200 PLANT PHYSIOLOGY 



turn downward within an hour from the time when its posi- 

 tion was changed.* The tip is carried downward by the 

 elongation and curvature that take place in the part most 

 rapidly growing, 3-4 millimetres back of the tip. In this 

 case gravity acts as a stimulus. Gravity cannot be the sole 

 force pulling the tip into the soil, for the tip is too light, 

 and the resistance of the soil is too great, for any such 

 result. 



Darwin believed that the tip of the root, like the brain, is 

 a sense organ, receiving the stimulus of gravitation and 

 sending back to the elongating part tha impulse to respond 

 to it. Sachs and others contended that only the growing 

 part received the stimulus and acted upon it. For years the 

 matter stood thus, and Kothert,t very carefully reviewing 

 the whole subject, had just published his opinion that it 

 could not be decided experimentally when Pfeffer J announced 

 the results of the ingenious experiments conducted under his 

 direction by Czapek. In order to decide the matter it was 

 necessary to employ means which would not in any way 

 injure the root or introduce any new factors into the experi- 

 ment ; decapitation, wounding, and the other devices re- 

 sorted to before being obviously open to objections serious 

 enough to invalidate the conclusions drawn from experi- 

 ments involving such procedure. This was accomplished by 

 taking advantage of the plasticity of the growing tip, caus- 

 ing the roots to grow into tubes, 3-4 millimetres long, of 

 thin glass, bent in the middle at a right angle. While the 

 tips were growing into these tubes, the plants were revolved 

 on a clinostat, so that no directive geotropic irritation 



* For experiments on roots, seeds may best be germinated in damp 

 tannin-free sawdust. (Stone. Bot. Gazette. XIX.. 1894.) Many experi- 

 ments are described in the laboratory manuals previously named. 



t Rothert, W. Die Streitfrage iiber die Function der Wurzelspitze. Flora. 

 1894. 



t Pfeffer, W. Cber die geotropische Sensibilitat der Wurzelspitze. Sit- 

 zungsber. d. K. Sachs. Gesellsch. d. Wissensch., Sitzung vom 2ten Juli, 1894. 

 Geotropic sensitiveness of the root-tip. Annals of Botany, vol. VIII., 

 1894. 



Czapek, F. Untersuchungen iiber Geotropismus. Jahrb. f. wiss. Bot., 

 Bd. XXVII.. 1895. 



