GROWTH OF PLANTS 



145 



node starts growing, thus forming a curvature and raising the 

 part of the stem situated above it. 



It is natural to attribute the regularity in the situation of the 

 axial organs of plants to the force that always and everywhere 

 acts in a strictly vertical direction, the force of gravity. The 

 ability to assume a definite position in relation to gravity is 

 called ''geotropism." Three types of geotropism are distin- 

 guished, viz.: positive geotro- 

 pism of roots, growing down- 

 ward; negative geotropism of' 

 the stems, growing upward; 

 and diageotropism of the leaves, 

 situated horizontally. That 

 gravity is the directing force in 

 geotropism was definitely 

 proved at the beginning of the 

 nineteenth century by Knight, 

 who solved the difficult problem 

 by means of a very clever 

 device. The difficulty of find- 

 ing sufficient proof in this 

 instance was due to the fact 

 that, because of our existence 

 on the earth's surface, we are 

 unable to escape the effect of 



the force of gravitation and ^^^^ 38.— The rising of a cereal plant 



cannot therefore observe the due to renewed growth and a curvature 



1,0 1 1 .1 , ' J formed by the nodes (after Nathansohn) . 



conduct 01 a plant, that is not 

 subject to the influence of gravity. 



Not being able to remove entirely the objects of his study from 

 the effect of gravity, Knight nevertheless succeeded in discover- 

 ing a simple method of avoiding its unilateral effect. He placed 

 the seedlings of various plants on a s;nall wheel, which was 

 rotated on a horizontal axis. This wheel was kept in motion 

 by means of a stream of water obtained from a brook, while the 

 seedlings, being dipped into the water at every rotation of the 

 wheel, were prevented from drying out. With each turn of 

 the wheel, the shoots passed through all the possible positions of 

 orientation to gravity at each revolution. As a result of such 

 continuous rotation, these plants no longer had any continuous 



