308 BOTANY 



PART I 



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 Avheel. Thus the 

 centrifugal force determined the orientation of the seedlings as gravity 

 does normally. 



In another experiment Knight allowed gravity and centrifugal 

 force to act simultaneously but in different directions on the seedlings. 

 The plants were fastened on a horizontal disc which rotated round a 

 vertical axis. When the distance of the plants from the centre and 

 the rapidity of rotation were so adjusted that the mechanical effects 

 of the centrifugal force and of gravity were equal, the roots grew out- 

 wards and downwards at an angle of 45'^ and the stem inwards and 

 upwards at the same angle. As the rapidity of rotation increased the 

 axis of the seedlings took a position approximating more to the 

 horizontal. It results from these experiments that the plant does 

 not discriminate between gravity and centrifugal force, and that the 

 one can be replaced by the other. Both these forces have this in 

 common that they impart to bodies an acceleration of mass. 



An essential addition to the fundamental researches of Knight 

 was given much later (1874) by the experiments of Sachs. In these 

 the plants were rotated round a horizontal axis as in Knight's first 

 experiment, but the rotation was slow, taking ten to twenty minutes 

 to effect one complete rotation. This is so slow that no appreciable 

 centrifugal force is develoi)ed. Since, however, by the continual 

 rotation any one-sided influence of gravity is eliminated, the roots and 

 shoots grow indifferently in the directions which they had at the 

 beginning of the exi)eriment. In this experiment Sachs employed 

 the KLINOSTAT (p. 305), which is so arranged that its axis of rotation 

 can be horizontal as well as vertical. 



The property of plants to take a definite position under the 

 influence of terrestrial gravity is termed GEOTROrisi\i. It has been 



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