ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 277 
belief that contact stimulation of roots growing in earth assists and 
renders more acute the downward curvature. He proposed the 
following two explanations for the failure of air and water grown roots 
to reach the vertical and for their subsequent growth straight ahead 
in an oblique position: 
1. That through long-continued geo tropic stimulation the root 
becomes "tired" and the autotropism of the organ gains the upper 
hand. This would mean that a position of rest is reached by such a 
root before it has curved clear to the perpendicular, except when 
some other stimulus such as contact assists the geotropic stimulus. 
2. That during their reaction the roots undergo such a change of 
geotonus that they become plagiotropic Hke secondary roots, the 
perpendicular position of rest being now replaced by a new position 
of rest to which the root tends to return after diversion therefrom. 
It is the latter of these explanations which Nemec favors. In the 
two papers cited and in a third (1904, S. 45-51) appearing three years 
later, he reports the results of experiments in support of his theory 
concerning the change of orthotropic roots to a plagiotropic condition. 
In these papers, he also reports his observations on the changes in 
the supposed perceptive apparatus, changes which take place simul- 
taneously with the taking on of the supposed plagiotropic condition. 
More detail and critical consideration of these conclusions of 
Nemec as well as of the contributions of Sachs and Elfving will be 
reserved for the main body of this paper. 
II. Methods 
The seedlings employed in this investigation were grown from 
seeds soaked for twenty-four hours in water, rinsed thoroughly and 
planted in uniformly moistened sawdust which had been well rubbed 
between the hands and which had been placed in pots without com- 
pression. A thin layer of moist sawdust was placed over the seeds 
and they were then permitted to germinate at a temperature of from 
18° to 20° C. In some of my experiments seedlings were grown for 
many days and even weeks in moist sawdust and on that account it 
is not out of place to comment upon the quality of the sawdust used. 
It was very uniform, absorbed a large amount of water and yet packed 
very slightly even when it had stood for many days. Roots which 
had grown in this medium to as great a length as 40 cm. appeared 
normal in every way. 
