354 



NATCKL 



\ylugust 10, 1882 



weeds, and Hawkweeds, all showing excito-contractility in the 

 same way, although the plants do not at all resemble each other 

 in external appearance. In order to make you acquainted with 

 the mechanism by which the excitable motions of plants are 

 brought about, I will confine myself to a very few examples, 

 selecting, of course, those which have been most carefully 

 investigated. 



Every one is acquainted with the general aspect of the sensi- 

 tive plant. Probably, also, most persons have observed the way 

 in which the leaves behave when one of them is touched, namely, 

 that the leaf, instead of being directed upwards, suddenly falls, as if 

 it had lost its power of supporting itself, and that the little leaflets 

 which spring from the side stalks fold together upwards (Fig. 1). 

 But perhaps every one has not observed exactly how this motion 

 is accomplished, namely, that by means of little cylindrical 

 organs the leaflets are jointed on to side-stalks, the side-stalk on 

 to the principal stalk, and the principal stalk on to the stem. In 

 those little cylinders, the powers of motion of the leaf have their 

 seat. They may, therefore, be called the motor organs of 

 Mimosa. 1 would ask your attention to their structure. 





Fig. 1. — Leaf of Mimosa; 



'., in the unexcited state : 

 (after Pfeffer). 



In my description I will confine myself to the relatively large 

 joint at the base of the principal leaf-stalk. If you make a 

 section through it in the direction of its length, you find that it 

 consists of the following parts. In the axis of the cylinder is a 

 fibro-vascular bundle ; above it are numerous layers of roundish 

 cells with thick walls and between these there exist everywhere 

 intercellular spaces, which in the resting — that is the excitable — 

 state of the organ, are filled with air. The surface is covered by 

 epidermis. Below the axial bundle there are equally numerous 

 layers of cells, but they differ from them in this respect, that 

 their walls are more delicate (Fig. 2). And now let us study the 

 mechanism of the motion. The literature of this subject is 

 voluminous. Substantially, however, we owe the knowledge 

 we possess to two observers — E. Btiicke, a who studied it in 1S4S, 



1 Brucke, " Ueber die Bewegung der Mimosa pudica." Midler's 

 " Archiv," 1848, p 434. 



and Pfeffer, 1 whose work appeared in 1S73. I must content 

 myself with the most rapid summary. 



Let me begin by noticing that Mimosa, in common with many 

 other excitable plants, exhibits that remarkable phenomenon 

 which we commonly call the sleep of plants, that i-, 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 differs fr im 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 downwards, is 

 still more or less resistent and elastic ; whereas in the nnexcitable 

 (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 ot sleep, but are still excitable, for when 

 they are touched they sink to an even lower position than that 

 of sleep, and at the :ame time bee >me limp. Hence you have, 

 as the result of excitation, two change*, namely (1), the change 

 of position, only to be ol served 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 

 when the plant is asleep. 





\^ 



Fto. 2.— Section of the motor organ as projected on the *< i 



1 umlle in the middle of the section consists of a cylinder of thick -walled 



fibres and vessels, surrounded by a layer (annular i:i 

 elongated cells. The parenchyma is thicker below than 



. bundle. The section fails to show that the walls of the cells of 

 the upper half have thicker walls. 



So much for the general nature of the excitatory change. 

 How do we discover what the mechanism is by which this re- 

 markable organ of motion acts? By a mode of experiment which 

 is well known to the physiologist. It may be called the n 

 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 

 the-e 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 prove that the motion of the whole leaf is depen 

 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. 1 he- 

 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 cel- 

 lular cushion expands and lengthens, showing that it is elastic, 

 and has a tendency to spring out when liberated. We have seen 

 that this resistent cushion consists of cells, that is, of h- tie 

 ' Pfeffer, " Physiulogische Untersuchungen," p. 9. 



