266 THE LIFE OF THE PLANT 



the lower ; the leaves are consequently widely spread 

 horizontally, or even slightly bent down, but at the 

 moment of irritation the tension of this upper part breaks 

 down, and the leaves, being left under the influence 

 of the tension of the lower part only, rise and draw 

 together in pairs. So the cause of this movement lies in 

 the sudden, almost instantaneous loss of turgidity in the 

 tissue of one of the two halves of the pulvinus : from 

 being turgid it suddenly becomes flaccid, the balance 

 between the two antagonistic halves of the organ is 

 upset, and the leaf or leaflet moves in the corresponding 

 direction. But how can we account for this sudden 

 collapse, this loss of tension ? The microscope reveals 

 the fact that the tissue which has this curious property 

 of losing its tension consists of cells with thinner walls 

 than the cells of the opposite antagonistic side of the 

 pulvinus ; and, moreover, that the cells of this irritable 

 tissue alternate with spaces filled with air. At the 

 moment of irritation these spaces become filled with 

 a liquid, as is easily proved. We have only to fix 

 our eyes on the thickened pulvinus at the base of the 

 petiole to see a sort of shade pass across that place 

 at the moment of irritation ; the spot suddenly becomes 

 darker. The same thing happens even more distinctly 

 if several pairs of leaves are suddenly but gently clutched 

 with both hands in such a way that they are irritated, 

 but at the same time not allowed to fold. We shall 

 notice, then, that the thickened pulvini, compared 

 above to the millet seeds and lying at the base of every 

 leaflet, will change in colour; from dull white they 

 become transparent green. The moment we let the 

 leaves go, they will fold. What causes this sudden 

 change in colour ? It is the same cause which produces 

 a dark spot on the white surface of the snow, a filter 

 paper, or ground glass when we sprinkle them with 

 water. The whiteness in all these cases depends upon 

 the reflection of light by countless minute surfaces in 



