1882.] on the Excitability of Plants. 153 



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. I would ask your attention to 

 their structure. 



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 sj)aces, 

 which in the restinoj — that is the excitable — state of the oro^an, are 

 filled with air. The surface is covered by epidermis. Below the axial 

 bundle there are equally numerous layers of cells, but they difter from 

 the others in this respect, that their walls are more delicate (Fig. 2). 

 And now let us study the mechanism of the motion. The literature 



Fig. 2. 



Section of the motor organ as projected on the screen. The vascular bundle in the 

 middle of the section consists of a cvlinder of thick-walled woodv iibres and 

 vessels, surrounded by a layer (annular in section) of elongated cells. The paren- 

 chyma is thicker below than above the vascular bundle. The section fails to 

 show that the cells of the upper half have thicker walls. 



of this subject is voluminous. Substantially, however, we owe the 

 knowledge we possess to two observers — E. Briicke,* who studied it 

 in 1848, and Pfeffer,t whose work appeared in 1873. 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 



* Briicke, " Ueber die Bewegmig der Mimosa pudica." Miiller's 'Archiv,' 

 184S, p. 434. 



t Pfeffer, ' Physiologische Untersuchungen,' p. 9. 



