MOVEMENTS OF IRRITATION. 541 



If the soil in which a Mimosa is rooted is not watered for a 

 long time, the plant being however exposed otherwise to normal 

 conditions, the capacity of the leaflets for response to shock or 

 contact falls off more and more. In an experiment made by me 

 at about 20 C., complete loss of sensitiveness to contact appeared 

 after four days. The leaflets in a state of dryness-rigor are 

 outspread, but by no means yet appear limp, and they regain 

 their irritability when the soil is again well supplied with water. 



If a Mimosa is exposed for a few hours to a temperature below 

 15 C. (e.g. 10 C.), the plant passes into a state of transitory 

 cold-rigor. The sensitiveness to contact and change of illumin- 

 ation disappears. The normal state returns, however, when the 

 plant is brought back to a place at a temperature above 15 C. 



To bring Mimosas into a state of temporary heat-rigor, we 

 place the plants in a suitable thermostat at a temperature of 40 C. 

 After about an hour (at 45 C. in a much shorter time) heat-rigor 

 sets in. The leaflets have in presence of light laid themselves 

 together upwards. With favourable conditions of temperature, 

 the normal state is regained in the course of a few hours. 1 



1 See especially Claude Bernard, Legons s. I. phenomenes d. I. vie, 1878 

 Sachs, Flora, 1863 ; and Pfeffer, Physiol. Untersuchungen, 1873. 



