1 68 RESEARCHES ON IRRITABILITY OF PLANTS 



copper sulphate or potassium cyanide solutions. The 

 choice of a strong poison was deemed advisable because the 

 absorption of even a small quantity might in such a case 

 prove effective in inducing depression or abolition of con- 

 duction. Application of an anaesthetic, like chloroform, has 

 the drawback that the escaping vapour renders the motile 

 organ insensitive. There is no such disadvantage in the 

 employment of a non-volatile poison like copper sulphate, 

 which by local application would affect the conductivity of 

 the selected portion of petiole without modifying the motile 

 sensibility of the pulvinus. The petiole of Biophytum was 

 found to be very suitable for these experiments. Among 

 the whorl of leaves, about half a dozen are found the leaflets 

 of which are fairly sensitive. Graduated stimulus is applied 

 to the stem by means of an electro-thermic stimulator. 

 This latter consists, as explained in a previous chapter, of 

 a v-shaped piece of platinum wire, through which an electric 

 current of suitable intensity could be sent, the circuit being 

 completed for a definite length of time by means of a 

 metronome. The electric current is so adjusted as to give 

 rise to a thermal shock without causing a burn. The effective 

 intensity can be gradually increased by taking advantage 

 of the additive effect of stimulus. Thus in a given case while 

 the thermal shock was singly ineffective, it became minimally 

 effective when repeated four times and maximally effec- 

 tive when repeated eight times. The excitatory impulse 

 originated at the stem, radiated subsequently to the leaves, 

 where the progress of the excitatory waves was visually 

 demonstrated by the serial fall of leaflets in a centrifugal 

 order. It should be mentioned here that the excitation 

 caused by thermal stimulus is most intense ; the block needs 

 to be very perfect to arrest the conduction of such an 

 excitation. 



Arrest of conduction by copper sulphate solution. — After 

 determining the value of an effective stimulus, poison was 

 applied to the portion of petiole which is next to the stem. 

 This was done by wrapping a strip of cloth 5 mm. in breadth 



