345 
CHAPTER XIIL 
OF IRRITABILITY. 
The vitality of plants seems to depend upon the existence 
of an irritability, which, although far inferior to that of ani- 
mals, is, nevertheless, of an analogous character. 
This has been proved by a series of interesting experiments 
by Marcet, of Geneva, upon the exact nature of the action 
of mineral and vegetable poisons. The subject of his observ- 
ations was the common kidney-bean ; 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 
twent-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, 
was found to produce effects similar to those of arsenic ; but 
no effect was produced upon a cherry tree, by boring a hole 
in its stem, and introducing a few globules of liquid mercury. 
Tin, copper, lead, muriate of barytes, a solution of sulphuric 
acid, and a solution of potash, were found to be all equally 
destructive of vegetable life ; but it was ascertained, by means 
of sulphate of magnesia, that those mineral substances which 
are innocuous to animals are harmless to vegetables also. In 
wie experiments with vegetable poisons, the bean plants were 
