SLEEP OF PLANTS. 



I made some experiments to see how they would be affected by 

 changes of temperature, but determined nothing further than 

 their mechanical sensibility to water, being a most extraordinarily 

 delicate Hygrometer, actively affected even by the vapour of the 

 hand at a considerable distance. 



It might have been something like this which Malpighi ob* 

 served. 



Although this may not throw any immediate light on the inter- 

 esting subject of the propulsion of the sap, yet the fact will 

 doubtless be thought worth noticing. 



It is most probable that the propulsion of the sap is 

 not .to be explained on any mechanical principles, but is 

 in some way the effect of vitality. 



IV. Sleep of Plants. 



Many leaves, especially the pinnate ones, of legumin- 

 ous plants, fold themselves together at night or droop 

 as if dying. It is a fact, very commonly observed, that 

 many flowers close at night without opening again un- 

 til the next day. Some flowers are open only in the 

 morning, closing before noon. Common names are 

 sometimes applied from the hour of their closing or 

 expanding ; as Four o'clock. The compound flowers 

 are very much disposed to close during the night. 

 The Dandelion and Marigold are examples. 



Shakspeare notices this fact in the Marigold in the 

 following lines, 



Her eyes like marigolds had sheathed their light, 



And canopied in darkness lay 



Till they might open to adorn the day. 



This change of the leaves and closing of the flowers 

 are what is called the sleep of plants. It seems to take 

 place in consequence of the absence of light. 



Light acts beneficially on the upper surface of leaves 

 and hurtfuily on the under side ; hence the upper sur- 

 face is always turned to the light in whatever situation 

 the plant may happen to be placed. 



The leaves of a great number of plants follow the 

 sun in his daily course. Clover is an example. 



Many flowers also follow the sun, as the Sunflower 

 11 



