300 
RICHARD M. HOLMAN 
in air for several days without showing any secondary curvature does 
indicate that in the soil some agency is operative which assists the 
root in its secondary geotropic reaction. Thus if a root which has 
grown for some time in the oblique position in air is placed in the 
horizontal it bends downward until the oblique position is again 
attained. If it is placed horizontally in earth, on the other hand, the 
secondary reaction does not cease when the oblique position is reached 
but continues until the elongating zone is in the perpendicular. That 
this influence of the medium is not dependent upon the material but 
rather upon its physical properties is indicated by the fact that roots 
in air and in water behave alike in this respect and those in earth, 
moist compact sawdust and compressed sphagnum alike curve cleat 
to the perpendicular. In the case of moist sawdust or sphagnum the 
acuteness of the secondary curvature can be widely varied by com- 
pacting the medium to different degrees. When the medium is firmly 
compressed the curvature of the roots growing in it is as prompt as 
when soil is employed as a medium. When the sawdust or sphagnum 
is rendered as loose as possible the downward curvature may be 
scarcely perceptible even after twenty-four to thirty-six hours. Refer- 
ence to figure 4 will make clear the difference in the secondary curva- 
ture in air and in loose and compact sawdust. The behavior of the 
root in compact sawdust (represented at C in figure 4) differs in no 
respect from that of roots in earth. 
What is the agency which reinforces the curvature of roots in earth 
and other firm media? 
In this section it is my object to consider what reinforcing agency 
it is which is responsible for the greater acuteness of the secondary 
curvature of roots growing in earth, sand, compacted sawdust, or 
compressed sphagnum than in air and in loose sawdust or sphagnum. 
The media such as moist sawdust or sphagnum the degree of com- 
pactness of which can be greatly varied will possess in varying degree, 
according to the extent to which they are compressed, the following 
properties which can be conceived of as reinforcing the root curvature : 
first, moisture content, the amount of water in unit volume of the 
medium would be increased by compression; second, content of dis- 
solved substance in unit volume of the medium, this would likewise 
be increased by compression; third, permeability to gases, the rate of 
gas interchange would become lower as the medium was rendered 
more compact; and fourth, the resistance offered to the passage of a 
