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 Biophytnm (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 



Fig. 670. — Records of simultaneous mechanical (M) and electrical (E) response in 

 Biophytum : the figures arc 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 has 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 i-nly one of a series, and curvature. 



