166 Dr. J. Burdon-Sanderson [June 9, 



In each, the diphasic character is distinct, and you see that the fii'st 

 or negative phase lasts less than a second, but that the positive, of 

 which the extent is much less, is so prolonged that before it has had 

 time to subside it is cut off bv another excitation. 



It would have been gratifying to me, had it been possible, to 

 exhibit to you other interesting facts relating to the excitatory process 

 in our leaf. It has, I trust, been made clear to you that the mechanism 

 of plant motion is entirely different from that of animal motion. 

 But obvious and well marked as this difference is, it is nevertheless 

 not essential, for it depends not on difference of quality between the 

 fundamental chemical processes of plant and animal protoplasm, but 

 merely on difference of rate or intensity. Both in the plant and in 

 the animal, work springs out of the chemical transformation of 

 material, but in the plant the process is relatively so slow that it 

 must necessarily store up energy, not in the form of chemical com- 

 pounds capable of producing work by their disintegration, but in the 

 mechanical tension of elastic membranes. The plant-cell uses its 

 material continunUij in tightening springs which it has the power of 

 letting off at any required moment by virtue of that wonderful 

 property of excitability which we have been studying this evening. 

 Animal contractile protoplasm, and particularly that of muscle, does 

 work only when required, and in doing so, uses its material directly. 

 That this difference, great as it is, is not essential, we may learn 

 further from the consideration that in those slow motions of the 

 growing parts of plants which form the subject of Mr. Darwin's 

 book, ' On the Movements of Plants,' there is no such storage of 

 energy in tension of elastic membrane, there being plenty of time 

 for the immediate transformation of chemical into mechanical work. 



I have now concluded all that I have to say about the way in 

 which plants and animals respond to external influences. In this 

 evening's lecture you have seen exemplified the general fact, applicable 

 alike to the physiology of plant and animal, that whatever knowledge 

 we possess has been gained by experiment. In speaking of Mimosa, 

 I might have entertained you with the ingenious conjectures which 

 were formed as to its mechanism at a time when it was thought that 

 we could arrive at knowledge by reasoning backwards — that is, by 

 inferring from the structure of a living mechanism what its function 

 is likely to be. In certain branches of physiology something has 

 been learnt by this plan, but, as regards our present investigation, 

 almost nothing, nor indeed could anything have been learnt. Every- 

 where we find that nature's means are adapted to her ends, and the 

 more perfectly the better we know them. But, with rare exceptions, 

 knowledge is got only by actually seeing her at work, for which 

 purpose, if, as constantly happens, she uses concealment, we must 

 tear off the veil, as you have seen this evening, by force. Have we 

 the right to assume this aggressive attitude ? Ought we not rather 

 to maintain one of reverent contemplation — waiting till the truth 

 comes to us? 



