TKANSACTIONS OF SECTION D. 677 



the klinostat, it becomes straight again. This fact suggested to Vochting his 

 conceptioD of rectipetality, a regulating power leading to growth in a straight line. 

 It may be objected that 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, tlie regulating 

 power reverses this state of things and brings the curving organ back towards 

 the starting-point. AVe have no means of knowing how this regulating power 

 acts in undisturbed growth. It is possible to imagine a type of irritability 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 matter 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 

 excessive 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 difierent 

 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 aberration 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 

 sldlful 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 acci- 

 dental deviation from a 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 tlie 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. 



