GEOTROPISM. II 



457 



direction of the hands of the clock. When the rotating end of the shoot has 

 passed through 360° in a direction contrary to the clock-hand, the end of the 

 inked line has twisted round through 360° in a direction conformable to the 

 motion of the clock-hand ; in other words, a complete revolution in one direc- 

 tion is accompanied by a complete twisting in the other, and in the end the 

 shoot is just as it was in the beginning and no torsion of any kind is observable 

 in the vertical part of the shoot. 



This rotatory movement has for long been considered autonomous, resulting 

 from the operation of internal factors, and, as a matter of fact, we shall presently 

 become acquainted with similar movements which are autonomous. The 

 revolutions of twining plants are, however, conditioned by gravity only ; in other 

 words, they are geotropic, but here we have to deal not with increased growth 

 on the upper or under surfaces of the organ but on its flanks ; in plants which 

 twine to the left it is the right flank (seen from above) that grows more 

 vigorously than the left, and vice versa. The whole shoot, however, does not 

 exhibit such 'lateral geotropic' reaction; on the contrary, it is limited to the 

 intermediate region between the erect and the horizontal portions of the shoot, 

 for the erect region is simply negatively geotropic while the horizontal region 

 is diageotropic. As soon, therefore, as the right flank of the bent portion is 

 induced to grow more rapidly by the stimulus of gravity the horizontal region 

 begins to rotate, and, in order to avoid torsion in the basal region, it must, as we 

 have seen, twist on its axis and turn another surface towards the right flank. 

 Thus new surfaces directed to the right are successively subjected to the lateral 

 geotropic stimulus. 



In proof of the geotropic nature of these rotating movements we may advance 

 the following evidence. 



1. These movements, as Schwendener (1881) fii-st pointed out, and as 

 Baranetzky (1883) afterwards confirmed, cease to be exhibited on the klinostat, 

 the axis straightens and performs only certain irregular oscillatory movements, 

 obviously due to internal causes (autonomous nutations. Lecture XLI). If the 

 revolving movements are to be correlated with these nutations and also termed 

 autonomous (Wortmann, 1886), and if it be said that gravity merely influences 

 their direction, then all geotropic movements must be considered autonomous, 

 since such nutations are universally distributed. As a matter of fact, C. Darwin 

 (1881) attempted to show that all movements in response to stimulus were 

 modified nutations, a contention which has not been verified. 



2. If we place the tip of the revolving stem against a stick, it stops the 

 movement and a tension arises in the shoot, because the right flank proceeds to 

 elongate more rapidly than the left. If it were the fact that after a time, for 

 inherent reasons, another, e. g. the under, surface proceeded to grow more vigor- 

 ously, the apex would be lifted up, the flank lying against the stick must 

 grow more rapidly and the tension would be counteracted. But nothing of 

 the kind is to be seen ; indeed Noll (1885) draws attention to the fact that in 

 his experiments the tensions were maintained for days. 



3. If we twist a revolving shoot of Calystegia through 180° and fix it so that 

 what was the upper surface is now the lower, and so that the previously convex 

 side now faces to the left and the concave side to the right, the curve becomes 

 flatter, and finally a new curving commences in the reverse direction. The rapid 

 reaction of a surface which is suddenly brought into the proper position to 

 receive the stimulus, and the absence of any after-effect in the previously stimu- 

 lated surface, must at least awake in us the suspicion that we have to deal here 

 with other conditions than those of ordinary orthogeotropism. 



The cause and the mechanism of the revolving movement ought now to 

 be generally obvious. We need not go into the complications which arise when 

 irregularities of movement occur in the apex of the shoot, but merely note 



