_Coal Gas: Experiment 66._--Coal gas induces a depression. It is curious that subjection to the action of this gas does not produce so evil an effect as one would expect. The introduction of the gas had reduced the growth-rate to more than half; but there was a recovery half an hour after the introduction of fresh air. _Sulphuretted Hydrogen: Experiment 67._--This gas not only exerts a depressing effect, but its after-effect is also very persistent. The plant experimented on was very vigorous and its rate of growth was depressed to half by subjection to the action of the gas for a short time. The record taken half an hour after the introduction of fresh air did not exhibit any recovery. ACTION OF POISONS. _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. Solution of ammonium sulphide induces increasing retardation of growth, with the strength of the solution. Copper sulphate solution acts as a toxic agent, retarding the rate of growth and ultimately killing the plant. XIII.--EFFECT OF VARIATION OF TURGOR AND OF TENSION ON GROWTH _By_ SIR J. C. BOSE. The movements of leaves of sensitive plants are caused by variation of turgor in the pulvinus induced by stimulus. The down movement or _negative_ response of _Mimosa_ is caused by a diminution or negative variation of turgor, while the erection or _positive_ response is brought about by an increase, or positive variation of turgor.