38 MOVEMENTS OF PLANTS 



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 seedUngs 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 observa- 

 tion 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 2o°-3o°. But this new 



