we may use a pot of mustard seedlings, which represents in miniature a forest of vertical stems. Now suppose the flower-pot upset and left lying on its side for a few hours: the seedlings will be found to have all recovered the vertical position, and they have done so by a bend which is just as much a case of movement as the flexure of a man’s arm, though it is effected by a very different mechanism. Not everyone realises how rapid this movement is. Fig. 2 is from a diagram made in the ordinary course of class-work at Cambridge, and illustrates this point. A shoot of Valerian was placed horizontally at 2.17, and a black line painted like a silhouette on a vertical sheet of glass to record its position at 2.30; similar lines were painted at intervals, forming a record of fairly rapid movement. If greater delicacy of observation had been practised, it would have been easy to show that the plant begins to curve up within a few minutes of being placed horizontally.
It is a remarkable fact that the plant should be stimulated, or stirred up, to a definite curvature by merely placing it horizontally. The curvature tends to bring the plant into the upright position, and when the whole stem has reached the vertical, the stimulus ceases to exist. It is as though the plant were in a condition of content when vertical, and of discontent in any other position, and as though the discontent expressed itself in curvature.
But the plant does not gain the vertical by a single continuous curvature; at first it overdoes the thing (see Fig. 2), and the end of the shoot may pass beyond the vertical by 20°–30°. But this new
position, inasmuch as it is not vertical, originates a new stimulus, and the new curvature which follows brings the shoot back towards the upright position. It may again overshoot the mark, but by repeated corrections it finally attains the normal upright posture.
It is this power of correcting the line of growth whenever it deviates from the upright that enables the pine tree to grow straight upwards. And this is what I meant when I said that its habit of growth depends on regulated curvature, to which no one can refuse the name of movement.
The pine and the seedling have, in fact, a wonderful kind of sensitiveness—a sensitiveness to the force of gravity. To those accustomed to think of Mimosa as the sensitive plant par excellence my words may sound strange. But the sensitiveness of Mimosa is crude by comparison with that of the seedling. A plant with a perception of the position of the centre of the earth and a power of growing
along the line so perceived is a much greater miracle than a leaf that closes its leaflets when burnt or cut or shaken.
I shall show that certain parts of the plant have the special quality of the perception of gravitation, but we are at present ignorant of how the act of perception is effected. We know something of the machinery of hearing or vision in animals, but in plants we can only guess that when a cell is placed horizontally a resulting change of pressure on the protoplasm produces that loss of equilibrium which is translated into curvature. It is, however, probable that Němec and Haberlandt are right, and that the stimulus depends on the pressure of solid particles, e.g. starch-grains, on the protoplasm. [40]
The use of this gravitational sensitiveness is clear enough. It is to the pine tree what a plumbline is to the builder, for neither plant nor man can build high unless he builds straight. A man has a general perception of the verticalness of his body and of surrounding objects, but he does not trust to this sense in placing brick on brick to make a house. He uses a plumb-line which tells him through his eye the precise line along which he must pile his bricks. The tree has also to pile one over another the cells or chambers in which its protoplasmic body lives, and this too must be done along a vertical line; but the plant is guided by the sensitiveness to gravity of which I have spoken.