darwin and modern science-第138章
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d 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…grass。 In both instances my father and I were wrong in assuming that the sensitiveness is confined to the tip； yet there is a concentration of irritability in that region and transmission of stimulus is as true for geotropism as it is for heliotropism。 Thus after nearly thirty years the controversy of the root…tip has apparently ended somewhat after the fashion of the quarrels at the 〃Rainbow〃 in 〃Silas Marner〃〃you're both right and you're both wrong。〃 But the 〃brain…function〃 of the root…tip at which eminent people laughed in early days turns out to be an important part of the truth。 （By using Piccard's method I have succeeded in showing that the gravitational sensitiveness of the cotyledon of Sorghum is certainly much greater than the sensitiveness of the hypocotylif indeed any such sensitiveness exists。 See Wiesner's 〃Festschrift〃； Vienna； 1908。）
Another observation of Darwin's has given rise to much controversy。 （〃Power of Movement〃； page 133。） If a minute piece of card is fixed obliquely to the tip of a root some influence is transmitted to the region of curvature and the root bends away from the side to which the card was attached。 It was thought at the time that this proved the root…tip to be sensitive to contact； but this is not necessarily the case。 It seems possible that the curvature is a reaction to the injury caused by the alcoholic solution of shellac with which the cards were cemented to the tip。 This agrees with the fact given in the 〃Power of Movement〃 that injuring the root…tip on one side； by cutting or burning it； induced a similar curvature。 On the other hand it was shown that curvature could be produced in roots by cementing cards； not to the naked surface of the root… tip； but to pieces of gold…beaters skin applied to the root； gold…beaters skin being by itself almost without effect。 But it must be allowed that； as regards touch； it is not clear how the addition of shellac and card can increase the degree of contact。 There is however some evidence that very close contact from a solid body； such as a curved fragment of glass； produces curvature： and this may conceivably be the explanation of the effect of gold…beaters skin covered with shellac。 But on the whole it is perhaps safer to classify the shellac experiments with the results of undoubted injury rather than with those of contact。
Another subject on which a good deal of labour was expended is the sleep of leaves； or as Darwin called it their NYCTITROPIC movement。 He showed for the first time how widely spread this phenomenon is； and attempted to give an explanation of the use to the plant of the power of sleeping。 His theory was that by becoming more or less vertical at night the leaves escape the chilling effect of radiation。 Our method of testing this view was to fix some of the leaves of a sleeping plant so that they remained horizontal at night and therefore fully exposed to radiation； while their fellows were partly protected by assuming the nocturnal position。 The experiments showed clearly that the horizontal leaves were more injured than the sleeping； i。e。 more or less vertical； ones。 It may be objected that the danger from cold is very slight in warm countries where sleeping plants abound。 But it is quite possible that a lowering of the temperature which produces no visible injury may nevertheless be hurtful by checking the nutritive processes （e。g。 translocation of carbohydrates）； which go on at night。 Stahl （〃Bot。 Zeitung〃； 1897； page 81。） however has ingeniously suggested that the exposure of the leaves to radiation is not DIRECTLY hurtful because it lowers the temperature of the leaf； but INDIRECTLY because it leads to the deposition of dew on the leaf…surface。 He gives reasons for believing that dew…covered leaves are unable to transpire efficiently； and that the absorption of mineral food…material is correspondingly checked。 Stahl's theory is in no way destructive of Darwin's； and it is possible that nyctitropic leaves are adapted to avoid the indirect as well as the direct results of cooling by radiation。
In what has been said I have attempted to give an idea of some of the discoveries brought before the world in the 〃Power of Movement〃 （In 1881 Professor Wiesner published his 〃Das Bewegungsvermogen der Pflanzen〃； a book devoted to the criticism of 〃The Power of Movement in Plants〃。 A letter to Wiesner； published in 〃Life and Letters〃； III。 page 336； shows Darwin's warm appreciation of his critic's work； and of the spirit in which it is written。） and of the subsequent history of the problems。 We must now pass on to a consideration of the central thesis of the book；the relation of circumnutation to the adaptive curvatures of plants。
Darwin's view is plainly stated on pages 3…4 of the 〃Power of Movement〃。 Speaking of circumnutation he says； 〃In this universally present movement we have the basis or groundwork for the acquirement； according to the requirements of the plant； of the most diversified movements。〃 He then points out that curvatures such as those towards the light or towards the centre of the earth can be shown to be exaggerations of circumnutation in the given directions。 He finally points out that the difficulty of conceiving how the capacities of bending in definite directions were acquired is diminished by his conception。 〃We know that there is always movement in progress； and its amplitude； or direction； or both； have only to be modified for the good of the plant in relation with internal or external stimuli。〃
It may at once be allowed that the view here given has not been accepted by physiologists。 The bare fact that circumnutation is a general property of plants （other than climbing species） is not generally rejected。 But the botanical world is no nearer to believing in the theory of reaction built on it。
If we compare the movements of plants with those of the lower animals we find a certain resemblance between the two。 According to Jennings （H。S。 Jennings； 〃The Behavior of the Lower Animals〃。 Columbia U。 Press； N。Y。 1906。） a Paramoecium constantly tends to swerve toward
