Ammonium Sulphide: Experiment 68.—This reagent in dilute solution retards growth, and in stronger solution acts as a poison. The following results were obtained with a wheat seedling under different strengths of solution:—
| Normal rate | 0.30 µ per sec. |
| 0.5 per cent. solution | 0.15 µ " " |
| 2.0 " " " | 0.08 µ " " |
Copper Sulphate: Experiment 69.—The effect of a solution of this reagent is far more depressing than the last. One per cent. solution acting for a short time depressed the rate from 0.45 µ to 0.13 µ per sec. Long continued action of the poisonous solution kills the plant.
SUMMARY.
The effect of a chemical agent is modified by the strength of the solution, the duration of application and the tonic condition of the tissue.
Dilute solution of hydrogen peroxide induces an acceleration of growth.
The action of dilute vapour of ammonia is a preliminary enhancement followed by depression of growth.
Ether vapour depresses the rate of growth. On the removal of the vapour there is a recovery of the normal rate.
The effect of carbonic acid is a great enhancement of the rate of growth; after this preliminary action, growth undergoes a decline. The effect described takes place equally in light or in darkness.
Coal gas induces a depression of the rate of growth from which there is a recovery after the removal of the gas. The action of sulphuretted hydrogen is far more toxic, the after-effect being very persistent.