ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 279 
a turning of it through 90°, the boxes with parallel walls were the 
more convenient. Boxes which were to be turned on edge or inverted 
during an experiment were provided with lids of parafhned heavy 
cardboard, perforated and held in place by wire or by means of plaster 
of Paris. 
When, in recording the results of an experiment, it was only of 
importance to know the position of the terminal portion of the root 
relative to the vertical, the angle was measured by means of a paper 
protractor pasted upon a semicircular piece of zinc plate and provided 
with a plumb and a thread so that when the straight edge of the pro- 
tractor was placed parallel with the root the thread indicated the angle 
of the root with the perpendicular. After a little practice the angle 
could be determined with an error not to exceed two degrees. When 
it was necessary to detect slight changes in the position of the root 
tip or to compare the form of the curvature of the root at intervals, a 
drawing camera which I have elsewhere described (Holman, 191 5) 
was employed. This camera had the distinct advantage that, by 
its use, drawings could be made of the roots without changing their 
position relative to gravity. The drawings were made on parchment 
paper and those made at different times could be superimposed so that 
when viewed by transmitted light the whole course of the root's curva- 
ture could be followed. 
Throughout this paper I have used the term "flattening" with 
reference to the increase in the radius of the geotropic curvature of 
a primary root in air which generally takes place after the geotropic 
curvature has reached a maximum. The word has been used in the 
same sense as Czapek, Simon and others have used the word "Aus- 
gleich." It has been found convenient to employ the term ''primary 
geotropic curvature" to designate the geotropic curvature of roots in 
air taking place before the beginning of the autotropic flattening as 
well as the geotropic curvature of any previously uncurved root 
regardless of the medium in which it executes the curvature. The 
curvature of roots subsequent to the primary geotropic curvature and 
to the flattening of the primary curvature has been designated in this 
paper as the "secondary geotropic curvature," regardless of the 
medium in which this secondary curvature takes place. 
