226 
Francis Darwin. 
of being externally determined as in tropic curves. This is implied 
in Pfeffer’s definition of nastism, according to which the stimulus 
is diffuse and therefore non-directive. 
Cases occur in which the nastic and tropic characters are 
combined. Thus certain nyctinastic plants are unable to carry out 
their sleeping movements on the klinostat, i.e., without the action 
of gravity. 1 Under normal conditions, i.e., when the plant is not on 
the klinostat, the sleep movement appears to be purely nastic, since 
it seems to depend simply on the diminished illumination. It is true 
that light (apart from its direction) determines when the leaves 
shall change their position, but the direction in which the leaves 
move is determined by gravity, which is essentially directive in 
character. Here the movement is photonastic according to Pfeffer’s 
definition, and yet it is not purely nastic, since the motor mechanism 
depends on the direction of a stimulus. 
Such cases must be referred to tentatively, since more research 
is needed before they can be relied on, but they certainly seem to 
be transitions to what has been referred to as changes of tone 
(Stimmung). Thus light (apart from its direction) affects the 
angle at which secondary roots grow, but the actual change is 
geotropic. 
Here I must leave a difficult question and pass on to purely tropic 
movements. The expression “ tropic ” is strictly applicable only to 
growth curvatures and to the variation movements of pulvini. But 
as Pfeffer 2 points out, there is frequently “no sharp line of demar¬ 
cation” between tropic and tactic movements. Indeed, if we take 
for our criterion the directive character of the stimulus, there is no 
reason why the majority of tactic movements should not be called 
tropic, since free swimming organisms move in the direction of 
an external stimulus, e.g., towards, or from, a source of light. 
But if this definition of tropism is strictly adhered to, we ought no 
longer to use the terms Jiapto- or thiginotropisin for curvatures due 
to contact, since the organ, e.g., a tendril, obviously does not place 
itself either parallel or at any definite angle with the direction of 
the stimulus. The haptotropism of a tendril, however, resembles 
the ordinary tropisms in as much as the stimulus determines the 
plane of curvature. Whether or no this form of irritability can 
lead to orientation depends on circumstances. If the 9>tatolith 
theory is correct, contact-irritability may lead to the most delicate 
1 Fischer, Bot. Zcitung, 1890. 
J Physiology (Eng. Tr.), Vol. iii., p. 154. 
