208 



Light 



taxis; orientation in the opposition direction is a negative tropism or 

 taxis. Occasionally a transverse tropism or taxis is displayed. 



Control of speed of locomotion by the intensity of the factor is 

 termed a kinesis. Gravity never changes significantly in intensity, 

 but the speed of swimming, flying, or creeping is influenced by altera- 

 tions in the strength of other factors; the common occurrence of 

 photokinesis has already been mentioned. 



The position of the main axis of the body is the primary orientation 

 of the plant or animal. This is usually determined by gravity, as is 

 seen in the upward growth (negative geotropism) of the shoot and 

 the downward growth (positive geotropism) of the root of a plant 

 seedling (Fig. 6.10). In the aquatic environment the buoyant action 



Fig. 6.10. Negative geotropism of shoots and positive geotropism of roots of four 



kernels of maize that have germinated in different positions. ( By permission from 



Botany by Sinnott, 1929, McGraw-Hill Book Co. ) 



of water often reduces the effect of gravity so that primary orienta- 

 tion is to light or to current. When the source of illumination is 

 from the side, the primary orientation of a land plant will be a 

 compromise between the influence of gravity and that of light. 



The primary orientation of motile organisms is also usually to 

 gravity, and a walking or a flying animal continually adjusts its posi- 

 tion to it. However, a fly on the ceiling or on a wall is quite uncon- 

 cerned as to which side up it is, since it is oriented primarily in rela- 

 tion to the surface upon which its feet rest. Sometimes orientation 

 is brought about passively by the resistance of the appendages as 



