Nomenclature and Method. 
225 
distinguished as essentially directive (or Tropic), and the non-directive 
or Nastic movement. 
The nastic movements were originally two only, viz., epinasty 
and hyponasty \ The essential feature was that the character of the 
curve is morphologically determined. Thus the epinastic leaves of 
Plantago media curve away from the axis when the plant grows on a 
steep bank just as they do on a level field. And the circinate 
(hyponastic) curve of a fern-frond is equally dependent on morpho¬ 
logical factors. Thus when the terms were first used, nastic was 
practically equivalent to autogenic. At present this is not the case : 
Pfeffer defines a nastic curve as one made in response to a diffuse 
stimulus. Thus to take photogenic reactions, a plant illuminated 
all round will give a nastic effect, whereas the great class of tropic 
reactions depend on the direction in which the light strikes the plant. 
The opening and closing of flowers due to changes in illumination 
or in temperature are described respectively as photo- and thermo- 
nastic, and nyctitropism has been re-christened nyctinastism. The 
paraheliotropic movements of leaves, i.e., the movements occurring 
in bright sunshine, are often described as though the leaves directed 
their edges to the sun. But it is only necessary to look at a Phaseolus 
in sunlight to see that this is not so. Ewart (Annals of Botany, 
XI., p. 447) shows that in the Leguminosae the movement is due to 
sunlight striking the pulvinus. He illuminated the pulvinus from 
below by means of a mirror and found that leaves so treated took 
up the normal characteristic position. The stimulus is therefore not 
directive, and strictly the phenomenon should be called para- 
helionastism. 
Among nastic curves the distinction between auto- and aetiogenic 
effects has been made, and with the difficulties already referred to. 
Thus in Ornithogalum the epinasty of the leaves is induced by light 
(photonasty), in Tulip it occurs in darkness and would, in Pfeffer’s 
terminology, be called autonastism, yet it cannot be doubted that 
the two cases are continuous. 
In one sense there is in most nastic curvatures an autogenic 
element. Thus in the photonastic movements of flowers the intensity 
of illumination determines whether the flower expands or closes. 
When it opens, the surface of the petal nearest the axis grows more 
rapidly than the other and vice versa when the flower closes, that is 
to say the relative positions of the alternating parts of the motor 
mechanism are morphologically (or internally) determined, instead 
1 See De Vries, Sachs’ Arbeiten i., p. 249. 
