APPARATUS OF VEGETABLE PHYSIOLOGY. 253 



admit that it is under the influence of this force that the 

 root grows centrifugally, and the stem centripetally that 

 is to say, that the direction of growth of each of these 

 organs depends upon a relation existing between a purely 

 physical force and their growing cells. 



Applying these results to the ordinary downward growth 

 of roots and upward growth of stems, we are at once led 

 to the conclusion that these organs assume their respective 

 directions of growth in consequence of the action of 

 gravity upon their growing cells. 



The mode in which gravity acts upon growing cells 

 may be to some extent rendered intelligible by means of a 

 diagram, (Fig. 1). Suppose A B to be a young plant of 



Fio. 1. 



which A represents the stem and B the root, each consisting 

 of a single cell. At first the position of both is horizontal, 

 but as growth proceeds A turns upwards and B down- 

 wards. This change in direction is due, in the case of A, 

 to the fact that the lower portion of the cell-wail has grown 

 more rapidly than the upper portion, and in the case of 

 B, to the fact that the upper portion of the cell-wall has 

 grown more rapidly than the lower. 



Why the growing cells of young roots should be so 

 affected by physical forces, such as gravity and centrifugal 

 force, that the direction of their growth should follow that 

 of the action of these forces, and why the growing cells 

 of young stems should be affected in a precisely opposite 

 manner, are questions to which no satisfactory reply can 

 be given at present. 



To this behaviour of the growing cells of plants to the 

 action of gravity the name geotropism has been given 

 those parts which obey it in the direction of their growth 

 being termed positively, those which oppose it negatively, 



