VITAL PRINCIPLE. 11 
The term of duration is very different in different spe- 
cies. While many mushrooms and insects are but the 
beings “ of a summer’s day,” the stately oak and the vora- 
cious pike outlive centuries. In general, when the pre- 
vious growth of an organized body has been slow, the pe- 
riod of decay is protracted in proportion ; and, when ma- 
turity has been quickly attamed, decay as rapidly suc- 
ceeds. 
2. PossEssED OF IRRITABILITY.—There is in this power 
which we are now considering, a disposition to be acted 
upon by different external objects, and to exhibit the influ- 
ence which these exercise by contractile or expansive move- 
ments. This faculty is termed Irritability. It appears to 
reside in the fibrous part of organized bodies. Plants ex- 
hibit this power in a very remarkable manner, upon expo- 
sure to the light of the sun, bending their stems, and turn- 
ing their leaves in various directions, according to the inten- 
sity and incidence of the rays. The pinnated leaves of cer- 
tain plants exhibit the same power, when touched by any ex- 
traneous body, the various leaflets collapsing in rapid suc- 
cession. ‘This is well displayed in Mimosa pudica and sen- 
sitiva, Smithia sensitiva, and some others. The Hedysarum 
gyrans, or Moving Plant, as it has been termed, exhibits 
a motion in its leaves of a still more remarkable kind, re- 
quiring a very warm still atmosphere for its production. 
The leaf is ternate, and the lateral leaflets approach and 
recede from one another, in a manner irregular as to time 
and co-operation. But the example of vegetable irritability 
which is most accessible in this country, is exhibited by the 
Barberry-bush. Sir James Epwarp Suiru, who first ob- 
served the phenomenon, thus expresses himself: ‘ In this 
(flower) the six stamens, spreading moderately, are sheltered 
under the concave tips of the petals, till some extraneous bo- 
dy, as the feet or trunk of an insect, in search of honey, 
