42 



NA TURE 



NoVEMliER 14, 1901 



which occur when ihe plant is displaced as regards the vertical 

 and cea^e when the habitual relation is reached, all these, 

 I say, seem to me only explicable on the theory that gravita- 

 tion does not act as a mechanical influence, but as a signal 

 which the plant may neglect entirely, or, if it notices, may 

 interpret in any way ; that is, it may grow along the indicated 

 line in either direction or across it at any angle. Vou may say 

 that this is no explanation at all, that it only amounts to saying 

 that the plant can do as it chooses. I have no objection to this, 

 if you will first define the meaning of the word " choice." 



I AM now going to deal with the subject of movement from a 

 somewhat different point of view, namely, to show that it is 

 possible to discover the part of the plant which reads the signal, 

 and this is not necessarily the part that executes the correlated 

 movement. In the reflex movement of an animal (for instance, 

 a cough produced by a crumb going the wrong way), we dis- 

 tinguish the irritation of the throat and the violent action of 

 the muscles of the chest and abdomen, and further, the nervous 

 machinery by which the stimulus is reflected or switched on, by 

 way of the central nervous system, from throat to coughing 

 muscles. In the plant, too, if we are to compare its movements 

 to the reflexes of animals (as has been done by C/.apek), we 

 must distinguish a region of percipience, another of motility, and 

 the transmission of an influence from the percipient to the motor 

 region. 



Transmission of a stimulus has long been known in Mimosa, 

 but in the far more important curvatures which we are now 

 considering it was not known to exist before the publication 

 of the "Power of Movement in Plants." There is an ex- 

 periment of Rothert's ' which we do in class work at 

 Cambridge, and which only difl'ers from my father's classical 

 experiment in the fact that a much more perfectly adapted 

 plant is employed. The plant in question is a grass, 

 Selayia, which has a remarkable form of seedling. When 

 the grain germinates it does not send up a simple cylin- 

 drical sprout like an oat, but a delicate stem terminating 

 in a pointed swollen part which looks like a little spear- 

 head. When a group of Sciarias is illuminated from one side, 

 they bend strongly over, with their little spear-heads all 

 pointing .straight at the light. The spear-heads do not bend ; 

 the whole movement is carried out by the stalk on which the 

 head is supported. But the remarkable thing is that it is the 

 sptar-liead and not the stalk which perceives the light. This is 

 easily proved by covering the lieads of a few Setarias with 

 opaque caps. Kor the result is that the blindfolded seedlings 

 remain vertical while tlfeir companions are pointing to the light. 

 Thus the part which bends is unaffected by illuinination, and 

 the part which is affected does not bend. The spear-head is 

 the percipient organ, the shaft or stalk is the motor region, 

 and from head to shaft an influence has clearly been transmitted. 

 My father and I made an attempt to prove the same thing 

 for the gravitation-sense of roots, that is, to prove that the tip 

 of the root is the region in which the force of gravity is per- 

 ceived by the plant. Our method of proof does not hold good, 

 but our conclusions are true after all. When gravitation is the 

 stimulus, the experiment is much more difficult th.an when 

 light is in question, because now that fairy godmothers are 

 extinct we must not hope for a substance opaque to gravitation, 

 a substance with which we might shelter the root-tips from 

 the force of gravity as the tips of the Sctaria seedlings were' 

 sheltered from light. 



The plan adojjted by us was simply to cut off the extreme tip 

 of the roots, and fortunately (or unfortunately) the result was 

 just what was expected — the tipless roots had lost the sense of 

 gravitation and were unable to curve downwards towards the 

 centre of the earth. It was surely natural to believe that the 

 tipless roots failed to bend because their sense-organs — their 

 percipient parts — had been removed. As a matter of fact they 

 had been removed, but it was fairly objected that the operation 

 of removing the delicate tissues at the lip of the root is a severe 

 one, and that the roots which refused to grow downwards 

 were suffering from shock and not from the absence of their 

 sense-organs. 



The subsequent history of the inquiry is an in.stance of the 

 unwisdom of prophesying unless you know. In 1S94 an able 

 summary of the question was published in a C'.erm.in journal, in 

 which Ihe impossibility of solving the problem of the gravita- 



tional sensitiveness of the root. lip was dwelt on, and immediately 

 afterwards Section K of this Association had the satisfaction 

 of hearing Pfeflfcr read a brilliant paper giving the long- 

 hoped-for proof that the tip of the root is a sense-organ for 

 gravitation.' 



Like many other experiments, it depends on a deception or 

 trick played on the plant. The root is forced to grow into a 

 glass tube closed at one end and sharply bent in the middle, 

 resembling, in fact, a little glass boot. The extreme tip 

 is thus kept at right angles to the main body of the root; 

 if the theory we are testing is the right one, a root 

 with its motor region horizontal and its tip vertical ought to 

 continue to grow horizontally, because the tip being vertical 

 is not stimulated by gravity ; it is in a quiescent, or, as it were, 

 a satisfied condition, and no bending influence is being sent to 

 the motor region. And this is what Pfeffer and Czapek found. 

 Fig. 4 A, if turned through a right angle, will represent such a 

 root. On the other hand, if the main body of the root points 

 vertically down while the sensitive tip is horizontal, a curvature 

 re.sults, because as long as the tip is horizontal it is stimulated, 

 and the stimulus is transmitted to the motor region. Kig. 4 a 

 shows the tip horizontal ; li shows the curvature which brings 

 the tip into the vertical once more. 



This experiment proves not only that the tip of the root is the 

 sense-organ for gravity, but also that the motile part is not 



1 Cohns lieilrng 

 NO. 1672, VOL. 65] 



1894. 



Flo. 4.— Roots in glass boots (from Pringsheini s Jahrbuchcr). 



directly sensitive ; in other words, that gravitation is perceived 

 exclusively in the tip of the root. Since the publication of 

 Pfeffer's and Czapek's papers I have been lucky enough to hit on 

 another way of testing the theory that the tip is the percipient 

 organ for gravitation,- and I am not without hopes that botanists 

 may become in this question as fertile as Cyrano with his seven 

 ways of flying to the moon. 



There is a certain kind of inverted action familiarly known 

 as the tail wagging the dog, and it is on this principle of inver- 

 sion that my experiment is designed. Inversion may in some 

 cases be practised without altering the final result. For in- 

 stance, it does not much matter whether the thre.ad goes to the 

 needle (the rational masculine plan) or vice z'ersii, as in the 

 orthodox feminine way of threading a needle. In other cases 

 you create what is practically a new machine by inversion, as in 

 a certain apparatus in which the hand of a clock stops still 

 while the clock itself rotates. The effect is still more striking 

 with my plants, for the inversion practised on them entirely 

 changes the character of their movement. 



The result may be shown with the seedling St/arias of 



1 Pfeffer, in the AMiinh of Botany, September 1S94. Further details in 

 Czapek's paper in Pringslieim's Ja/tri., 1805. 

 - F. Darwin, Annah of liotnuy, December, 1899. 



