GROWTH AND MOVEMENT 
429 
plasts occurs, and this may or may not be apparent. It expresses itself 
by a change of position in the leaf of Biophytum (fig. 670), or of Mimosa 
because there is at the base of the leaf a cushion of cells, whose lower 
ones, on account of the stimulation, exude some of the water that kept 
them tense more readily than do the upper ones. Again, upon stimu- 
lation there may be a change in the rate or amount of some function or, 
more rarely, a change in the character of a function. Thus, the proto- 
plasm of a gland may be caused to secrete more or less rapidly than 
before, or the protoplasm in a growing cell may have its growth accel- 
erated or retarded. Further, a gland may have the character of its 
2! 
FIG. 670. Records of simultaneous mechanical (M) and electrical (E) response in 
Biophytum: the figures are seconds; dotted lines show the moment of application of a 
stimulus, and the solid lines the deflection of the leaflet or of the galvanometer needle. 
After BOSE. 
secretion profoundly altered by excitation, or a part not growing may have 
its cells set again into active division and growth. 
Sensitive plants. The fact that certain plants, having a special 
mechanism, respond to a stimulus quickly by a mechanical movement 
has given them an undeserved reputation as " sensitive plants " par ex- 
cellence; but they are not really more sensitive than others. Whether 
a plant exhibits movements or not depends on whether it Las an ap- 
propriate mechanism to permit the protoplasmic contractions to propel 
it through the water, or the changed turgor to displace an organ, or the 
changed rate of growth to cause a curvature. Movements, then, are 
favorable for a study of sensitiveness merely because they are obvious 
reactions that can often be observed without apparatus. They do not 
signify unusual sensitiveness, nor does immobility imply its lack. Every 
plant responds appropriately to a sufficient stimulus, and every plant is 
therefore a sensitive plant. 
Propagation of the excitation. The reaction specially observed is 
not usually the only one. It may be only one of a series, and curvature, 
