158 ON THE POWER OF MOVEMENT IN PLANTS. 



Wales, and others testify to this. Sir J. W. Hooker says, that in 

 railway-journeys through these parts, we can tell at once any 

 change in direction by noting the position of these leaves. 



" Look at this delicate plant, that lifts its head from the 

 meadow. 



See how its leaves all point to the north, as true as the 

 magnet ; 



It is the compass-flower, that the hand of God has sus- 

 pended 



Here on its fragile-stalk, to direct the traveller's journey 



Over the sea-Hke, pathless, hmitless waste of the desert." 



Thus does Longfellow in ' Evangeline,' tell us in poetry, of a 

 beautiful fact in nature ! 



I need scarcely say, that the sleep of the flowers is, like that of 

 leaves, a provision for securing them from the ill effects of too great 

 radiation, and that the Directto?t, and not the Intensity, of light is 

 that which governs both positive and negative Heliotropism. 



I have no space left to speak of Positive and Negative Geotrop- 

 ism, directing most stems upwards or away from the earth in the 

 latter, and roots downwards and to the earth in the former, nor of 

 the effects of positive Geotropism in securing a decent and useful 

 burial for the pods of subterranean Clover, and some other flowers. 



Our second division of Class III — viz.— 



B. — Induced Movements — i.e., those brought about by 

 mechanical stimuli, and in no way Periodic — I must also leave 

 untouched. 



I can only indicate briefly what this division includes by 

 referring to — 



I. — Liduced Movements in Leaves, such as may be seen in 

 Mimosa and Desmodium (already noted as also illustrating Periodic 

 motion), and in Sundew and Venus' Fly-trap {Dionosa), where 

 insects are the agents, sacrificing their own lives in the process. 

 Also to such cases as the leaves of Schinus and Rhns, which can 

 be made to execute a literal dance by throwing them into water. 

 In Utricularia, or Bladderwort, the fine, hair-like leaves are 

 furnished with floating bladders ; these possess valves which close 

 with a certain and fatal snap when the wanderings of aquatic 



