darwin and modern science-第99章
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ing about a horizontal axis by means of the instrument known as a klinostat。 Under these conditions circumnutation becomes irregular or ceases altogether。 When the same experiment is made with a plant which has twined spirally up a stick； the process of climbing is checked and the last few turns become loosened or actually untwisted。 From this it has been argued that Darwin was wrong in his description of circumnutation as an automatic change in the region of quickest growth。 When the free end of a revolving shoot points towards the north there is no doubt that the south side has been elongating more than the north； after a time it is plain from the shoot hanging over to the east that the west side of the plant has grown most； and so on。 This rhythmic change of the position of the region of greatest growth Darwin ascribes to an unknown internal regulating power。 Some modern physiologists； however； attempt to explain the revolving movement as due to a particular form of sensitiveness to gravitation which it is not necessary to discuss in detail in this place。 It is sufficient for my purpose to point out that Darwin's explanation of circumnutation is not universally accepted。 Personally I believe that circumnutation is automaticis primarily due to internal stimuli。 It is however in some way connected with gravitational sensitiveness； since the movement normally occurs round a vertical line。 It is not unnatural that； when the plant has no external stimulus by which the vertical can be recognised； the revolving movement should be upset。
Very much the same may be said of the act of twining； namely that most physiologists refuse to accept Darwin's view （above referred to） that twining is the direct result of circumnutation。 Everyone must allow that the two phenomena are in some way connected； since a plant which circumnutates clockwise； i。e。 with the sun； twines in the same direction； and vice versa。 It must also be granted that geotropism has a bearing on the problem； since all plants twine upwards； and cannot twine along a horizontal support。 But how these two factors are combined； and whether any （and if so what） other factors contribute； we cannot say。 If we give up Darwin's explanation； we must at the same time say with Pfeffer that 〃the causes of twining are。。。unknown。〃 （〃The Physiology of Plants〃； Eng。 Tr。 （Oxford； 1906）； III。 page 37。）
Let us leave this difficult question and consider some other points made out in the progress of the work on climbing plants。 One result of what he called his 〃niggling〃 （〃Life and Letters〃； III。 page 312。） work on tendrils was the discovery of the delicacy of their sense of touch； and the rapidity of their movement。 Thus in a passion…flower tendril； a bit of platinum wire weighing 1。2 mg。 produced curvature （〃Climbing Plants〃； page 171。）； as did a loop of cotton weighing 2 mg。 Pfeffer （〃Untersuchungen a。d。 Bot。 Inst。 z。 Tubingen〃； Bd。 I。 1881…85； page 506。）； however； subsequently found much greater sensitiveness： thus the tendril of Sicyos angulatus reacted to 0。00025 mg。； but this only occurred when the delicate rider of cottonwool fibre was disturbed by the wind。 The same author expanded and explained in a most interesting way the meaning of Darwin's observation that tendrils are not stimulated to movement by drops of water resting on them。 Pfeffer showed that DIRTY water containing minute particles of clay in suspension acts as a stimulus。 He also showed that gelatine acts like pure water； if a smooth glass rod is coated with a 10 per cent solution of gelatine and is then applied to a tendril； no movement occurs in spite of the fact that the gelatine is solid when cold。 Pfeffer （〃Physiology〃； Eng。 Tr。 III。 page 52。 Pfeffer has pointed out the resemblance between the contact irritability of plants and the human sense of touch。 Our skin is not sensitive to uniform pressure such as is produced when the finger is dipped into mercury （Tubingen 〃Untersuchungen〃； I。 page 504。） generalises the result in the statement that the tendril has a special form of irritability and only reacts to 〃differences of pressure or variations of pressure in contiguous。。。regions。〃 Darwin was especially interested in such cases of specialised irritability。 For instance in May； 1864； he wrote to Asa Gray （〃Life and Letters〃； III。 page 314。） describing the tendrils of Bignonia capreolata； which 〃abhor a simple stick； do not much relish rough bark； but delight in wool or moss。〃 He received； from Gray； information as to the natural habitat of the species； and finally concluded that the tendrils 〃are specially adapted to climb trees clothed with lichens； mosses； or other such productions。〃 （〃Climbing Plants〃； page 102。）
Tendrils were not the only instance discovered by Darwin of delicacy of touch in plants。 In 1860 he had already begun to observe Sundew （Drosera）； and was full of astonishment at its behaviour。 He wrote to Sir Joseph Hooker （〃Life and Letters〃； III。 page 319。）： 〃I have been working like a madman at Drosera。 Here is a fact for you which is certain as you stand where you are； though you won't believe it； that a bit of hair 1/78000 of one grain in weight placed on gland； will cause ONE of the gland…bearing hairs of Drosera to curve inwards。〃 Here again Pfeffer （Pfeffer in 〃Untersuchungen a。 d。 Bot。 Inst。 z。 Tubingen〃； I。 page 491。） has； as in so many cases； added important facts to my father's observations。 He showed that if the leaf of Drosera is entirely freed from such vibrations as would reach it if observed on an ordinary table； it does not react to small weights； so that in fact it was the vibration of the minute fragment of hair on the gland that produced movement。 We may fancifully see an adaptation to the capture of insectsto the dancing of a gnat's foot on the sensitive surface。
