40 



NATURE 



[November 14, 1901 



the soulh. They also probably carried on a trade by sea. In 

 325 li.c , Pythias, llie first explorer of Kiitain known to fame, 

 was sent at the head of an expedition (roni Massilia, working 

 his way along the Atlantic coast and wintering somewhere near 

 Dover. Krom this point he sailed to the Orkneys and Scan- 

 dinavia, reluming by way of the amber coast at the mouth of 

 the Elbe. The tireek influence was also felt from the northern 

 borders of Greece through Germany. In Britain a coinage 

 which was copied from the Slater of Philip of Macedon marks 

 the close of the prehistoric Iron Age, when the (Jreek influence 

 was dominant. In Ireland, it is worthy of note, none of these 

 coins have been met with, and it is likely, therefore, that the 

 Greek influence was never felt in that island. 



From this outline it is clear that the principal artistic de- 

 velopment in Britain, in the Bronze and prehistoric Iron Ages, 

 was due to the art of the .south, and that it was derived mainly 

 from the Mediterranean civilisation, including under that term 

 Mycen.xan, .ligean, Etruscan and Dalmatian art, and that in 

 later times it was aided by intercourse with the Greek.s. 



W. Boyd Dawkins. 



THE MOVEMENTS OF PLANTS} 

 TTis sometimes asserted that the power of movement is a 

 character distinguishing animals from plants. This statement 

 arises to some extent from an obvious confusion of thought. 

 Trees are stationary, they are rooted to one spot ; but they are 

 not, therefore, motionless. We think thein so because our eyes 

 are dull — a fault curable with the help of a microscope. And 

 when we get into the land of magnification, where the little 

 looks big and the slow looks quick, we see such evidence of 

 movement that we wonder we do not hear as well as see the 

 stream of life that flows before our eyes. 



In speaking of the cells of which plants are built, Mr. Huxley 

 said that a plant is " an animal enclosed in a wooden box, and 

 Nature, like Sycorax, holds thousands of delicate Ariels imprisoned 

 in every oak." It is this delicate prisoner, the living protoplasm, 

 that we may watch pacing round its prison walls. And we may 

 see it stop as though frightened at our rough usage, and then, 

 after a hesitating twitch or two, we see it recover and once more 

 flow round the cell. Or we can see under the microscope minute 

 free-swimming plants rushing across the field of view, all one 

 way, like a flock of little green sheep that we can drive to and 

 fro with a ray of light for a sheep dog. 



But I am not going to speak to-night of microscopic matters, 

 but rather of things on a bigger scale which can be seen with 

 the naked eye. I will begin by trying to show that very obvious 

 movements are to be seen in every kitchen garden or in every 

 garret window where a scarlet runner is grown for its red flowers' 

 sake. 



If you will examine a scarlet runner, you see that the shoot 

 is not comjiletely vertical, Ijut bends over to one side. To 

 record the movements of the plant a series of photographs may 

 be taken vertically from above the plant, so that the end of 

 the shoot .shows like the hand of a watch against a sort of clock- 

 face on which Ihe points of the compass are marked. These 

 photographs will show how the shoot swings round in its 

 instinctive search for another stick to climb up. 



This well-known movement is performed by a co-ordinated 

 .series of curvatures the exact nature of which need not trouble 

 us now. Let us rather consider the less obvious power of 

 coordination which enables a plant to grow upwards in a 

 straight line. Think of a forest of pine trees, hundreds of 

 thousands of them, all growing vertically up towards Ihe 

 sky. Here is a clear case of movement, for the leading 

 shoots were once but a few inches from the ground, and 

 now they are crawling along veriical lines loo feet up in the 

 air. Itmaybe .said that this is mere increase in size, not movement 

 in the ordinary sense. But I can show you that the trees could 

 not grow in this way had Ihey not a power of curvature to 

 which the name of movement cannot be refused. 



As it is not easy to experiment on pine trees, we will 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 



1 Evening lectu 

 Association, Sepic 



deli 



ivercd at Ific Glasgow meeting of the Britisli 

 6, by Francis Darwin. F R S. 



NO. 1672, VOL. 65] 



flexure of a man's arm, though it is effected by a very dilTerent 

 mechanism. Not everyone realises how rapid (his movement 

 is. Fig. I is from a diagram made in the ordinary course of 

 class-work at Cambridge, and illustrates this point. A shoot of 

 I'alerian 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 inicrvals, form- 

 ing a record of fairly rapid movement. If grealtr delicacy of 

 observation had been practised, it would have been easy to 

 show that the plant begins 10 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 hori- 

 zontally. The curvature tends to bring the plant into the upright 

 l)osition, 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 Ihe plant does not gain the vertical by a single continuous 

 curvature ; at first it overdoes the thing (see Fig. i)and the end 

 of the shoot may pass beyond the vertical by 20°- 30°. But Ihis 

 new position, inasmuch as it is not veriical, originates a new 

 stimulus, and the new curvature which follows brings the shoo! 

 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 



Fig. I. — K Valerian stem curving geotrop'c.illy. 



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 sirange. But the sensitiveness 

 of Mimo.sa 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 hope I shall be able to prove to you that we can point to 

 certain parts of the plant which 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 some- 

 thing of the machinery of hearing or vision in animals, but 

 in plants we can only guess that when a cell is placed horizon- 

 tally a resulting change of pressure on the protopl.asm produces 

 that loss of equilibrium which is translated into curvature.' 



The use of this gravitational sensitiveness is clear enough. It 

 is to the pine tree what a plumb-line is to the builder, for 



' It is, however, prob.-iblethat Nemec and Haberlandt are right, and that 

 the stimulus depends on the pressure of solid particles, e.s; starch-grains, on 

 the protopl.i.>m. See Iheir papers in the Diiilsch Hot. Ces., 1900. 



