Cuap. IV. THE EFFECTS OF HEAT. 73 
power of movement due to heat is called by Sachs* 
heat-rigidity ; and this in the case of the sensitive- 
plant (Mimosa) is induced by its exposure for a few 
minutes to humid air, raised to 120°—122° Fahr., or 
49° to 50° Cent. It deserves notice that the leaves of 
Drosera, after being immersed in water at 130° Fahr., 
are excited into movement by a solution of the car- 
bonate so strong that it would paralyse ordinary 
leaves and cause no inflection. 
The exposure of the leaves for a few minutes even 
to a temperature of 145° Fahr. (62°7 Cent.) does not 
always kill them; as when afterwards left in cold 
water, or in a strong solution of carbonate of ammo- 
nia, they generally, though not always, become in- 
flected; and the protoplasm within their cells under- 
goes aggregation, though the spheres thus formed are 
extremely small, with many of the cells partly filled 
with brownish muddy matter. In two instances, when 
leaves were immersed in water, at a lower temperature 
than 130° (54°-4 Cent.), which was then raised to 145° 
(62°:7 Cent.), they became during the earlier period 
of immersion inflected, but on being afterwards left 
in cold water were incapable of re-expansion. Ex- 
posure for a few minutes to a temperature of 145° 
sometimes causes some few of the mote sensitive 
glands to be speckled with the porcelain-like appear- 
ance; and on one occasion this occurred at a tempera- 
ture of 140° (60° Cent.). On another occasion, when 
a leaf was placed in water at this temperature of only 
140°, and left therein till the water cooled, every 
gland became like porcelain. Exposure for a few 
minutes to a temperature of 150° (65°5 Cent.) gene- 
rally produces this effect, yet many glands retain a 
* ¢Traité de Bot.’ 1874, p. 1034, 
