340 BOTANY PART i 



centrifugal force determined the orientation of the seedlings as gravity 

 does normally. 



In another experiment KNIGHT allowed gravit}^ and centrifugal 

 force to act simultaneously but in different directions on the seed- 

 lings. The plants were fastened on a wheel 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 developed. 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 experiment. In this experiment SACHS employed 

 a piece of apparatus termed the KLINOSTAT. 



The property of plants to take a definite position under the 

 influence of terrestrial gravity is termed GEOTROPISM. It has been 

 seen that there are not only orthotropous organs which place them- 

 selves in the direction of gravity, and grow positively geotropically 

 (downwards) or negatively geotropically (upwards), but also plagio- 

 tropous organs which take up a horizontal or oblique position. The 

 positions assumed by the lateral organs are also though as a rule 

 not exclusively determined by gravity. 



All vertically upward -growing organs, whether stems, leaves 

 (Liliiflorae), flower-stalks, parts of flowers, or roots (such as the 

 respiratory roots of Amcennia (Fig. 188), Palms, etc.), are negatively 

 geotropic. When such negatively geotropic organs are forced out of 

 their upright position, they assume it again if still capable of growth. 

 In negatively geotropic organs, growth is accelerated on the side 

 towards the earth ; on the upper side it is retarded. In consequence 

 of the unequal growth thus induced, the erection of the free-growing 

 extremity is effected. The actual course of the directive movement 

 of geotropism, as will be seen from the adjoining figure (Fig. 278), 

 does not consist merely of a simple, continuous curvature. The 

 numbers 1-16 show, diagrammatically, different stages in the geotropic 

 erection of a seedling growing in semi -darkness and placed in a 



