154 Dr. J. Bur don-Sanderson [June 9, 



we commonly call the sleep of plants, that is, that as night approaches 

 the leaf-stalks sink, and the leaflets fold up, the whole leaf assuming 

 a position closely resembling that which it assumes when it is 

 irritated. All that time will allow me to say on this subject is 

 that although the leaf assumes the same position in sleep as after 

 excitation, the two effects are not identical. The state of sleep ditfers 

 from that in which the plant finds itself after it has been irritated in 

 two particulars. The first is, that in the state of sleep it is still 

 excitable, and responds to stimulation exactly in the same way, 

 although from being already depressed the extent of its motion is 

 diminished ; the other is, that in sleep, the joint, although bent down- 

 wards, is still more or less resistent and elastic ; whereas in the 

 unexcitable (or, what comes to the same thing excited) state, all 

 elasticity has disappeared. In a word, in the motor organ of Mimosa, 

 in common with all other excitable structures, the characteristic of 

 the excited state is limpness. All the Mimosa plants on the table are 

 in the state of sleep, but are still excitable, for when they are 

 touched they sink to an even lower position than that of sleep, and at 

 the same time become limp. Hence you have, as the result of 

 excitation, two changes, namely (1) the change of position, only to be 

 observed when the plant is awake, and (2) the loss of stiffness, 

 dependent, as we shall see, on a vital change in the protoplasm of the 

 cells, which is also observed when the plant is asleep. 



So much for the general nature of the excitatory change. How 

 do we discover what the mechanism is by which this remarkable organ 

 of motion acts? By a mode of experiment which is well known to 

 the physiologist. It may be called the method of ablation. We 

 have here a mechanism which consists of several distinct parts, each, 

 we may presume, having a distinct purpose ; and the only method 

 which will enable us to discover what these several purposes are is to 

 observe how each acts alone — or, on the other hand, how the rest act 

 after it has been taken away. 



To j)rove that the motion of the whole leaf is dependent on the 

 motor organ at the base of its stalk, requires no experiment. We see 

 that the leaf descends, the joint bends, while the stalk remains rigid, 

 and we know from its structure that the latter contains no mechanism 

 by which it can act mechanically on the joint, as I act on my wrist 

 by the muscles of my fore-arm. 



The question therefore is — What part of the joint is essential ? 

 We begin by taking away the upper half, leaving the axial bundle 

 and the lower half, and find that the leaf assumes a higher position 

 than before. When touched, it falls. The function of the upper part, 

 therefore, is merely auxiliary. The essential part is the lower, which 

 in the unexcited state is capable of bearing the weight of the leaf. When 

 it is excited it suddenly becomes weak, and the leaf falls. How does it 

 do this ? We will proceed to remove the axial bundle. The cellular 

 cushion expands and lengthens, showing that it is elastic, and has a 

 tendency to spring out when liberated. We have seen that this 



