Natural History 643 



is gently shaken, instantly the leaf-stalks droop, the secondary 

 stalks fall together, the leaflets fold up ; the whole scaffolding of 

 green light screens has collapsed. In nature, a wandering animal 

 or a shower of rain brings about the same result, a series of move- 

 ments so rapid that the reaction is complete in a few seconds. 

 Soon a reverse sets in, and a quarter of an hour later the plant 

 is in its normal condition again. 



A violent shock is not necessary ; touch the lower half of the 

 joint of the leaf -stalk and it droops; a little later the secondary 

 stalks come together, then the leaflets fold up in succeeding pairs. 

 Singe the apical leaflet and the chain of reactions is reversed. 

 Make a cut in the stem, and first the nearest leaf reacts, then the 

 next, then one further away. The shock-stimulus is passed on. 

 It is conducted at the rate of an inch in the second; this is a 

 thousand times slower than the conduction along an animal's 

 nerve, but a hundred times as quick as the rate commonly de- 

 veloped in plants. The movement itself is slow compared with 

 the wink of an eyelid, but very fast in comparison with that of 

 an inverted root bending downwards. 



The movement is different from that of a curving root or 

 stem. It depends on a sudden diminution of pressure in the cells 

 of the lower half of the joint, which thus loses its rigidity and 

 collapses. This allows of a very rapid fall; it also allows of the 

 occurrence of the movement long after growth has ceased. 



Of what use to the plant is this spectacular reaction? It 

 has been suggested that the rapid closing shakes off small insects 

 feeding on the leaves. It has been suggested that the plant thus 

 avoids damage by violent rain or hail. It has been suggested 

 that browsing animals are frightened by the sudden change in 

 the appearance of their pasture and repelled by the thorny 

 appearance of the closed plant. It cannot be said that these 

 explanations are convincing. They do not apply at all to a 

 dozen other plants which show similar but less active leaf move- 

 ments. The only animal that shows a distaste for the Sensitive 



