432 



TRANSFORMATION OF ENERGY 



course of the root be watched through a transparent sheet of mica, on which circles 

 have been scratched, it will be found that it takes the form of an arc of about 

 15 mm. radius. D shows the same root seven hours after the commence- 

 ment of the experiment, and now it will be seen that the marks i and 2 have 

 already moved past the index, and that the root as a whole has elongated more 

 than 4 mm., the individual increments measured on the convex side being : — 



A 



2 5 ^J_ 



B 



z 3 



The curvature is further sharpened ; the radius of the convex arc, which was 

 15 mm. in C., is reduced to 10 mm. in D. The curve 

 corresponds to the arc of a circle, in the formation of 

 which all the growing parts take part up to mark 5, al- 

 ,^^^ , , though apparently the zones 1 1 and III are more sharply 



rf^"^"'"^"^ bent than I, IV, and V. E illustrates the root after 



X twenty-three hours, and the curvature now exhibits 



^ T> two changes ; in the first place, it is no longer repre- 



sented by an arc of a circle, the curvature is much 

 greater between marks 2 and 3 than in the region in 

 front or behind ; in the second place the radius of 

 the curve between 2 and 3 is still further reduced, 

 viz. to about 8 mm. In stage D the apex of the root 

 lies at an angle of about 45° with the horizontal, in 

 E it is at right angles to the horizontal, and we can 

 see that the cause, but not the only cause, of the down- 

 ward direction taken by zones II and I is the bending 

 and growth of zone III (between 2 and 3). In zone II 

 curvature is still apparent, which decreases towards 

 mark i, while the bending in zone I is scarcely observ- 

 able at all. From mark 3 to the apex the form of the 

 root approaches a parabola whose apex lies somewhere 

 near 3 (Sachs, 1873 b, p. 440). 



If we now inquire why it is that at stage D the 

 growing region does not show equal curvature in all 

 zones, we shall find that the reason hes in the dif- 

 ferent intensities of growth in the separate zones and 

 also the different positions assumed by them. Zone IV, 

 after seven hours, has increased markedly less than 

 III, and V is already full grown. The capacity for 

 bending has thus ceased in zone V, while in IV it is ob- 

 viously much less than in III. Zones I and II, however, 

 which in the end grow much more rapidly than III, in 

 a very short time attain the vertical, that is, succeed 

 in reaching a position where the geotropic stimulus 

 can no longer affect them. It would appear, however, 

 that another important condition must be taken into 

 account in considering the localization of the chief 

 region of curvature in zone III, a condition the value of 

 which may be estimated with the greatest certainty in organs with longer growing 

 regions than roots have. The straightening taking place in zone I must, as 

 shown in the figure, be partly the result of elongation, for a curved organ must 

 become gradually flatter, as simple geometrical considerations show us, if it 

 grows equally both on the convex and concave sides. 



From what we have now seen we may conclude that the bending of the 

 root is limited to the growing zone, but our observations have taught us nothing 



Geotropic curvature 

 After Sachs, from 

 Smaller Practical 

 , 1903. (The index in 

 ; a little more to the 



