CONDITIONS FAVOURABLE TO MECHANICAL RESPONSE 45 



The isotropic condition is seen in the case of radial organs, 

 like cylindrical stems or peduncles. One way of showing 

 this is to try to bend such a stem in all directions, when it 

 will be found that equal forces produce equal bending in any 

 direction. This, however, is not the case with dorsi-ventral 

 organs, such as the pulvinus. This is more pliable in a 

 vertical than in a lateral plane. 



The mass of cells which constitute the lower half of the 

 pulvinus, in Mimosa for example, is larger than that of the 

 upper. The lower half is also the more excitable. The leaf 

 remains in a balanced horizontal position, under the action of 

 two opposing forces, the tensions of the opposite halves of the 

 pulvinus, these tensions being modified by the turgidity of 

 the cells. Pfeffer and Sachs have shown that under stimula- 

 tion there is an expulsion of water from these excitable cells. 

 This may be seen if we watch the cut end of a pulvinus very 

 attentively after the disappearance of the excitation due to 

 cut, and during the application of a new stimulus. The 

 application of this stimulus is followed by a visible escape of 

 water from the cut end. A more striking demonstration of 

 this fact will be given in Chapter XXI. 



Stimulus, then, induces diminution of the turgidity of the 

 organ, by expulsion of water from the excited tissue, and I 

 shall show by experiment how this negative variation of 

 turgidity, owing to the dorsi-ventral inequality of the organ, 

 causes the depression or fall of the leaf. 



Response by artificial turgidity-variation. — We may 

 fix air-tight the cut end of a branch of Mimosa bearing leaves 

 in a U-tube filled with water. When this is done, a quantity 

 of water is sucked up, and owing to this increase of turgidity 

 the leaves will be forced to assume a highly erect or almost 

 vertical position. After several hours this excessive turgidity 

 will disappear, and the leaf will then assume a more or less 

 horizontal position. The other end of the tube may now be 

 connected alternately with a vacuum and with a force-pump, 

 by means of which a diminution or increase of internal pres- 

 sure may be induced at will. When connected with the 



