/,(.)2 MECHANICAL LAWS OF GROWTH. 



outwards and the apices of the stem inwards (towards the centre of rotation). If 

 the rotation takes place in a horizontal plane, gravitation acts, in addition to 

 centrifugal force, on the growing parts, and the direction of the stem and root 

 becomes oblique. But when the rotation is very rapid, it is possible to increase 

 the centrifugal force to such an extent that the axis of growth remains nearly 

 horizontal. If, on the contrary, the seedlings are fixed to a disc rotating in a ver- 

 tical plane, each side of the growing part is in turn directed for a short time 

 upwards, downwards, to the right, and to the left. The action of gravitation there- 

 fore affects all sides equally ; /. e. the growth of the organ is practically inde- 

 pendent of gravitation. Centrifugal force is therefore the only force that acts on 

 the growing parts; and the root takes an outward radial direction even when the 

 disc is not rapidly turned, the stem an inward radial direction. If however the disc 

 is made to turn very slowly in a vertical plane (round a horizontal axis), so that 

 there is in fact no centrifugal force (as by intermittent turns, one revolution in ten 

 to twenty minutes with a radius of from 5 to 10 cm.), I have shown^ that the organs 

 then grow neither in the direction of gravitation nor in that of the centrifugal force, 

 but just in those direcdons in which they had happened to be placed when fixed in 

 the vessel. Under such conditions parts which normally grow straight often curve 

 in a- plane quite independently of external forces, and this can only be due to 

 internal causes of growth which are distributed unequally round the axis of 

 growth. Thus, for example, primary roots and stems of germinating seeds (Faba, 

 Pisum, Fagopyrum, Brassica), will not lie in a straight line, but their respective axes 

 of growth will intersect at any angle up to a right angle, the anterior side of the 

 base of the stem growing more rapidly than the posterior side, and thus causing a 

 curvature. It is clear that the direction of the secondary roots which spring from 

 the primary root, as well as that of the leaves on the stem, is also, under these 

 conditions, affected only by internal causes of growth. It is only in this way that 

 we can explain the directions and forms assumed by parts of plants when unin- 

 fluenced by gravitation, centrifugal force, or heliotropic curvatures, which could not 

 occur in these experiments. 



CHAPTER IV. 

 THE MECHANICAL LAWS OF GROWTH. 



Sect. ii. Definition. The growth of crystals consists in an increase of their 

 volume by the apposition of homogeneous particles in definite directions. In plants 

 the process which we call growth is much more complicated ; and the term is 

 employed in different senses, according as we are speaking of the grov»th of a grain 

 of starch or of chlorophyll, of part of a cell-wall, of a whole cell, or of a multi- 



Wiirzburger Med.-Phys. Gcsellschaft, March 16, iS; 



