82 
WILLIAM H. BROWN 
in a stretched condition. Closing movement in Mimosa appears to be 
due largely to a contraction of the cells of the concave region, while 
closing movement in Dionaea is due largely to an expansion of the 
cells of the convex half. In Dionaea there is probably also an out- 
ward passage of water from the cells of the concave region, as in 
Mimosa, but in Dionaea this water appears to pass into the cells of 
the concave region instead of into intercellular spaces. Another case 
in point is the rapid contraction movement of the stamens of the 
Cynareae, due, according to Pfeffer (1906), to passage of water from 
cells to intercellular spaces, as in Mimosa. 
Superficially, the mechanism of movement in Dionaea resembles 
more closely that exhibited by tendrils than it does the mechanism of 
the movement just mentioned. Fitting (1903) observed that curva- 
ture in tendrils is due to change in the rates of growth on the opposite 
sides of the organ. According to this writer there is here, as in 
Dionaea, a pronounced temporary acceleration of the growth of the 
convex half, while, also as in Dionaea, the average rate of growth is 
also increased. After a temporary stimulation the tendril straightens 
and at the same time ceases to elongate. The opening of Dionaea 
leaves is also due to an increased rate of growth on the concave side. 
The geotropic and heliotropic curvatures of growing organs are 
also due to unequal rates of growth on the opposite sides (Pfeffer, 
1906). In this case growth is increased on the convex side but, accord- 
ing to the measurements of Sachs, there is a retardation of the average 
rate. If, as seems likely, there is a decrease in osmotic pressure in 
the concave region of Dionaea leaves and an increase of pressure in 
the opposed region, this feature of the movement in Dionaea is similar 
to the geotropic movements of pulvini that have ceased growing. 
According to Pfeffer (1906) Milliard found, by plasmolysis, that geo- 
tropic curvature in such organs is accompanied by a decrease in osmotic 
pressure on the concave side and an increase on the convex side. 
If, on the other hand, the increase in the dimensions of the convex side 
of the leaf of Dionaea is connected with an increased extensibility of 
the cell walls, this is similar to the stretching of the convex side of 
growing organs showing geotropic and heliotropic curvatures, where 
the stretching is soon fixed by growth. 
It thus seems that the phenomena connected with the mechanism 
of movement in Dionaea leaves are, except for the rate at which move- 
ment occurs, very similar to those shown in geotropic and heliotropic 
