2] GEOTROPISM 395 



than 1 to 1.5 mm. This terminal millimeter or so, then, acts 

 like a sense organ in a vertebrate, which receives the sensation 

 at some distance, it may be, from the responding organ. 



From this time on, two schools may be distinguished, of 

 which the first, following DARWIN, regarded the stimulus as 

 received at the root tip and transmitted to the region of 

 growth ; while the second conceived the stimulus to act di- 

 rectly upon the root in the bending region and to arise from 

 the difference in the pressures on the upper and lower surfaces 

 of the radicle. The second school persistently denied the 

 validity of the decapitation experiments, WIESNER ('81) in 

 particular maintaining that decapitation inhibited growth and 

 consequently growth curvatures, but did not necessarily remove 

 the sensitive organ. The first school was forced to new experi- 

 ments. FRANCIS DARWIN ('82) maintained on the basis of 

 such experiments that it is not the cutting per se which 

 inhibits geotropism ; for the cutting of the root tip, as for 

 example, lengthwise, without its removal, permits the normal 

 response. But it was easy to reply to this that a transverse cut 

 might well affect growth more seriously than a longitudinal 

 one. 



The satisfactory solution of this difficulty required a method 

 by which, without mutilating the root, gravity should act 

 horizontally upon the chief growing part of the root without so 

 acting upon the root tip. A method for accomplishing this- 

 was invented by PFEFFER ('94). A radicle of a bean or other 

 species, fixed to a klinostat, was made to grow into a small 

 glass tube, closed at its further end and bent so as to form two 

 arms, at right angles to each other, and each about 1.5 to- 

 2.0 mm. long. The preparation was now 

 turned until the root tip was directed ver- 

 tically downwards, while the rest of the 

 root, and especially its chief growing re- 



gion, was horizontal (Fig. 107). This region FIG. 107. Diagram ir- 

 was then subjected to the full transverse Crating the method 



J employed in PFEF- 



action of gravity, while the tip was not so FER'S experiment, 

 acted upon. Meanwhile the normal growth 



processes were not interfered with, for as the root lengthened 

 it backed out, so to speak, from the bent tube, the apex remain- 



