RESPONSIVE CURVATURES — NEGATIVE GEOTROPISM 499 



and that the expansion and convexity of the lower must be 

 merely the subsidiary effect. But it is usually supposed, as 

 we have seen, that the predominatingly active factor in these 

 gravitation-curvatures is the accelerated growth of the convex 

 side. Hence the crucial experiment by which the correctness 

 of one hypothesis or the other may be determined will 

 evidently consist in demonstrating which of the two, contrac- 

 tion or expansion, is actually the essential element in the 

 responsive growth-curvature. And if, further, we should 

 succeed in proving that contraction was the essential element, 

 we should then have established a unity, as between the 

 phenomena of response to gravitation and those which are 

 the results of other forms of stimulation. But before I 

 describe this particular investigation I must explain the 

 method of continuous record, which I employ for observing 

 gravitational curvatures. 



Record of curvature induced by gravitation. — We have 

 in the Optical Lever a means by which the responsive effect 

 of gravitation-curvature and its variations may be recorded 

 quickly and continuously, and with as great a magnification 

 as is desirable. The horizontally laid specimen, say the scape 

 of Uriclis Lily, has its terminal upper end attached to one arm of 

 the Lever, the other arm being weighted with a slight counter- 

 poise. A continuous record is then taken, on a revolving 

 drum, from which we obtain the responsive curvature and its 

 time-relations. It will be seen from the record that the scape 

 first bent down during a period of forty minutes, after which 

 the effect of gravitation was seen by the reversal of the 

 curve, which indicated the curving up proper to gravitation 

 (fig. 207). 



It is sometimes thought that this preliminary lapse of 

 time before the appearance of the gravitation-effect, some- 

 times known as the presentation time, is the interval necessary 

 for the statoliths to fall ; but I shall presently describe some 

 experiments which will show that the time taken for variation 

 in response to the varying action of gravitation is very much 

 shorter, probably less than a minute. In the present case we 



K K 2 



