GEOTROPISM. I 433 



as to the more immediate course of growth. In this relation there are the 

 following possibilities to be taken into account : — 



1. Growth proceeds on one side with uniform rapidity. 



(a) This side is the concave side, but an increased growth must then 

 occur on the convex side. 



(b) This side is the convex side, but the rate of growth must be reduced 

 on the concave side. 



2. Growth alters on both sides, decreasing on the concave and increasing on 

 the convex. 



In the second alternative the decrease of growth on one side may be as 

 great as the increase on the other, and then the rate of growth in the axis 

 of the root, which is equidistant from the convex and concave sides, does not 

 alter at all ; but in the former possibility growth of the axis must always alter, 

 showing an acceleration in (a) and a retardation in (b). In order to demonstrate 

 this point clearly Sachs (1873 b) calculated the increments of growth on 

 the convex and concave sides, and also in the axis of roots which had for 

 some hours undergone geotropic curvature, and, for the sake of comparison, 

 corresponding measurements were made on a root which was allowed to grow- 

 straight. The following is a summary of the results obtained : — 



, . \ Root No. T 



Increase in 4 zones ivt 



^ ( ,, jNo. 2 



Increase in 3 zones \ 



320 18.4 25.2 30-5 



This table shows that the curvature both on the average and in each individual 

 case is due to a slight acceleration of growth on the convex side and a marked 

 retardation on the concave side ; axial growth is more restricted than in the 

 root allowed to grow normally. [According to Luxburg's (1905) measure- 

 ments Sachs's results are not to be depended on. This author holds that 

 decrease of growth does not take place in the middle line.] 



Negative geotropic curvature in a stem is illustrated at Fig. 134 (Sachs, 

 1888). The region in this example which is capable of growth is about 50 cm. 

 in length. It has been divided into five zones by indian-ink lines, the four lower 

 (5-2) being each 100 mm., the uppermost (i) only 50 mm. long. The stem was 

 laid horizontally at noon (a). After 3^ hours {b) curvature had taken place 

 in all the zones ; zone No. i had shown the greatest curvature (radius = 16 cm.), 

 the least curved was zone No. 5. After 5 J hours (c) the greatest curvature 

 was observable in zones Nos. 3 and 4, while zone No. i, which had already bent 

 beyond the vertical, had begun to straighten itself. After twenty-two hours {d) 

 zones 1-3 had become erect and the chief curvature (with 7 cm. radius) lay between 

 the bottom of 4 and the apex of 5. There are two phenomena worthy of 

 note in this experiment. In the first place, the removal of the region of most 

 vigorous curvature to the still growing base from the zone of maximum growth 

 near the apex, where it first appears, and, in the second place, the supra-curvature 

 of the apical region, occasioned notonlyby the after-effect of the geotropic stimu- 

 lus but also by the basal progression of the bending. This supra-curvature is in 

 some cases much more apparent than in the case of Cephalaria, as may be 

 seen from a glance at Fig. 135. The supra-curvature is, however, ver}- soon 

 neutralized, for a new geotropic stimulus begins to operate in the opposite 

 way, and for other reasons which we have already hinted at (p. 432), but of 

 which we shall have to speak later on. 



The final result is invariably that a definite basal curving takes place 



lOST F f 



