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n of a stimulus was discovered by him in Sundew in 1860； so that in 1862 he could write to Hooker （〃Life and Letters〃； III。 page 321。）： 〃I cannot avoid the conclusion； that Drosera possesses matter at least in some degree analogous in constitution and function to nervous matter。〃 I propose in what follows to give some account of the observations on the transmission of stimuli given in the 〃Power of Movement〃。 It is impossible within the space at my command to give anything like a complete account of the matter； and I must necessarily omit all mention of much interesting work。 One well…known experiment consisted in putting opaque caps on the tips of seedling grasses （e。g。 oat and canary…grass） and then exposing them to light from one side。 The difference； in the amount of curvature towards the light； between the blinded and unblinded specimens； was so great that it was concluded that the light…sensitiveness resided exclusively in the tip。 The experiment undoubtedly proves that the sensitiveness is much greater in the tip than elsewhere； and that there is a transmission of stimulus from the tip to the region of curvature。 But Rothert （Rothert； Cohn's 〃Beitrage〃； VII。 1894。） has conclusively proved that the basal part where the curvature occurs is also DIRECTLY sensitive to light。 He has shown； however； that in other grasses （Setaria； Panicum） the cotyledon is the only part which is sensitive； while the hypocotyl； where the movement occurs； is not directly sensitive。
It was however the question of the localisation of the gravitational sense in the tip of the seedling root or radicle that aroused most attention； and it was on this question that a controversy arose which has continued to the present day。
The experiment on which Darwin's conclusion was based consisted simply in cutting off the tip； and then comparing the behaviour of roots so treated with that of normal specimens。 An uninjured root when placed horizontally regains the vertical by means of a sharp downward curve； not so a decapitated root which continues to grow more or less horizontally。 It was argued that this depends on the loss of an organ specialised for the perception of gravity； and residing in the tip of the root； and the experiment （together with certain important variants） was claimed as evidence of the existence of such an organ。
It was at once objected that the amputation of the tip might check curvature by interfering with longitudinal growth； on the distribution of which curvature depends。 This objection was met by showing that an injury； e。g。 splitting the root longitudinally （See F。 Darwin； 〃Linnean Soc。 Journal （Bot）。〃 XIX。 1882； page 218。）； which does not remove the tip； but seriously checks growth； does not prevent geotropism。 This was of some interest in another and more general way； in showing that curvature and longitudinal growth must be placed in different categories as regards the conditions on which they depend。
Another objection of a much more serious kind was that the amputation of the tip acts as a shock。 It was shown by Rothert （See his excellent summary of the subject in 〃Flora〃 1894 （Erganzungsband）； page 199。） that the removal of a small part of the cotyledon of Setaria prevents the plant curving towards the light； and here there is no question of removing the sense…organ since the greater part of the sensitive cotyledon is intact。 In view of this result it was impossible to rely on the amputations performed on roots as above described。
At this juncture a new and brilliant method originated in Pfeffer's laboratory。 （See Pfeffer； 〃Annals of Botany〃； VIII。 1894； page 317； and Czapek； Pringsheim's 〃Jahrb。〃 XXVII。 1895； page 243。） Pfeffer and Czapek showed that it is possible to bend the root of a lupine so that； for instance； the supposed sense…organ at the tip is vertical while the motile region is horizontal。 If the motile region is directly sensitive to gravity the root ought to curve downwards； but this did not occur： on the contrary it continued to grow horizontally。 This is precisely what should happen if Darwin's theory is the right one： for if the tip is kept vertical； the sense…organ is in its normal position and receives no stimulus from gravitation； and therefore can obviously transmit none to the region of curvature。 Unfortunately this method did not convince the botanical world because some of those who repeated Czapek's experiment failed to get his results。
