296 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



certain region of the rotated plant lies underneath for a short time, this region 

 soon comes to live above for the same period, so that gravitation acts succes- 

 sively in opposite directions upon each portion of the plant, and a tendency to 

 bend toward one side is offset by an equal tendency to bend toward the 

 opposite side. Thus no geotropic bending occurs in such an experiment. 

 Geotropic bendings are due to uneqaul growth on the two sides of the bend- 

 ing organ, and they occur only in the growing regions of stems and roots; after 

 the tissues have become mature and have ceased to grow these bendings are no 

 longer possible. Also, the more rapidly an organ is growing the more quickly 

 it bends in response to gravitation, and all conditions that retard growth also 

 retard the geotropic response. 



Fig. 139. — Pfeffer's clinostat. A, arranged for rotation of potted plant on horizontal axis; 

 B, glass moist chamber, for rotating germinating seeds, etc. 



The effect of gravitation upon the geotropically stimulated plant is to release 

 certain chemical and physical reactions, and these, in their turn, lead to the bend- 

 ing itself, but only after a certain time has elapsed. The time period extending 

 from the beginning of the application of the stimulus to the beginning of the 

 visible response is termed the reaction time, and its length varies with different 

 organs and plants, from about forty minutes to several hours. It is not neces- 

 sary, however, that the stimulus be continued throughout all of this reaction 

 period. If a plant is stimulated for a period shorter than its reaction time, as 

 by lying quietly on its side, and is then rotated on the clinostat so as to equalize 

 the lateral pull of gravity, geotropic bending finally occurs, providing the original 

 period of stimulation was of adequate length. The shortest possible time of 

 stimulation that is sufficient to bring about the later response is called the 

 minimum presentation time of the geotropic stimulus. Generally this is only 

 from two to four minutes, rarely longer, and the fact that this period is so short 



