THE PLANT AND THE ANIMAL 265 



Apparently we have here an impetuous movement, 

 caused by some external stimulus, reminding us very 

 strongly of the movement of an animal when it tries to 

 avoid some irritating contact. Can we give any 

 explanation of this phenomenon ? Yes and no. We can 

 indicate the proximate mechanism of the movement, 

 but as yet cannot explain the nature of the stimulation, 

 caused by irritation and in its turn producing the move- 

 ment. This movement takes place at the points where 

 the leaflets are attached to the stalks, where these are 

 attached to the main petiole, and, lastly, where the 

 latter is attached to the stem. At all these points, 

 the joints or articulations, special swellings or cushions 

 are found. These leaf cushions or pulvini are formed 

 of a tissue, the cells of which are overfilled with sap, 

 and as a result these parts are in a constant state of 

 tension. The moment the leaves are irritated the tension 

 is suddenly released ; it is even reversed. Thus, for 

 instance, the tension of the tissues of the lower half of 

 the pulvinus which forms the base of the leaf-stalk 

 supports it horizontally, and even in a rather uplifted 

 position (as on the right-hand side of the figure). But 

 whenever irritated this part of the cushion loses its 

 tension, becomes floppy, loses its turgidity ; it is then 

 unable to support the petiole, which sinks or rather is 

 bent down by the upper half of the pulvinus, which 

 has preserved its turgidity. Thus the two parts of the 

 cushion — the upper and the lower — are in constant 

 antagonism. In a normal condition the tension of the 

 lower part predominates and the petiole is held up ; 

 at the moment of irritation, when the tension of the 

 lower part ceases, the preponderance is in favour of the 

 upper part which bends the leaf down. At places 

 where individual leaflets are attached to the stalks, the 

 reverse phenomenon happens : the upper part of the 

 pulvinus (appearing here in the form of a white knob 

 the size of a millet seed) is always more strained than 



