ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 3II 
illustrate this point. So little resistance was offered by the medium 
in the case of the root illustrated in the figure that the sawdust was 
pushed aside by the curved root tip and thus a channel was formed 
considerably wider than the root. (The more compact the sawdust 
the narrower this channel until in very compact sawdust the channel 
is not wider than the root and there is no free space below the older 
parts of the root such as is shown in figure 6.) Not only was the root 
only slightly depressed in the manner described above but also the 
progressive flattening of the curvature of the tip was almost complete. 
It is clear that in a medium which would considerably hinder or 
prevent the flattening of the curvature of the tip of the root the 
normal position would be reached more promptly and by a more 
acute curvature than in the case represented in the figure. 
The extent of the reinforcement of the geotropic curvature due 
to the resistance offered by the medium depends upon two factors: 
the sharpness of the tip curvature and the consistency of the medium. 
We would expect the rate of curvature to become less as the root 
approaches the vertical for as the tip approaches the normal per- 
pendicular position its curvature becomes less acute. This is fre- 
quently the case when very loose sawdust is the medium used, al- 
though the greater compactness of the medium in the lower part of 
the Sachs's box results in a greater resistance being offered to the 
advance of the root and frequently makes the difference in the rate 
of downward curvature inappreciable. It was not found possible to 
vary the resistance offered by the soil sufficiently to secure such 
flat secondary curvatures as those in loose moist sawdust or sphagnum 
except by mixing other materials with the soil. However, roots 
placed horizontally or inclined upward in fine, moderately moist 
soil so that the tip was not more than 10 to 15 mm. below the surface 
often curved very slowly downward during the first 2 to 3 cm. of 
growth. In these cases the slight weight of soil above caused the 
soil about the roots to offer little resistance to the advance of the 
root tip. When a somewhat deeper level was reached, where, owing 
to the weight of the earth above, a greater resistance was offered to 
the root's advance, the curvature became more acute and the per- 
pendicular position was soon attained. Sometimes, however, the 
curvature was so slow that the roots actually emerged from the soil 
and grew upward into the air. 
In this section we have considered a number of points of difference 
