3o8 
RICHARD M. HOLMAN 
be followed by an upward curvature or, if the thigmotropism of the 
root were weak, at least in a decrease in the intensity of the down- 
ward curvature. This is, however, contrary to the observed facts. 
It seems rather to be a passive depression of the downward curved 
root tip due to the resistance offered to its advance by the medium 
which reinforces the secondary curvature. The downward bent or 
asymmetrical root tip which is thrust passively forward from behind 
by the increase in length of the elongating region cannot follow a 
straight course in a firm medium because of the non-symmetrical 
application of the considerable force opposing its advance. This 
is made clearer by the accompanying diagram (figure 5). The outline 
dicular. For further explanation see the text. 
below and to the left is from a camera drawing of the tip of a root 
of Vicia faba and in the diagram the same outline has been used except 
that a portion of the upper surface has been represented as if it were 
a plane (the solid portion of the line BC.) XA represents a force 
acting at A and tending to thrust the tip forward in a direction parallel 
to the axis of the elongating region. This force results from the 
increase in length of the region of elongation. YA and Y'A represent 
