ACTION OF GRAVITATION ON GROWTH IN LENGTH. 847 



side more rapid than is the case when both these parts are in the normal erect 

 position, is answered in the affirmative not only by the direct measurement just men- 

 tioned, but also by some previous results which I obtained in another way, namely, hy 

 taking a number of similar shoots, splitting up some of them at once into strips of 

 tissue which were measured, and by splitting up and measuring the others in the same 

 way after continued growth, some in a vertical, some in a horizontal direction, finally 

 comparing these measurements with each other \ It became apparent in all cases 

 that of any two similar strips of tissue, the one belonging to the lower convex side had 

 grown more rapidly, the one belonging to the upper concave side less rapidly than 

 the corresponding strips of tissue in an erect shoot within the same time. As a con- 

 sequence of this, the difference in length between the cortex and the pith belonging 

 to the upper (concave) half of the upwardly curved shoot is increased, and that 

 between the cortex and pith of the lower (convex) half is diminished, so that the 

 upward curvature causes an increase of the tension between the tissues of the upper 

 half and a diminution of the tension between those of the lower half. This may be 

 illustrated by the following example. Twelve pieces of stem of Sida napcBa cut off 

 above and below, the leaves having been removed, each consisting of six or seven 

 internodes and 300 mm. in length, were taken : of these, four were at once split up 

 into strips of tissue, four were laid horizontally in damp sand in a box, and four were 

 placed nearly erect upon moist sand in a cylinder. The two following tables give 

 the mean measurements of the strips of tissue belonging to four pieces of stem : — 



Lengths of the strips of tissue in Millimetres. 



If a shoot which has lain for some time {}i—'2 hours) in a horizontal position and 

 has begun to show the first traces of an upward curvature be placed vertically, or be 

 moved so that the plane of curvature becomes horizontal, the commencing curvature 

 increases ; hence it appears that the action of gravitation has a persistent effect which 

 may continue as long as three hours, and may produce considerable curvature. In the 

 second of the two cases the curvature lies in a horizontal plane, and simultaneously 

 with its increase an elevation of the free apex occurs in consequence of the geotropism 

 induced by the new position. The persistent effect manifests itself even when the 

 shoot is strongly curved upwards. 



My observations afford the following information as to the form of the curvature 

 of a shoot which is assuming the erect position under the action of gravity. 



On experimental as well as on theoretical grounds it appears that the curvature 

 (with a few exceptions) is not and cannot be a segment of a circle: this is only the 



» Sachs, Arb. d. bot. Inst. Wlirzburg, 1872, Heft II. p. 194. 



