MECHANICS OF GROWTH, yy^' 



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 (Fada, 

 Fi'sum, Fagopyrum, Brassicd) 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 MECH^ICS 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 multicellular 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 



1 Wurzburger Med.-Phys. Gesellschaft, March 16, 1872: [also, Ueb. Ausschliessung der geo- 

 tropischen und heliotropis ,hen Kriimmungen wahrend des Wachsens, Arb. d. bot. Inst, in Wiirzburg, 

 II. 2, 1879. Sachs calls the apparatus used for this purpose a Clinostat.'] 



