202 == Structural and Physiological Botany. 
the tissues thus becoming flaccid, and the parts becom- 
ing elongated and bent. These facts can easily be 
established by experiment, very clearly by simple observa- 
tion of rapidly growing plants shaken by violent winds, 
when, though apparently injured, they are not generally so 
in reality. In other cases of a similar kind the extensibility 
of the parenchyma is diminished, and a flaccidity, shorten- 
ing, and curvature of the organs occasioned. These changes 
are probably the cause of the movements of zrrztabzlity, as 
it is termed, of some leaves; although these phenomena 
may also be due to other causes, such as electrical shocks or 
currents. The most remarkable instance occurs in the leaves 
of the sensitive plant, I/imosa pudica. The bipinnate leaves 
of this plant are united to the stem by an articulation, as 
also are the four pinne and the separate leaflets to their 
petioles (see Fig. 517, p. 403). The primary petiole has the 
greatest scope for movement, since it can rise or fall to 
the extent of a semicircle. The pinnz move upwards and — 
laterally, so that they lie upon one another lke parts of a 
fan. ‘The secondary pinnules move upwards, and close up 
like the wings of a butterfly at rest. The sensitive plant — 
closes its leaves spontaneously in the evening, and also in 
the day-time when excited by any irritation: This irm- 
tability is only manifested when the temperature of the 
surrounding air is above 15°C. ; it 1s still weak between 16° 
and 18°, and appears to reach its maximum at 30°C. At 
that temperature the plant is so sensitive that the move- 
ment is communicated to a number of leaflets almost simul- 
~ taneously ; while, when the sensitiveness is less, the move- 
ment advances regularly from the irritated spot. The 
closed leaflets again assume their normal position when the 
irritation ceases, if it has not been too strong and conse- 
quently had an injurious effect ; and this takes place sooner 
or later according to the vigour of the plant, commencing 
sometimes in as short a time as five minutes. Poisons 
often kill the sensitive plant; anzesthetics, such as ether 
