EFFECT OF CARBON DIOXIDE 



303 



carried out with a small piece of a shoot of Mimosa bearing 

 a lateral leaf ; the sub-petioles may also be cut off, thus 

 reducing the weight of the organ. In order to prevent 

 drying, the cut ends of the stem and of the petiole are 

 covered with small pieces of moist cloth. The sensibihty 

 of the pulvinus is fully restored in the course of an hour, 

 as shown by the normal fall of the petiole under mechanical 

 stimulation. The advantage of a cut specimen is that it 

 can be easily manipulated and held in the inverted position 



(fig- 179)- 



Fig. 179. Method of record of geotropic response of jSIimosa 

 held in an inverted position with the more excitable half of 

 pulvinus B facing upwards. 



Experiment 166. Effect of CO2 on geotropic response 

 of Mimosa.- — The first part of the record in fig. 180 shows 

 the uniform erectile geotropic response in the inverted 

 position. The plant-chamber was next filled with carbon 

 dioxide. This caused an arrest and a subsequent reversal 

 of the geotropic response, which occurred 2 minutes after 

 the application of the gas. By this reversal the leaf was 

 brought below its original position and maintained there. 

 Substitution of fresh air brought about a restoration, and 

 normal geotropic response was renewed in the course of 

 4 minutes. 



Experiment 167. Erythrina indica. — The pulvinus of 

 Erythrina is less sensitive than that of Mimosa ; the charac- 

 teristic effects are, in other respects, the same in the two 



