7^8 MECHANICAL LAWS OF GROWTH, 



A knowledge of the different capacity for growth possessed by the anterior and 

 posterior sides of bilateral organs lies at the root of an understanding of the fact that 

 leaves, lateral shoots, and many secondary roots, although they are heliotropic and 

 geotropic, yet assume definite positions with respect to the horizon, but without 

 growing vertically upwards or downwards. When multilateral primary stems and 

 roots grow vertically, the essential cause is their growth being uniform on all sides 

 of the axis of growth ; the different sides of the organ are in equilibrium with one- 

 another. Every deviation from the vertical position, to the right, left, front, or back, 

 is counterbalanced by geotropism ; the growing part continues to grow until the free 

 apex stands erect, in which position the action of gravitation is again equal on all 

 sides. In the same manner light acts equally strongly on all sides of such organs. 

 If therefore one side is exposed to stronger light, a heliotropic curvature takes place 

 which finally brings the free part into a position in which all sides receive equally 

 strong light on all sides, and therefore grow uniformly without any further curvature. 

 The case is different with bilateral organs the anterior and posterior sides of which 

 possess independently different capacities for growth, and which therefore exhibit a 

 tendency for their more rapidly growing side to become convex. If the growth is 

 very strongly hyponastic or epinastic, the curvature thus caused may take place in spite 

 of the opposing action of light and gravitation, supposing the organs to be actually 

 heliotropic or geotropic. Organs which grow horizontally or obliquely to the horizon 

 must not be assumed to be on that account wanting in heliotropism or geotropism ; 

 still less is it necessary to suppose in these cases any special or altogether abnormal 

 relations to light and gravitation. It is sufficient, as de Vries has clearly shown, to 

 suppose that light and gravitation act in the ordinary way on the growth of bilateral 

 organs, in order to explain their directions of growth, if only it is borne in mind 

 that their heliotropism and geotropism cooperate with their hyponastic and epinastic 

 properties, and thus bring about positions of the organs which must be considered 

 as the resultants of these different forces. The weight of the overhanging part must 

 however also be taken into account, its tendency being always to change the lateral 

 direction of the organ into a more horizontal or even pendulous one ; and this must 

 occur more decidedly the less the elasticity of the organ. When large leaves assume 

 oblique or horizontal positions, it is because their epinasty tends to make them con- 

 cave downwards as they unfold, while their positive heliotropism tends to make them 

 concave upwards. The result is consequently a more or less flat expansion of the 

 leaf, the position of which depends on the relation of the weight of the lamina to 

 the flexibility of the petiole and mid-rib. The same phenomena are observable 

 in horizontal or oblique lateral shoots, in which however the hyponasty of the axis 

 often counterbalances the greater mass of the pendent parts (as in Primus avium^ 

 Ubnus awipcstris, Corylus Avellana^ Picea nigra, &c.). As soon as the position re- 

 sulting from these forces is attained, it becomes permanent, from the mature parts 

 becoming Hgnified, rigid, and hard, and thus in a condition to maintain the weight 

 of the pendent parts. 



If leaves which are unfolding or still growing have their under side turned 

 upwards or towards the light, very strong curvatures take place, generally combined 

 with torsions, by which the lamina finally resumes more or less completely its 

 normal position ; and the impression is given as if the under side were more 



