1897 ] THE CURVATURE OF ROOTS 325 
from cell division to cell enlargement, and where the walls 
exhibit the greatest extensibility. The forward edge of this 
zone lies at a distance of 2 to 2.4™™ from the forward limit of 
the punctum vegetationis in Zea. The measurements were made 
of sections of roots which had been under geotropic excitation 
for three hours and were then killed in chromic acid. During 
this time the region forward of the motor zone had doubtless 
increased in length at its usual rate, and the measurements thus 
include an increment of growth amounting to 10 to 15 per cent. 
of the total length given. This fact has been wholly disregarded 
in the determinations hitherto made of the location of the 
motor zone. The distance from the tip to the region of curva- 
ture often measures 8 to 20™™ twenty-four hours after excitation. 
The excitation sets certain forces in play in a region at a certain 
distance from the tip at the time of excitation. The apical 
region continues to elongate, and by the time the motion 
becomes visible the apex has extended its own length consider- 
ably. 
That the curvature does not extend over the entire region of 
growth according to its condition is to be seen in a comparison 
of the curvatures obtained mechanically and those resulting 
from the geotropic reactions. Sachs has urged as objection to 
the localization of the motor zone the argument that many of 
the results pointing to this conclusion have been obtained from 
abnormal conditions, the foremost of which he assumes as the 
placing of the root in such position that its tip projects above 
the horizontal. He assumes that the greatest geotropic stimu- 
lation is obtained when the tip is horizontal. This has been 
disproven by recent investigations, which have demonstrated 
that geotropic excitation increases in force as the tip approaches 
the vertical pointing upward. 
Sachs urged that the curvature obtained by roots placed in 
Such position underwent minor excitation, in accordance with 
his theory that the entire growing region is geotropically sensi- 
_tive as well as motile. The recent confirmation of Darwin’ 
theory of the localization of the irritable cells in the apex of the 
