THE MOVEMENTS AND SENSITIVITY OF PLANTS 339 



pairs (Fig. 140). If the plant is severely shaken, then all its leaves 

 will droop at once. A very strong irritation of a separate leaf, as 

 by injuring through cauterization or by a stroke, will result in a 

 rather rapid transmission of the irritation throughout the plant. 

 As the wave of irritation spreads through the branch, the leaves 

 will close one after the other. This wave may be transferred from 



Fig. 140. — Mimosa branch. On the left, two open leaves. On the right, a leaf 

 lowered and folded, due to irritation (after A r athansohn). 



one branch to another and thus finally will involve the whole 

 organism. 



If the temperature is sufficiently high, above 25 to 30° C, then 

 the leaves of Mimosa will fold and droop within a few seconds. 

 Under these conditions the irritation received is also transmitted 

 further with great rapidity, as fast as 15 mm. per second. Such 

 an extremely rapid response is not characteristic of plants in 

 general. Naturally, then, Mimosa, which was first brought to 

 Europe from Brazil in the seventeenth century, very soon became 

 widely known not only in scientific circles, but among the public as 

 well. The sensitivity of this plant is very much akin to the irri- 



