398 MacDougal. — The Mechanism of 
tissues equal to the stretching force previously acting upon 
them. The contraction is not due directly to a muscular 
contraction of the protoplasts as has been suggested by 
MacFarlane in connexion with his work upon Dionctea (XI), 
but to the release of an expansive force exerted up to the 
moment of the movement of contraction. The action of the 
stimulus in either instance doubtless results in the sudden 
release of chemical energy, due to new affinities set up, and is 
accompanied by the excretion of carbon dioxide, water, and 
other substances. The great volume of the intercellular 
spaces of the motor tissues would admit of an extension of 
a much greater quantity of water than that necessary to allow 
the ordinary amount of contraction, and the return of the 
motor cells to their normal stature would be accompanied by 
the reabsorption of the intercellular fluid. The greater density 
of the protoplasm of the concave side , the richness of its granular 
contents , the formation of aggregation-bodies , and the alterations 
in stature and volume of the parenchymatous cells of the concave 
side , lead to the conclusion that it is to the action of these cells 
that contact curvatures are due. 
Recapitulation. 
The contents of this paper may be briefly summarized as 
follows : — 
1. The power of curvature is characteristic of organs of 
such widely different morphological value and physiological 
function that it cannot be assumed a priori that the 
mechanism of reaction is similar or identical in them all. 
2. Two general methods of curvature are known to prevail 
in various organs of different species. The curvature of 
stems, petioles, and peduncles, in response to heliotropic 
and geotropic stimuli, is supposed to be due to elongation of 
the convex sides of these organs. The movements of pulvini, 
tentacular hairs of Drosera and the leaves of Dionaea , and 
similar phenomena in other plants, is known to be due to 
the action of cells on the side becoming convex. 
