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 1 2 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 P or rum 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 



3 I 



