MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 



533 



as the turgidity of the cells increases in one of the halves of the pulvinus, in con- 

 sequence of an increased afflux of water, this half elongates, bulges out, becomes 

 convex, and exerts a stronger pressure than the opposite half, so that the latter 

 becomes concave and much contracted. The supple portion of the bundle-strand 

 between the two halves of the cushion becomes bent, and the leaflet, whose stiff 



Fig. 133.— Alteration of Position of Leaflets in Compound Leaves. 



! Leaf of Mimosa Lindheimeri, seen from above, in day position. 2 The same in night position, a Leaf of Amorpha fruticosa 

 in day position. * The same in night position, s Leaf of Coronilla varia in day position. « The same in night position. 

 ' Leaf of Tetragonolobus siliquosus in day position. ' The same in night position. 



midrib is formed by the continuation of the bent bundle-strand, is inclined over 

 in the direction of the concave half of the pulvinus. If the increase of turgidity 

 occurs alternately first in the one and then in the other half of the pulvinus, the 

 leaflet will also be bent now to the one, then to the other side; and if the leaf- 

 support has a horizontal position, an alternate rising and sinking of the leaflet will 

 occur. It is to be noticed here that the leaflet itself remains actually quite passive, 

 and that the pressures which have come into play only operate in the pulvinus. 

 The commonest stimulation to periodic alteration of the turgidity in the pulvini 