Darwin was fond of telling how when he demonstrated the sensitiveness of Drosera to Mr Huxley and （I think） to Sir John Burdon Sanderson； he could perceive （in spite of their courtesy） that they thought the whole thing a delusion。 And the story ended with his triumph when Mr Huxley cried out； 〃It IS moving。〃
Darwin's work on tendrils has led to some interesting investigations on the mechanisms by which plants perceive stimuli。 Thus Pfeffer （Tubingen 〃Untersuchungen〃 I。 page 524。） showed that certain epidermic cells occurring in tendrils are probably organs of touch。 In these cells the protoplasm burrows as it were into cavities in the thickness of the external cell…walls and thus comes close to the surface； being separated from an object touching the tendril merely by a very thin layer of cell… wall substance。 Haberlandt （〃Physiologische Pflanzenanatomie〃； Edition III。 Leipzig； 1904。 〃Sinnesorgane im Pflanzenreich〃； Leipzig； 1901； and other publications。） has greatly extended our knowledge of vegetable structure in relation to mechanical stimulation。 He defines a sense…organ as a contrivance by which the DEFORMATION or forcible change of form in the protoplasmon which mechanical stimulation dependsis rendered rapid and considerable in amplitude （〃Sinnesorgane〃； page 10）。 He has shown that in certain papillose and bristle…like contrivances； plants possess such sense… organs； and moreover that these contrivances show a remarkable similarity to corresponding sense…organs in animals。
Haberlandt and Nemec （〃Ber。 d。 Deutschen bot。 Gesellschaft〃； XVIII。 1900。 See F。 Darwin； Presidential Address to Section K； British Association； 1904。） published independently and simultaneously a theory of the mechanism by which plants are orientated in relation to gravitation。 And here again we find an arrangement identical in principle with that by which certain animals recognise the vertical； namely the pressure of free particles on the irritable wall of a cavity。 In the higher plants； Nemec and Haberlandt believe that special loose and freely movable starch…grains play the part of the otoliths or statoliths of the crustacea； while the protoplasm lining the cells in which they are contained corresponds to the sensitive membrane lining the otocyst of the animal。 What is of special interest in our present connection is that according to this ingenious theory （The original conception was due to Noll （〃Heterogene Induction〃； Leipzig； 1892）； but his view differed in essential points from those here given。） the sense of verticality in a plant is a form of contact…irritability。 The vertical position is distinguished from the horizontal by the fact that； in the latter case； the loose starch…grains rest on the lateral walls of the cells instead of on the terminal walls as occurs in the normal upright position。 It should be added that the statolith theory is still sub judice； personally I cannot doubt that it is in the main a satisfactory explanation of the facts。
With regard to the RAPIDITY of the reaction of tendrils； Darwin records （〃Climbing Plants〃； page 155。 Others have observed movement after about 6〃。） that a Passion…Flower tendril moved distinctly within 25 seconds of stimulation。 It was this fact； more than any other； that made him doubt the current explanation； viz。 that the movement is due to unequal growth on the two sides of the tendril。 The interesting work of Fitting （Pringsheim's 〃Jahrb。〃 XXXVIII。 1903； page 545。） has shown； however； that the primary cause is not （as Darwin supposed） contraction on the concave； but an astonishingly rapid increase in growth…rate on the convex side。
On the last page of 〃Climbing Plants〃 Darwin wrote： 〃It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement。 It should rather be said that plants acquire and display this power only when it is of some advantage to them。〃
He gradually came to realise the vividness and variety of vegetable life； and that a plant like an animal has capacities of behaving in different ways under different circumstances； in a manner that may be compared to the instinctive movements of animals。 This point of view is expressed in well… known passages in the 〃Power of Movement〃。 （〃The Power of Movement in Plants〃； 1880； pages 571…3。） 〃It is impossible not to be struck with the resemblance between the。。。movements of plants and many of the actions performed unconsciously by the lower animals。〃 And again； 〃It is hardly an exaggeration to say that the tip of the radicle。。。having the power of directing the movements of the adjoining parts； acts like the brain of one of the lower animals； the brain being seated within the anterior end of the body； receiving impressions from the sense…organs； and directing the several movements。〃
The conception of a region of perception distinct from a region of movement is perhaps the most fruitful outcome of his work on the movements of plants。 But many years before its publication； viz。 in 1861； he had made out the wonderful fact that in the Orchid Catasetum （〃Life and Letters〃； III。 page 268。） the projecting organs or antennae are sensitive to a touch； and transmit an influence 〃for more than one inch INSTANTANEOUSLY；〃 which leads to the explosion or violent ejection of the pollinia。 And as we have already seen a similar transmission of a stimulus was discovered by him in Sundew in 1860； so that in 1862 he could write to Hooker （〃
