12 THE SIGNS OF LIFE [LECT. 



the eyeball from fundus to cornea and I make sure of this 

 direction once more by touching the fundus electrode with a 

 bit of zinc, and seeing that the spot moves to your right. 



Let me make one more pair of trials to assure you that 

 it has been luminous radiation in particular, and not thermal 

 radiation, that has excited the retina of this eyeball. I repeat 

 the exposure just as before, with the standard candle at un- 

 altered distance (5 feet), but with an alum cell interposed to 

 filter off most of the heat, and, as you see, the response is no 

 smaller than before. Finally, I take away the alum cell, and 

 expose the retina to the very sensible heat of a black hot 

 surface of metal, and, as you see, there is no response at all 

 to heat without light. 



7. TIic mechanical excitability of vegetable protoplasm. The 

 fourth and last experiment that I wish to show is a very simple 

 one, but in my judgment a fundamental experiment in general 

 physiology. It relates to the mechanical excitability of vegetable 

 protoplasm, and is calculated to convince you of the essential 

 identity between the excitatory responses of vegetable and of 

 animal protoplasm. Those of you who are students of physi- 

 ology or of physiological botany, are no doubt acquainted with 

 various instances of movements in plants consequent upon 

 mechanical excitation ; and you probably know from the observa- 

 tion by Burdon-Sanderson on Dionxa, that these movements 

 are accompanied by electrical effects. You may take it that the 

 excited plant-stuff is the seat of chemical change, that this 

 change is of the nature of a disintegration from large complex 

 molecules to smaller, simpler molecules, that this disintegration 

 has involved an increased osmotic pressure, whence turgor of 

 certain cells, whence movement of petioles and leaves. And 

 with these chemical changes there are of course associated 

 electrical changes. 



But these notions are to be extended, and we are to recognise 

 further that any and every living vegetable protoplasm, when 

 excited, undergoes chemical, and therefore electrical, change, 

 whether it actually moves or not. 



I have used all kinds of vegetable protoplasm, usually during 



