684 CLIMBING PLANTS. 



entirely resembles a torsion, but which, however, as a matter of fact, is 

 connected only with a successive bending to all the points of the compass, and 

 with no actual spiral twisting whatever. This movement may also be seen on a 

 switch fixed in the ground, and, generally, in any pliant shoot, by bending down 

 the top in all directions successively, so that the point describes a circle; thus 

 it can be easily demonstrated that no spiral torsion in the tissue of the shoot 

 is caused by the successive bending on all sides. This movement has received 

 the name of circumnutation. 



We may now proceed to inquire into the series of changes within the stem 

 which cause it thus to bend in all directions, what must go on in the cells along 

 one line in this stem to cause it elongate, aloncr another to make it contract, 

 and to bring about this successive elongation and contraction in all the peripheral 

 longitudinal rows. Here unilateral pressure from outside, which so often causes 

 curvature in other cases, is shown to be just as little the reason as unilateral 

 illumination, which it is known also produces a curving of leafy stems towards 

 the incident sunlight. When we see that the young branches of beeches are 

 overhanging under their burden of leaves, we may think of explaining the 

 matter by gravity; but how can we thus explain the enigmatical advance of 

 the inclination towards all the points of the compass, which is the point at 

 issue here, and which has to be accounted for? The phenomenon has also been 

 referred to growth, and it has been said that it was caused by the various 

 longitudinal lines on the circumference of the shoot successively growing more 

 actively than the sides opposite to them. But even supposing that the whole 

 matter was only a phenomenon of growth (which is certainlj^ not the case, since 

 many shoots make these revolutions without showing the slightest increase in 

 length), the question why it happens that the stronger growth is transferred from 

 one longitudinal row to another, would still remain to be answered. 



The fii'st step in an attempt at explanation is to consider similar phenomena 

 where the conditions are much simpler and where the investigation is hindered 

 neither by simultaneous growth nor by simultaneous torsion. As such phenomena 

 we may regard the rotating movements of protoplasmic threads in swimming 

 swarm-spores, the cii'cular movements of the threads of Oscillatoriea3 composeC 

 of disc-shaped cells like rolls of coins, and the similar movements of the whip- 

 like filaments of numerous species of Dasyactis and Euactis. Here we may 

 disregard the question of the end to be attained by these movements. This 

 much is certain (1) that in the one case protoplasmic threads, and in the other 

 simple rows of cells, exhibit in their revolving movements those advancing, opposed 

 strains which we have just noted in the rotating switch; (2) that the elongation 

 on the one side and the contraction on the other in all these filamentous structures 

 are not produced by a direct external stimulus. This elongation and contraction, 

 this enigmatical advance of inclination towards all the points of the compass can 

 therefore be caused only by internal forces, and we must suppose that the living 

 protoplasm of the whip-like thread spontaneously elongates and contracts, bends 



