August 27, 189] 



NA TURE 



415 



alternate bendings in opposite diractions, according as one or 

 other of the components is temporarily the stronger. 



iii. Wiesner allows that circumnutation does exist in some 

 cases. This last class he considers a small one ; he states, 

 indeed, that "nearly all, especially the clearly perceptible 

 circumnutations," are combined movements belonging to the 

 second of the above categories. 



Although I have perhaps no right to such an opinion without 

 repeating Wiesner's work, yet 1 must confess that I cannot give 

 up the belief that circumnutation is a widely-spread phenomenon, 

 even though it may not be so general as we supposed. 



If, then, circumnutation is of any importance, we are forced to 

 ask what is its relation to growth-curvatures. It was considered 

 ly my father lo be "the basis or groundwork for the acquire- 

 ment, according to the requirements of the plant, of the most 

 diversified movements" ("Power of Movement," p. 3). He 

 also wrote {loc. cit., p. 4) : — " A considerable difficulty in the 

 way of evolution is in part removed, for it might be asked how 

 (lid all these diversified movements .... first arise? As the 

 case stands, we know that there is always movement in progress, 

 .ind its amplitude, direction, or both, have only to be modified 

 for the good of the plant in relation to internal or external 

 stimuli." 



Those who have no belief in the importance of circumnutation, 

 and who hold that movements may have arisen without any such 

 basis, may doubtless be justified in their position. I quite agree 

 tliat movement might be developed without circumnutation 

 having anything to do with the matter. But in seeking the 

 Dfigin of growth-curvatures it is surely rational to look for a 

 widely-spread movement existing in varying degree?. This, as 

 I believe, we have in circumnutation : and here comes in what 

 >cems to me to be characteristic of the evolution of a quality 

 such as movement. In the evolution of structure, each indi- 

 vidual represents merely a single one of the units on which 

 selection acts. But an individual which executes a number of 

 movements (which may be purposeless) supplies in itself the 

 material out of which various adapted movements may arise. 

 I do not wish to imply that tentative movements are of the same 

 order of importance as variations, but they are undoubtedly of 

 importance as indication of variability. 



The problem may be taken back a stage further ; we may ask 

 \\hy circumnutation should exist. In the "Power of Move- 

 ment " (p. 546) we wrote : — " Why every part of a plant whilst 

 it is growing, and in some cases after growth has ceased, should 

 have its cells rendered more turgescent and its cell-walls more 

 extensile first on one side then on another ... is not known. 

 It would appear as if the changes in the cells required periods 

 of rest." Such periods of comparative rest are fairly harmonious 

 with any theory of growth ; it is quite conceivable by intussus- 

 ceptionists and appositionists alike that the two stages of elonga- 

 tion and fixation should go on alternately/ but this would not 

 necessarily lead to circumnutation. It might simply result in a 

 confused struggle of cells, in some of which extension, in others 

 elongation, was in the ascendant ; but such a plan would be an 

 awkward arrangement, since each cell would hinder or be 

 hin<lered by its neighbour. Perfection of growth could only be 

 attained when groups of contiguous cells agreed to work 

 together in gangs — that is, to pass through similar stages of 

 growth synchronously. Then, if the different gangs were in 

 harmony, each cell would have fair play, elongation would 

 proceed equally all round, and the result would be circumnuta- 

 tion.'- Whether or no any such origin of circumnutation as is 

 here sketched may be conceived, there can be no doubt that it 

 had its origin in the la«s of growth apart from its possible 

 utilization as a basis for growth- curvature. 



It is, however, possible to look at it from a somewhat different 

 point of view — namely, in connection with what Vochting has 

 called rectipetality ("Die Bewegung der Bliithen und Friichte," 

 1882). He made out the fact th.it when an organ has been 

 allowed to curve geotropically, heliotropically, &c., and is then 

 removed from further stimulation by being placed on the 

 klinostat, it becomes straight again. This fact .suggested to 

 Vochting his conception of rectipetality, a regulating power 

 leading to growth in a straight line. It may be objected that 



' Strasburger, " Histolog Beitrage," p. 195, speaks of the pause that 

 must occur after the formation of a cellulose lamella. Hofmeister, IVarttem- 

 burg. Jalireshefte, 1874, describes the growth in length n{ Spir jgyra a« made 

 up of short intervals uf rapid growth alternating with long pauses of slow 

 growth. 



