6i6 



NATURE 



[July 15, 1920 



reached a limit, the record becoming once more 

 horizontal. The obvious precaution to be taken 

 in the study of variation of growth under change 

 of temperature is to wait for several minutes for 

 the attainment of steady temperature. The 

 elongation caused by physical change abates in 

 a short time, whereas the physiological variation 

 in the rate of growth is persistent. 



In Fig. 2a is given a record of growth of 

 Scirpus kysoor ; the growth per second magnified 

 ten thousand times is 95 mm. The absolute rate 

 of growth per second is therefore 000095 mm., 

 or 0-95/X, where /x or micron is 0001 mm. 



Effect of Stimulus on Growth. — A generalisa- 

 tion was obtained that all forms of stimuli, 

 mechanical, electrical, or radiational, induce a re- 

 tardation of the rate of growth; under increasing 

 intensity or duration of stimulus this retardation 

 may culminate in an arrest of growth or even in 

 actual contraction of the organ. As regards radia- 

 tion, all rays of the vast aithereal spectrum (with 



Fig. 2. — Crescographic records, a, Successive records of growth at intervals of i second ; X 10,000, with a stationary plate. 

 Effect of temperature : </, N, normal rate of growth ; C, retarded rate under cold ; H, enhanced rate under warmth ; 

 6, record on moving plate where diminished slope of curve denotes retarded rate under cold ; c, horizontal record 

 showing absence of growth in dead branch ; physical expansion on application of warmth at arrow followed by 

 horizontal record on attainment of steady temperature. 



the exception of red and yellow rays which cause 

 photo-synthesis) are found to cause response by 

 modifying the rate of growth of the plant. I 

 have thus been able to obtain records of response 

 of plants to long aether waves employed in signal- 

 ling through space. (Nature, October 30, 1919.) 

 Effect of Sub-minimal Stimulus. — A very unex- 

 pected result was obtained under the action of 

 sub-minimal stimulus, which induced an accelera- 

 tion of growth instead of retardation under 

 moderate intensity. This I find to be true of 

 stimulation as diverse as that caused by electric 

 shock, by light, and by chemical agents. A strik- 

 ingly practical result was obtained with certain 

 poisons which in normal doses killed the plant, 

 but which in quantities sufficiently minute acted 

 as an extraordinarily efficient agent for stimulat- 

 ing growth, the treated plants growing far more 

 vigorously and flowering much earlier. It is only 

 by the discovery of laws of growth that any 

 marked advance in scientific agriculture will be 

 NO. 2646, VOL. 105] 



rendered possible. We have been using a fev; 

 stimulating agents, whereas there are thousands 

 of the action of which we have no conception. 

 The rule-of-thumb methods often employed in the 

 application of a few chemical agents and of elec- 

 tricity have not been uniformly successful. The 

 cause of the anomaly is found in the discovery of 

 an important factor — namely, the dose of applica- 

 tion — which had not hitherto been taken suffi-- 

 ciently into account. 



The Balanced Cresco graph. —The high sensi- 

 tiveness already secured has been very greatly 

 enhanced by the employment of the Null Method 

 or the Method of Balance, where the rate of up- 

 movement of growing tip is exactly compensated 

 by the down-movement of the plant. A train of 

 revolving clock-wheels, actuated by the fall of a 

 weight, lowers the plant at the required rate. The 

 exact adjustment is obtained by the right- or left- 

 handed turning of a screw which regulates the 

 governor. In this way the rate of growth becomes 

 exactly compen- 

 sated, and the re- 

 corder now dots a 

 horizontal line in- 

 stead of the former 

 curve of ascent. 

 The turning of the 

 adjusting screw 



also moves an index 

 against a circular 

 scale so graduated 

 that its reading at 

 once gives the rate 

 at which the plant 

 is growing at the 

 moment. When 

 balanced, the re- 

 cording apparatus 

 is extremely sensi- 

 tive, the effect of 

 any change in the environment, however slight, 

 being at once indicated by the upset of balance 

 with the up or down movement of the indicator. 

 I have in this way been able to detect induced 

 variation in the rate of growth so exceedingly 

 minute as i^c-OTnTinj-o (T<t ^^- P^i" second. An illus- 

 tration of the delicacy of the method will be 

 found in the record given in Fig. 3, on the 

 effect of carbonic acid gas on growth ; there 

 is an immediate acceleration of growth (up- 

 record), which continues for two and a half 

 minutes ; this is followed by retardation, as 

 shown by the down curve. With diluted 

 carbonic acid the acceleration may persist for a 

 considerable time. As another instance of the 

 delicacy of the method of balance, I obtained a 

 decided response of the plant to the light so fleet- 

 ing as that of a single electric spark the duration 

 of which is of the order of xTJ^tnnnr second. 



The Magnetic Crescograph. — There is a limit 

 to the magnification obtained by a compound 



