It must be clearly understood that gravity does not act directly on the growth of plants. It does
not act as a magnet acts on iron, or to take a better example, it does not simply act as gravity acts on the plumb-line in which the string is kept in a vertical line by the weight. It might be supposed that in some occult way the stem was mechanically kept straight like the string, and this indeed was the view formerly held about such roots as grow straight down into the earth. But it is not so; the thing is not explicable mechanically. Gravitation is nothing more than a sign-post or signal to the plant—a signal which the plant interprets in the way best suited to its success in the struggle for life, just as what we see or hear gives us signals of changes in the exterior world by which we regulate our conduct.
It may be said that this is hard to prove, 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 them 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.