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 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 concave downwards as they unfold, while their 
positive hehotropism 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, Ulmus campestris, Corylus 
Avellana, 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 does 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.] 