Czapek （〃Berichte d。 Deutsch。 bot。 Ges。〃 XV。 1897； page 516； and numerous subsequent papers。 English readers should consult Czapek in the 〃Annals of Botany〃； XIX。 1905； page 75。） has devised another interesting method which throws light on the problem。 He shows that roots； which have been placed in a horizontal position and have therefore been geotropically stimulated； can be distinguished by a chemical test from vertical； i。e。 unstimulated roots。 The chemical change in the root can be detected before any curvature has occurred and must therefore be a symptom of stimulation； not of movement。 It is particularly interesting to find that the change in the root； on which Czapek's test depends； takes place in the tip； i。e。 in the region which Darwin held to be the centre for gravitational sensitiveness。
In 1899 I devised a method （F。 Darwin； 〃Annals of Botany〃； XIII。 1899； page 567。） by which I sought to prove that the cotyledon of Setaria is not only the organ for light…perception； but also for gravitation。 If a seedling is supported horizontally by pushing the apical part （cotyledon） into a horizontal tube； the cotyledon will； according to my supposition； be stimulated gravitationally and a stimulus will be transmitted to the basal part of the stem （hypocotyl） causing it to bend。 But this curvature merely raises the basal end of the seedling； the sensitive cotyledon remains horizontal； imprisoned in its tube； it will therefore be continually stimulated and will continue to transmit influences to the bending region； which should therefore curl up into a helix or corkscrew…like form；and this is precisely what occurred。
I have referred to this work principally because the same method was applied to roots by Massart （Massart； 〃Mem。 Couronnes Acad。 R。 Belg。〃 LXII。 1902。） and myself （F。 Darwin； 〃Linnean Soc。 Journ。〃 XXXV。 1902； page 266。） with a similar though less striking result。 Although these researches confirmed Darwin's work on roots； much stress cannot be laid on them as there are several objections to them； and they are not easily repeated。
The method whichas far as we can judge at presentseems likely to solve the problem of the root…tip is most ingenious and is due to Piccard。 （Pringsheim's 〃Jahrb。〃 XL。 1904； page 94。）
Andrew Knight's celebrated experiment showed that roots react to centrifugal force precisely as they do to gravity。 So that if a bean root is fixed to a wheel revolving rapidly on a horizontal axis； it tends to curve away from the centre in the line of a radius of the wheel。 In ordinary demonstrations of Knight's experiment the seed is generally fixed so that the root is at right angles to a radius； and as far as convenient from the centre of rotation。 Piccard's experiment is arranged differently。 （A seed is depicted below a horizontal dotted line AA； projecting a root upwards。） The root is oblique to the axis of rotation； and the extreme tip projects beyond that axis。 Line AA represents the axis of rotation； T is the tip of the root just above the line AA； and B is the region just below line AA in which curvature takes place。 If the motile region B is directly sensitive to gravitation （and is the only part which is sensitive） the root will curve （down and away from the vertical） away from the axis of rotation； just as in Knight's experiment。 But if the tip T is alone sensitive to gravitation the result will be exactly reversed； the stimulus originating in T and conveyed to B will produce curvature （up towards the vertical）。 We may think of the line AA as a plane dividing two worlds。 In the lower one gravity is of the earthly type and is shown by bodies falling and roots curving downwards： in the upper world bodies fall upwards and roots curve in the same direction。 The seedling is in the lower world； but its tip containing the supposed sense…organ is in the strange world where roots curve upwards。 By observing whether the root bends up or down we can decide whether the impulse to bend originates in the tip or in the motile region。
Piccard's results showed that both curvatures occurred and he concluded that the sensitive region is not confined to the tip。 （Czapek （Pringsheim's 〃Jahrb。〃 XXXV。 1900； page 362） had previously given reasons for believing that； in the root； there is no sharp line of separation between the regions of perception and movement。）
Haberlandt （Pringsheim's 〃Jahrb。〃 XLV。 1908； page 575。） has recently repeated the experiment with the advantage of better apparatus and more experience in dealing with plants； and has found as Piccard did that both the tip and the curving region are sensitive to gravity； but with the important addition that the sensitiveness of the tip is much greater than that of the motile region。 The case is in fact similar to that of the oat and canary…g