\ 
they are withdrawn. Light, heat, and electricity, are the great stimuli 
common to vegetable and animal life. The agency of the first causes 
fi and the con 
certain plants to expand their blossoms ata part 
tinuance of its action causes them to close at an hour, quite as definite in - 
many instances. On the contrary, both or either of these phenomena, 
the expansion and the closing of the blossoms, may be retarded beyond 
the usual period, by altered conditions with regard to light. This is 
well known and acknowledged by those who contend, that in all this 
there is no proof of any thing beyond the mechanical action of an 
agent, on a mass of peculiar capillary tubes. Sir James Edward Smith, 
however, long ago, pointed out a wide distinction between caloric 
acting as a mechanical agent upon a mere mass of matter, and asa 
stimulus upon a living vegetable. Had it acted as a mechanical agent 
upon a mere mass of matter, then its effects must have been equal 
when applied in the same degree at different times to the same plant. 
. This, however, is notoriously untrue, in regard of the agency either 
of heat or of light. The effect is diminished according to the amount 
of excitement to which the individual had been exposed before the 
experiment began, exactly as occurs, when a stimulus is applied to an 
animal; an observation made by Mr. James M‘Nab on Thysanotus 
proliferus, illustrates another similarity in the phenomena of irrita- 
bility in the two living kingdoms of nature. Every medical man 
knows that certain spasmodic actions are continued by habit in an ani- 
mal, recurring regularly at certain periods; but that if this regularity be 
broken, by medicine or otherwise, and one paroxysm retarded, the 
hour of accession becomes irregular for a while. It is notorious 
that this is also true with respect to various healthy periodical actions 
in the animal economy. The observation by Mr. M‘Nab, shows that 
precisely the same thing occurs with our plant. I have mentioned the 
hours at which, in clear weather, the opening and closing of the flowers 
occurred with great precision. A flowering plant was in the evening 
removed, by Mr. M‘Nab, from the stove, and placed in a dark room. 
Next day its flowers were not expanded till twelve o'clock, and did not 
- close till late in the evening. The plant was then (that is in the evening — 
after the flowers had closed) taken back to its former position in the 
stove, but next morning the flowers did not expand before nine o'clock, 
as formerly, they expanded only at twelve, but they closed, as usual, at 
two o’clock;—in one day more, the natural periods were re-established, 
