ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 3O9 
respectively the resistances offered by a relatively loose and a relatively 
compact medium to the advance of the root tip. These forces act 
at right angles to the surface represented by BC. The resultants of 
the forces XA, YA and XA, Y'A are respectively ZA and Z'A and 
their direction indicates a passive depression of the tip. There has 
been left out of account the friction of the root surface against the 
particles of the medium, a force which would act parallel to the surface. 
This force must be small compared with the other forces concerned. 
It would lessen somewhat the tendency to downward depression of 
the root tip but probably to only a slight extent. 
This effect of the resistant medium in altering the direction of 
the root's growth by passive displacement of the curved tip is com- 
parable to the effect observed when a stake sharpened to a chisel 
edge is driven into soil. To drive such a stake straight into hard soil 
is very difficult and indeed quite impossible if the driving force is 
applied in a direction parallel to the long axis of the stake. The 
behavior of the root during its secondary curvature in a firm medium 
can be illustrated by means of a model root, having three parts; a tip 
having the form of the curved root tip, an extensible portion repre- 
senting the region of elongation of the root and a rigid portion, repre- 
senting the part of the root in which growth has ceased. I constructed 
such a model in which the extensible portion was formed of small and 
very elastic rubber tubing. A piece of fine spring wire, which was 
not very stiff extended through a glass tube which formed the part of 
the model corresponding to the older part of the root and through the 
rubber tubing and was attached to the glass "root tip." By means 
of this wire the "elongating region" could be increased in length and 
the root tip advanced. When the whole model was "planted" in 
earth behind the glass wall of a Sachs's box and the tip was pushed 
forward by means of the wire which extended through a hole in the 
end wall of the box the "root tip" and the adjacent portion of the 
"elongating region" were depressed and the "root" bent downward. 
Just as in the case of the living root, the greater the resistance which 
the medium offered the sharper was the downward curvature of the 
root model. 
In the case of roots advancing through the soil or other compact 
media, the resistance of the medium tends to intensify the curvature 
in still another way. The tip curvature of roots growing obliquely 
downward, as shown in figure 2 (^3, A/\., and ^5) is constantly being 
