em and push forth all round,
keeping slightly below the horizontal; the tertiaries take it for granted
that their predecessors have done the usual thing, and that they can
satisfactorily occupy the spaces left among their elders by random
growth. The fact that the tertiary roots have no specialised
sensitiveness of gravitation shows that their unregulated growth is good
enough for the necessities of the case. For among organised beings
necessity is the mother of development, and what their brethren of second
rank have developed they too could assuredly have gained. To this point
of view I shall return, but first I should like to give a few more
instances of actions carried out in response to the signal of gravity;
and these examples shall be from stem-structures.

The ripe flower-heads of a clover (_T. subterraneum_) bury themselves in
the ground, thus effectually sowing their own seeds, and they are guided
to the ground by their unusual capacity of curving down and directing
themselves like a primary root towards the centre of the earth.

Other flower-stalks are guided by gravitation for quite different
purposes. Take, for instance, a common narcissus. In the young
condition there is a straight shaft ending in a pointed flower-bud; but
as the flower opens the stalk bends close to the top and brings the
flower-tube into a roughly horizontal position, where it shows off its
brightly coloured crown to the insects that visit it. The flowers are
guided to the right position by the gravitational sense, and they
increase or diminish the angular bend in their stalk till the right
position is attained, as shown in Fig. 3.

All these cases of plants executing certain useful curvatures, which
occur when the plant is displaced as regards the vertical, and cease when
the habitual, relation is reached, all these, I say, seem to me only
explicable on the theory that gravitation does not act as a mechanical
influence, but as a signal which the plant may neglect entirely, or, if
it notices, may interpret in any way; that is, it may grow along the
indicated line in either direction or across it at any angle. It may be
said that this is no explanation at all, that it only amounts to saying
that the plant can do as it chooses. I have no objection to this, if the
meaning of the word 'choice' be defined.

I am now going to deal with the subject of movement from a somewhat
different point of view, namely, with the object of showin