DIV. ii PHYSIOLOGY 339 



plants. As in the case of the latter, their significance lies in the attainment of 

 favourable conditions of life. The effective stimulus, the positive and negative 

 modes of reaction, and the alternation from one to the other are completely 

 analogous to the phenomena already described in relation to tactic movements. 



1. GEOTROPISM (") 



It is a matter of experience that the trunks in a Fir wood are 

 all vertical, and therefore parallel to one another ; the branches and 

 leaves of those trees, on the other hand, take other positions. If, 

 instead of a tree, we consider a seedling, for example of the Maize, 

 \ve find that, at any rate to begin with, the organs stand in the 

 vertical line. At the same time we here observe more readily than 

 in the case of a ,tree the totally different behaviour of the roots and 

 the stem, the former growing vertically downwards and the latter 

 upwards. If we bring the seedling from its natural position and 

 lay it horizontally we find that a curvature takes place in both 

 organs ; the root curves downwards, and the shoot of the seedling 

 upwards. Since these curvatures are not effected at the region 

 where the root passes into the shoot, but in the neighbourhood of 

 the apices of the two organs, a region of variable length remains 

 horizontal, and only the two ends of the plant are brought by 

 the curvature back into their natural directions, and continue to 

 grow in them. That this vertical growth of the main root and main 

 stem is due to gravity is apparent from direct observation, which 

 shows that these organs are similarly oriented all over the globe, and 

 lie in the direction of radii of the earth. The only force acting 

 everywhere in the direction of the earth's radius that we know of is 

 gravity. Not, however, as a result of this line of thought, but from 

 the experiments of KNIGHT (1806), was this knowledge introduced 

 into our science. KNIGHT'S experiments rest on the following 

 consideration. It is evident that gravity can only cause the root to 

 grow downwards, and the stem to grow upwards, if the seed is at 

 rest and remains in the same relative position to the attractive force 

 of the earth. From this KNIGHT conjectured "that this influence 

 could be removed by the constant and rapid change of position of 

 the germinating seed, and that we should further be able to exert an 

 opposite effect by means of centrifugal force." 



He therefore fastened a number of germinating seeds in all 

 possible positions at the periphery of a wheel, so that the root on 

 emerging would grow outwards, inwards, or to the side, and he 

 caused the wheel to rotate round a horizontal axis. Since this 

 rotation was very rapid, not only was the one-sided action of gravity 

 excluded, but at the same time a considerable centrifugal force was 

 produced, which in its turn influenced the seedlings. The result of 

 the experiment was that all the roots grew radially away from, and 

 all the shoots radially towards the centre of the wheel. Thus the 



