MECHANICS OF GROWTH. 
773 
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 directions 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, 
Pisunij 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 MECHANICS OF GROWTH. 
Sect, i i. 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 growth of a grain 
of starch or of a chlorophyll-granule, of part of a cell-wall, of a whole cell, or of a 
multicellular organ. The common point in all these processes is that they depend 
at last on the intercalation of new micellae between those already in existence, in 
other words on intussusception, as has already been explained in the first section 
of Book III. But even in structures so simple as grains of starch or parts of 
cell-walls, we are met with insurmountable difficulties when we attempt to explain 
the mechanical process of growth in all its details ; and the present state of our 
knowledge by no means enables us to propound a connected theory of the 
growth of the entire cell or of a muldcellular organ. We are in fact at present 
able only to follow empirically the processes of growth in detail, their causes 
and results. After this we may attempt to form definite ideas of the separate 
^ Würzburger Med.-Phys. Gesellschaft, March 16, 1872: [also, Ueb. Ausschliessung der geo- 
tropischen und heliotropis ,hen Krümmungen während des Wachsens, Arb. d. bot. Inst, in Würzburg, 
II. 2, 1879. Sachs calls the apparatus used for this purpose a Clmodat.'] 
