ACTION OF GRAVITATION ON GROWTH IN LENGTH. 849 
still later a very gentle curvature of the basal portion of the growing region, for 
the rate of growth diminishes from the apex towards the base whereas the thickness 
increases, and the further from the apex any portion is the greater the weight which it 
has to raise when curving. In consequence of the continued action of gravitation, or in 
consequence of its persistent effect, the curvature rapidly increases, but more rapidly in 
the apical than in the other portions. 
As the result of this, the apical portion, and then the middle portion, becomes more and 
more nearly erect, and the inclination to the vertical of the shoot is less the nearer these 
portions are to the apex. For instance, a line drawn tangentially to the apical portion 
will very nearly coincide with the vertical, whereas a tangent to the centre of the middle 
portion will be inclined to it at an angle of about 45°, and a tangent to the centre of the 
basal portion deviates not more than perhaps 5-10° from the horizontal. Consequently 
the apical portion will not be affected any longer by the action of gravitation, whereas 
the middle portion will continue to curve considerably, for its growth is still tolerably 
rapid and it is in a position which is favourable for curvature : the basal portion grows 
but slowly, but its position is very favourable for curvature. In consequence of the 
continuing curvature of the middle and basal portions the now erect apical portion 
becomes bent over out of the vertical, and this is increased by the persistent effect 
of the action of gravitation. Thin very rapidly-growing stems acquire this form of cur- 
vature in from 3 to 5 hours, thicker ones in from 12 to 15 hours, and very thick ones in 
from 24 to 30 hours. 
After this condition has been attained a remarkable change of the form of the 
curvature begins. Whilst the apical portion which is erect or has curved even beyond 
the vertical is straightening itself in consequence of the more rapid growth of its 
concave side, the basal portion continues to curve slowly upward by reason of its still 
nearly horizontal position. In consequence of this the middle portion is passively 
elevated, in addition to its own active curvature, so that it comes to assume, like the 
apical portion, a position which is unfavourable to its geotropism, and like it, it begins 
to straighten itself (at least in its anterior part). Finally, the whole anterior part 
(including the apical and middle portions) stands erect, whilst the mature portion, lying 
behind the basal portion, is horizontal, the two being connected by the sharply-curved 
basal portion of the growing region. 
It appears, therefore, that the greatest curvature occurs first in the thin quickly- 
growing apical portion, then in the thicker middle portion which grows less rapidly, and 
finally in the still thicker slowly-growing basal portion. 
If, on the other hand, we consider a scape of Allium Cepa or of Allium Porrum in which 
the growth is basal, the first effect of being placed in a horizontal position is that the 
greatest curvature is exhibited by that part of the basal region which is growing most 
rapidly, the mature apical portion remaining straight and being passively elevated. The 
curvature of the basal portion takes place but slowly, for it is very thick and it has to 
support the overhanging weight of the anterior portion. In this case also the apical 
portion may be elevated beyond the vertical, since the transverse sections of the basal 
portion which lie behind the region of greatest curvature continue to curve slowly and 
the position of the whole of the scape which lies in front of them is passively altered. 
If a conical shoot with apical growth, the growth being more active toward the 
apex, be placed in such a position that the apex is directed downwards in a direction 
which deviates but little from the vertical, all the parts are at first in a position which is 
very unfavourable for geotropism, since gravitation acts upon the shoot at a very acute 
angle. The time which will elapse before the first appearance of curvature must there- 
fore be greater than when the shoot is lying horizontally. It must be borne in mind 
that, as the curvature proceeds, the parts which are affected by it come to occupy a more 
favourable position for geotropism, for they approach the horizontal more and more 
closely ; the action of gravitation will therefore increase as the curvature increases. 
The apical portion comes, at length, to occupy a horizontal position ; it commences to 
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