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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 towards the aboral side of its body owing to certain peculiarities in the set and power of its cilia。 But the tendency to swim in a circle； thus produced； is neutralised by the rotation of the creature about its longitudinal axis。 Thus the direction of the swerves IN RELATION TO THE PATH of the organism is always changing； with the result that the creature moves in what approximates to a straight line； being however actually a spiral about the general line of progress。 This method of motion is strikingly like the circumnutation of a plant； the apex of which also describes a spiral about the general line of growth。 A rooted plant obviously cannot rotate on its axis； but the regular series of curvatures of which its growth consists correspond to the aberrations of Paramoecium distributed regularly about its course by means of rotation。 （In my address to the Biological Section of the British Association at Cardiff （1891） I have attempted to show the connection between circumnutation and RECTIPETALITY； i。e。 the innate capacity of growing in a straight line。） Just as a plant changes its direction of growth by an exaggeration of one of the curvature…elements of which circumnutation consists； so does a Paramoecium change its course by the accentuation of one of the deviations of which its path is built。 Jennings has shown that the infusoria； etc。； react to stimuli by what is known as the 〃method of trial。〃 If an organism swims into a region where the temperature is too high or where an injurious substance is present； it changes its course。 It then moves forward again； and if it is fortunate enough to escape the influence； it continues to swim in the given direction。 If however its change of direction leads it further into the heated or poisonous region it repeats the movement until it emerges from its difficulties。 Jennings finds in the movements of the lower organisms an analogue with what is known as pain in conscious organisms。 There is certainly this much resemblance that a number of quite different sub…injurious agencies produce in the lower organisms a form of reaction by the help of which they； in a partly fortuitous way； escape from the threatening element in their environment。 The higher animals are stimulated in a parallel manner to vague and originally purposeless movements； one of which removes the discomfort under which they suffer； and the organism finally learns to perform the appropriate movement without going through the tentative series of actions。
I am tempted to recognise in circumnutation a similar groundwork of tentative movements out of which the adaptive ones were originally selected by a process rudely representative of learning by experience。
It is； however； simpler to confine ourselves to the assumption that those plants have survived which have acquired through unknown causes the power of reacting in appropriate ways to the external stimuli of light； gravity； etc。 It is quite possible to conceive this occurring in plants which have no power of circumnutatingand； as already pointed ou