298 



HUTCHINSON'S POPULAR BOTANY 



a little cushion-like swelling, called the pulvinus (the Latin word for 

 "cushion"), which contains a woody centre surrounded by parenchymatous 

 cells, rich in water. When one of the pinnate leaflets is touched, the 

 effect is transmitted to the pulvinus by the threads of protoplasm, with 

 the result that the water passes from the cells on the lower to those on the 

 upper side, causing the former to pass from a distended into a flabby state. 

 They thus become temporarily unfit to support the leaf-stalk, which in 



[E. Step. 



FIG. 3C4. HOLLY (Ilex aquifolium). 

 Showing the spiny, dentate leaves and the clusters of red berrii 



consequence falls of its own weight. By and by the water gains its original 

 distribution, and then the leaf-stalk resumes its horizontal position. 



The celebrated Telegraph-plant (Desmodium gyrans) is even more 

 interesting than the Mimosas. It is an East Indian plant with violet flowers 

 and trifoliate leaves, and the latter are in motion night and day. In bright 

 sunshine the two lateral leaflets jerk up and down, and from side to side, in 

 a remarkable manner, while the large terminal leaflet goes through similar 

 though less perceptible movements (fig. 358). Should these movements be 

 artificially checked for a while, the leaf will start again with increased 

 velocity directly the retarding influence is removed. During darkness the 



