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, by 
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 300mm. 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. 
At the commencement. After 20 hours. 
(Growing erect.) (Curved upwards.) (Standing oWiquely.) 
Concave cortex 298*0 310*5 318*8 
„ pith 308-8 337*5 341*5 
Convex pith 308*8 342-9 342*0 
„ cortex 298*0 328*2 319*6 
Differences of the lengths of Pith and Cortex. 
Of the concave side io*8 27*0 22*7 
„ convex „ 10*8 14*7 22*4 
Increments of length in 20 hours. 
Curved upwards. Standing obliquely. 
Of the concave cortex 12*5 20*8 
„ „ pith 28*7 32*7 
„ convex pith 34*1 33*2 
„ „ cortex 30*2 2 1*6 
If a shoot which has lain for some time {}4, — 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 efl'ect 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. Würzburg, 1872, Heft II. p. 194. 