' I purposely omit the circumnutation of pulvini. 



NO. I 139, VOL. 44] 



such a power is nothing more than the heredity, which moulds 

 the embryo into the likeness of its parent, and by a similar 

 power insists that the shoot or root shall take on the straight 

 form necessary to its specific character. But the two cases are 

 not identical. The essence of rectipetality is the power of 

 recovering from disturbance caused by external circumstances. 

 When an organ has been growing more quickly on one side than 

 another, the regulating power reverses this state of things and 

 brings the curving organ back towards the starting-point. We 

 have no means of knowing how this regulating power acts in 

 undisturbed growth. It is possible to imagine a type of irrit- 

 ability which would insure growth being absolutely straight, 

 but it is far more easy to conceive growth as normally made up 

 of slight departures from a straight line, constantly corrected. 

 In drawing a line with a pencil, or in walking towards a given 

 point, we execute an approximately straight line by a series of 

 corrections. If we may judge in such a manner by our own 

 experience, it is far more conceivable that the plant should 

 perceive the fact that it is not growing absolutely straight, and 

 correct itself, than that it should have a mysterious power of 

 growing as if its free end were guided by an external force 

 along a straight-edge. The essence of the matter is this : we 

 know from experiments that a power exists of correcting exces- 

 sive unilateral growth artificially produced ; is it not probable 

 that normal growth is similarly kept in an approximately straight 

 line by a series of aberrations and corrections? If this is so, 

 circumnutation and rectipetality would be different aspects of 

 the same thing. 



This would have one interesting corollary : if we fix our 

 attention on the regulating power instead of on the visible 

 departures from the straight line, it is clear that we can imagine 

 an irritability to internal growth-changes existing in varying 

 intensities. With great irritability very small departures from 

 the straight line would be corrected. With a lower irritability 

 the aberrations would be greater before they are corrected. In 

 one case the visible movement of circumnutation would be very 

 small, in the other case large, but the two processes would be 

 the same. The small irregular lateral curvatures which Wiesner 

 allows to exist would therefore be practically of the same value 

 as regular circumnutation, which he considers comparatively 

 rare. 



The relation between rectipetality and circumnutation may be 

 exemplified by an illustration which I have sometimes made use 

 of in lecturing on this point. A skilful bicycle-rider runs very 

 straight, the deviations from the desired course are comparatively 

 small ; whereas a beginner "wobbles" or deviates much. But 

 the deviations are of the same nature ; both are symptoms of 

 the regulating power of the rider. 



We may carry the analogy one step further : just as growth-^ 

 curvature is the continuance or exaggeration of a nutation in a 

 definite direction, so when the rider curves in his course he does 

 so by wilful exaggeration of a " wobble." 



It may be said that circumnutation is here reduced to the rank 

 of an accidental deviation from the right line. But this does 

 not seem necessarily the case. A bicycle cannot be ridden at 

 all unless it can " wobble," as every rider knows who has 

 allowed his wheel to run into a frozen rut. In the same way it 

 is possible that some degree of circumnutation is correlated with 

 growth in the manner suggested above, owing to the need of 

 regular pauses in growth. Rectipetality would thus be a power 

 by which irregularities, inherent in growth, are reduced to order 

 and made subservient to rectilinear growth. Circumnutation 

 would be the outward and visible sign of the process. 



I feel that some apology is due from me to my hearers for the 

 introduction of so much speculative matter. It may, however, 

 have one good result, for it shows how difficult is the problem 

 of growth- curvature, and how much room there still is for work 

 in this field of research. 



NOTES. 

 The German Leopold-Caroline Academy at Halle has con- 

 ferred the degree of Doctor of Philosophy on the Director of the 

 Royal Gardens, Kew, 



Messrs. Macmillan and Co. hope to publish before Christ- 

 mas a series of popular sketches in the history of astronomy 

 from the earliest times to the present day, in the form of a 



