152 THE VEGETATIVE ORGANS 



apply their upper surfaces each to the opposite one, and 

 point their tips to the sky. At night they droop and apply 

 their under surfaces together. Thus they obtain the 

 advantages of favourable light, escape the injurious effect 

 of intense light, and, on assuming the night-position, reduce 

 the loss of heat from radiation. 



Rapid movements in Sensitive Plants. Some plants exhibit 

 the power of movement to such a degree as to have earned 

 the name of Sensitive Plants. The most familiar example 

 is Mimosa pudica, the Sensitive Plant. Its leaves are 

 bipinnate, i. e. each leaflet or pinna is again pinnately 

 divided into similar segments or pinnules. The end of each 

 leaflet has a pair of pinnules. At the base of each leaflet 

 and pinnule, and also at the base of the leaf-stalk, there is 

 an organ of movement, and the leaves exhibit sleep-move- 

 ments such as are seen in the Clover and Wood Sorrel. 

 So sensitive are the leaves, that a very slight stimulus 

 causes the leaflets to droop in the daytime. If a lighted 

 match be held under the end of a leaf, the heat-stimulus 

 produces a series of remarkable changes. Not only do the 

 heated pinnules droop, but the stimulus is transmitted 

 from one to another, pinnules and leaflets drooping in 

 succession, until, eventually, the stimulus reaching the 

 cushion on the leaf-base, the whole leaf hangs down lan- 

 guidly. There it remains until the shock has passed off, 

 when it gradually regains its former position. The leaves 

 of the Venus' Flytrap close up in a similar manner, but very 

 rapidly, in response to a contact-stimulus (see p. 366). 



These movements are due to rapid changes in the tur- 

 gidity of the cells of the cushions ; the effect of a stimulus 

 is to cause water to escape from the turgid cells of the 

 cushion into the neighbouring air-spaces. Later, as the 

 cells once more become turgid, the leaves and leaflets resume 

 their ' awake ' position. 



Movements of flowers and fruits. Flower-movements 



