CHAP. XV. 
IRRITABILITY. 
4-21 
which is larger than those at the side, does not move, except 
to sleep ; but the lateral ones, especially in warm weather, 
are in continual motion, both day and night, even when the 
terminal leaflet is asleep. External stimuli produce no effect ; 
the motions are very irregular ; the leaflets rise or fall more 
or less quickly, and retain their position for uncertain periods. 
Cold water poured upon it stops the motion, but it is imme- 
diately renewed by warm vapour. 
To this class of irritability ought, perhaps, to be referred 
the curious phenomenon well known to exist in the fruit of 
Momordica Elaterium, the Spirting Cucumber. In this plant 
the peduncle, at a certain period, when the fruit has attained 
its perfect maturity, is expelled, along with the seeds and the 
mucus that surrounds them, with very considerable violence. 
Here, however, endosmose appears to offer a satisfactory ex- 
planation. According to Dutrochet, the fluid of the placen- 
tary matter in this fruit gradually acquires a greater density 
than that which surrounds it, and begins to empty the tissue 
of the pericarpium : as the fruit increases in size the same 
operation continues to take place; the pulpy matter in the 
centre is constantly augmenting in volume at the expense of 
the pericarpium ; but, so long as growth goes on, the addition 
of new tissue, or the distention of old, corresponds with the 
increase of volume of the centre. At last growth ceases, but 
endosmose proceeds ; and then the tissue that lines the walls 
of the central cell is pressed upon forcibly by the pulp that it 
encloses, until this pressure becomes so violent that rupture 
must take place somewhere. The peduncle, being articulated 
with the fruit, at length gives way, and is expelled with vio- 
lence ; at the same time the cellules of tissue lining the cavity 
all simultaneously recover their form, the pressure upon them 
being removed, and instantly contract the space occupied by 
the mucous pulp ; the consequence of which is that it also is 
forced outwards at the same time as the peduncle. It has 
been found by measurement, that the diameter of the central 
cavity is less after tlie bursting of the fruit than before. 
Movements produced by touch, or by external violence, 
are very frequent. 1 he Sensitive Plant (Mimosa pudica), 
which will rapidly fold up its leaves as if in a state of sleep, 
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