SENSITIVE PLANTS. 
29 
Dr. Grolding Bird, who showed that the curvature which takes 
place when the stalks of herbaceous plants are divided vertically 
is due to osmotic action. Similar phenomena were observed by 
Morren in the mobile styles of Goldfussia and Stylidium. 
As regards the sensitive plant, it has been shown by isolating 
the cellular from the fibro-vascular portions, by means of in- 
cisions, above, below, and to either side of the central cord, 
that the cellular tissue so liberated becomes lengthened ; hence, 
all the while it remained attached to the central cord it must 
have acted like a spring, or rather as a double spring, one on 
one side, one on the other. When the force exerted by the one 
equals that of the other the leaf is retained in the horizontal 
position, but if the equilibrium be destroyed, the leaf falls. 
This equilibrium may be destroyed either by the superior energy 
of the upper spring, or by the diminished force or temporary 
paralysis of the lower one, owing to which the upper spring 
is able to push down the leaf and keep it down till a renewal of 
force in the lower cells replaces the leaf in its natural position. 
This latter explanation is the one that is generally accepted by 
physiologists. 
Anatomy, then, points to the rush of liquids from certain 
cells, and to the turgescence of others, as the cause of the 
movements, equilibrium being restored when the fluids again be- 
come diffused. Assuming that this unequal distension of certain 
cells is at least a general concomitant with the motions in 
question, it remains now to trace the causes that set them in 
action. Some of these, such as light, especially the blue rays, 
and heat have been already alluded to; but we have still to 
allude to the effect of what may be termed mechanical stimuli, 
including in that term agents that act chemically. Sensitive 
plants have been subjected to almost as many experiments as 
rabbits or frogs. A volume would be required to give the details ; 
all that it is necessary to do in this place is to state the general 
results. The effects of contact with the finger or other object, 
the result of a puff of wind or a drop of water, on sensitive plants 
are well known. Touch them, and they shrink from the blow 
by whatever means it be inflicted. Vibration, even without 
actual contact, is sufficient to set the leaves in motion. A foot- 
step in their native country makes the leaves close; in other 
cases, as we have seen, the touch of an insect ensures the motion 
of the leaf or of the stamen. Sensitive plants, however, are apt, 
like other creatures to get more or less accustomed to external in- 
fluences, to get exhausted, and so after a time become indifferent 
and lose their mobility. A curious instance of this is seen in 
the experiments performed by Desfontaines, who carried a Mi- 
mosa about with him in his carriage. The poor plant at first 
manifested its usual signs of sensibility, but by-and-by it ceased 
to respond to the stimulus, and its leaves became motionless. 
