150 GENERAL VITALITY [BOOK n. 



that a different action exists in all plants by day and night, 

 although it is less visible in some than in others ; thus plants 

 of Corn, in which there is little indication of sleep when 

 grown singly, exhibit that phenomenon very distinctly when 

 observed in masses; their leaves become flaccid, and their 

 ears droop at night. These effects have been attributed to 

 the action of light ; and it is probable that that agent con- 

 tributes to produce them; for a flower removed from the 

 shade will often expand beneath a lamp, just as it will beneath 

 the sun itself, and De Candolle found that he could induce 

 plants to acknowledge an artificial day and night, by alternate 

 exposure to the light of candles. But it is obvious that there 

 must be some cause beyond light, because many flowers will 

 close in the afternoon while the light of the sun is still play- 

 ing upon them, and the petals of others will fold up under a 

 bright illumination, and that cause may be reasonably sup- 

 posed to be an excitable vital fluid. The experiments of 

 Macaire and Marcet prove indeed conclusively that whatever 

 the true seat of Vegetable vitality may be, it is similar in its 

 nature to that of the Animal Kingdom. 



This has been proved by Marcet, of Geneva, who instituted 

 a series of experiments upon the exact nature of the action of 

 mineral and vegetable poisons. The subject of his observa- 

 tions was the common Kidneybean ; and, in each experiment, 

 a contrast was formed between the plant operated upon and 

 another watered with spring water. A vessel containing two 

 or three Bean plants, each with five or six leaves, was watered 

 with two ounces of water, containing twelve grains of oxide 

 of arsenic in solution. At the end of from twenty-four to 

 thirty-six hours the plants had faded, the leaves drooped, and 

 had even begun to turn yellow. Attempts were afterwards 

 made to recover the plants, but without success. A branch 

 of a Rose tree was placed in a solution of arsenic ; and in 

 twenty-four hours ten grains of water and 0*12 of a grain of 

 arsenic had been absorbed. The branch exhibited all the 

 symptoms of unnatural decay. In six weeks a Lilac tree was 

 killed, in consequence of fifteen or twenty grains of moistened 

 oxide of arsenic having been introduced into a slit in one of 

 the branches. Mercury, under the form of corrosive sublimate, 



