action of heat, and it was only after long immersion that 

 the responses were entirely abolished. Drying was in fact 

 found, significantly enough, to be an easier method than the 

 application of heat, to kill, and therefore to abolish the 

 responsiveness of, the wood. If the wood be first dried, and 

 then soaked in water, it entirely ceases to manifest electrical 



