48 CHAP. V. EFFECT OF CHEMICAL AGENTS 



rapidly over the surface, and the organ exhibits rapid 

 wilting. 



It is instructive to compare the death-spasm under an 

 anaesthetic with the parallel effect of stimulation by a 

 strong electric shock. The plant recovers from moderate 

 stimulation and responds again to fresh stimulation. But 

 under excessive stimulation, such as that induced by 

 strong electric shocks, the excitatory contraction passes 

 from a reversible to an irreversible condition associated with 

 death. Both pulvinated and growing organs exhibit a 

 violent contraction from which there is no recovery. In 

 the case of anaesthetics likewise, a mild dose induces a 

 contraction, recovery taking place after substitution of 

 fresh air. But under a stronger dose the violent con- 

 tractile spasm proves to be the spasm of death. 



Effect of Poisonous Agents 



This refers to gases or liquids which cause depression 

 and subsequent death of the plant. 



Sulphuretted Hydrogen 



Experiment 23. — This gas not only exerts a great 

 depressing effect, but is also toxic in its action, which 

 accounts for the impossibility of growing many plants in 

 a town, the air of which contains traces of this gas. For 

 example, Biophytum sensitivum, which flourishes and main- 

 tains its high sensitivity seven miles out of Calcutta in the 

 suburban area, soon succumbs when brought within the 

 town. The record (fig. 21) shows the marked retardation 

 of growth when the plant is exposed to HoS even for 

 15 minutes. In the present case the growth-rate was 

 reduced to half. 



Experiment 24. Ammonium Sulphide. — This reagent 

 in dilute solution retards growth, and in stronger and long- 

 continued dose acts as a poison. The following results 



