_Effect of carbonic acid on Balanced growth: Experiment 100._--I have already shown that carbonic acid diluted with air induces an enhancement of the rate of growth, but its long continued action induces a depression (p. 185). I shall now employ the Method of Balance in studying the effect of CO_{2} on growth. It should be remembered in this connection that the horizontal record indicates the balance of normal rate of growth. An up-curve exhibits the induced enhancement and a down-curve, a depression of growth. In the present experiment after obtaining the exact balance, pure carbonic acid gas was made to fill up the plant-chamber at the point marked with an arrow (Fig. 96). It will be seen that this induced an almost immediate acceleration of the rate, the latent period being less than five seconds. The acceleration continued for two and half minutes; the accelerated rate then slowed down, became enfeebled, and the growth returned for a short time to the normal as indicated by the horizontal portion at the top of the record; this proved to be the turning point of inversion from acceleration into retardation of growth. The stronger is the concentration of the gas the earlier is the point of inversion. With diluted carbonic acid the acceleration may persist for an hour or more. EFFECT OF ANÆSTHETICS. _Effect of Ether: Experiment 101._--Dilute vapour of ether is found to induce an acceleration of rate of growth which persist for a considerable length of time. This is seen in the upsetting of the balance upwards on the introduction of the vapour (Fig. 97a.). [Illustration: FIG. 97.--(_a_) Effect of ether, acceleration of growth, (_b_) effect of chloroform preliminary acceleration followed by depression.] _Effect of Chloroform: Experiment 102._--The effect of chloroform vapour is relatively more depressing than ether. Application of chloroform is seen to induce at first an acceleration which persisted for 50 seconds, but after this depression set in (Fig. 97b). Prolonged application of the anæsthetic is followed by the death of the plant. SUMMARY. In the Method of Balance the movement of growth upwards is compensated by an equal movement of the plant downwards, with the result that the record remains horizontal. The effect of an external agent is immediately detected by the upsetting of the balance, up-record representing acceleration above normal, a down-record the opposite effect of depression below the normal rate. The latent period and the after-effect of stimulus may thus be obtained with the highest accuracy. The sensitiveness of the Method of Balance may be raised so as to indicate a variation of rate of growth smaller than 1/1000 millionth of an inch per second. XXIII.--ON TROPIC MOVEMENTS _By_ SIR J. C. BOSE.