ove, and indeed, like other
biological theories, it can only be shown to be true by its explaining a
number of facts. It is interesting to try to explain the facts without
the assumption in question. If gravity does not act indirectly as a
signal it must act directly; and we must find a reason why, in the case
of the mustard seedling above referred to, the stem has grown up and the
root down. There is absolutely nothing in their structure or manner of
growth to help us to see why this difference of behaviour under identical
conditions should exist. And if, instead of placing the mustard seedling
in the dark we had grown it near the window, {41} we should have come
across another remarkable phenomenon, namely, that the stem grows
towards, the root away from, the light--and this is equally inexplicable
on a mechanical basis.

But it may be urged that it is not fair to compare a root and a stem
which are structurally unlike. Let us, therefore, keep to roots. When
the root of a bean has grown vertically down into the soil for some
distance it begins to bud forth into side roots. These are exactly like
the primary root from which they spring; there is no difference in
structure or in machinery of growth. Yet the secondary roots do not grow
vertically down, but obliquely, or in some cases horizontally. There is
one more striking fact about the roots of the bean. The secondary, like
the primary roots, give off branches, and these--the tertiaries--behave
differently from both the elder generations of roots. For instead of
directing themselves vertically or horizontally, they simply treat the
force of gravity with contempt and grow where fancy leads them. The
point on which I wish to insist is, that it is impossible to explain on
any theory of the _direct_ action of gravity why the three orders of
roots have three distinct modes of growth. They may remind us of three
generations, grandfather, father, and son, all of one blood and yet
behaving towards the universe in three distinct ways--a fact not unknown
in human society.

On the other hand, it would not be difficult to show that the behaviour
of the three orders of roots is well suited to the plant's needs, and
therefore we can understand how the power of reacting in three different
ways to the same signal has been evolved. The main root takes the
shortest course to the deeper layers of earth; the four or five ranks of
secondary roots divide the world between th