858 MECHANICS OF GROWTH. 



fore exhibit a tendency for the more rapidly growing side to become convex. If the 

 growth is very strongly hyponastic or epinastic, the curvature thus caused may take 

 place in spile 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 concave downwards as they unfold, while their 

 positive heliotropism tends to make them concave upwards. The result is con- 

 sequently 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 Prunus Avium, Uhnus campesln's, Corylus 

 Avellanay Picea nigra, &c.). As soon as the position resulting from these forces is 

 attained, it becomes permanent, from the mature parts becoming lignified, 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 

 sensitive to the influence of light, and the upper side to that of gravitation than 

 the reverse. But this hypothesis is superfluous if it is borne in mind that in this 

 case epinasty works concurrently with heliotropism and geotropism, and hence 

 much stronger curvatures must take place than in the normal position where the 

 former acts in opposition to the two latter forces. 



The results here described are derived from the experiments of de Vries, which have 

 been already quoted. For the following I am also indebted to him. 



(a) Leaves. If a strongly developed mid-rib is separated from a leaf in active growth, 

 it curls up concavely on the under side, showing that a tension exists between it and 

 the lamina. De Vries found this to be the case in nearly two hundred species, with 

 only a few exceptions. This curvature docs not take place equally strongly at all ages ; 

 in leaves which have but just emerged from the bud it does not occur at all; it 

 increases with age, and attains its maximum when the leaf is nearly fully grown, then 

 decreases, and altogether disappears when the leaf has reached full maturity. This 

 tendency to curve is at first apparent along the whole length of the mid-rib ; it dis- 

 appears first of all at the base, the part capable of curvature becoming constantly 



* [See note 2 on p. 854 with reference to diaheliotropism and diageotropism.] 



