ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 275 
irritability of any single organ of the plant. Nevertheless, a number 
of problems in this connection are still without satisfactory solution. 
The influence of the medium upon the orientation of the terrestrial 
root is a problem which has received only slight attention from plant 
physiologists and it is to this problem that the present paper is largely 
devoted. The investigation was prompted by the following questions: 
Why do primary roots, placed horizontally or directed obliquely 
upward in air, exhibit, after a growth of one or two days, a very flat 
curvature such that the growing region forms a considerable angle 
with the perpendicular, whereas, in earth, roots similarly placed curve 
acutely and arrive at the normal perpendicular position? Why do 
these roots in air frequently fail to bend to the perpendicular but 
grow for days in a direction oblique to the direction of the stimulus 
of gravity, while roots in earth always return to the perpendicular 
when displaced therefrom? 
The problem was suggested to me by Professor Wilhelm Pfeffer 
while I was a student in the Leipzig Botanical Institute. I wish to 
express my very sincere thanks to Professor Pfeffer for his constant 
interest and encouragement in my work while I was a student in his 
institute. The greater part of the work was done in Leipzig, but 
certain experiments were made at the botanical laboratory of the 
University of California. I am particularly indebted to Dr. William 
A. Setchell, professor of botany at the University of California, for 
his kindness in securing for me an excellent centrifugal apparatus. 
Privatdocent Johannes Buder, of the Leipzig Institute, and Dr. T. H. 
Goodspeed, of the Department of Botany of the University of Cali- 
fornia, also furnished advice and assistance. 
As far as I have been able to determine, the first reference to the 
difference in behavior, relative to gravity, of roots growing in air and 
earth is to be found in Hofmeister's paper (1869, S. 35, ff.) in defense 
of his conception of the "mechanics of the penetration of the root into 
the soil" which had been so ably attacked by Frank. Hofmeister 
explained the difference in curvature of roots in air and soil as resulting 
from an earlier loosening of the cells of the root cap when the root was 
constantly wet, as he believed it to be when growing in soil, than when 
the root was growing in air, where, even if occasionally wetted, it was 
not constantly in contact with liquid water. In the latter case he 
supposed so little of the "plastic" root tip to be free from the encircling 
root cap that the root curved only very slowly while the shorter root 
