THE METABOLISM OF PLANTS. 3OI 



ments exhibited by plants are discontinuous : the only con- 

 tinuous movement is, apparently, the streaming movement 

 of the protoplasm. Again, most of the movements are spon- 

 taneous, but some of them, as in the case of the leaves of 

 the Sensitive Plant, of the stamens of Berberis, of Helian- 

 themum, of the Cynareae, &c., are induced by some external 

 cause. Continuous movement is the expression of a cor- 

 responding continuous evolution of energy; discontinuous 

 movement implies that it is only at certain intervals that 

 the necessary evolution of energy takes place. We have 

 already learned (page 7) that the living protoplasm of cer- 

 tain cells is automatic, that is, that it gives rise to internal 

 stimuli which find their outward expression in spontaneous 

 movement : we may now go further and say that these in- 

 ternal stimuli determine the evolution of energy which makes 

 movement possible. We have also learned that protoplasm 

 is irritable ; we can now interpret this by saying that, in the 

 case of organs exhibiting induced movements, the necessary 

 evolution of energy is determined by a stimulus acting from 

 without. 



Now with regard to the nature of the stimuli. Con- 

 cerning the internal stimuli we know nothing. Stimulation 

 from without can be effected in a variety of ways. The 

 external stimulus may be mechanical, simply the contact 

 of a foreign body, or electrical, or chemical ; a sudden change 

 from light to darkness, or a variation in the intensity of 

 the illumination, will sometimes act as a stimulus. Then 

 there is the further question, how the stimulus acts upon 

 the protoplasm. To this we can give no definite answer, 

 but since, as we have seen, movement can only take place 

 when destructive metabolism is active, it is very probable 

 that the immediate effect of stimulation is to cause de- 

 structive metabolism. We may accept Pfliiger's dictum, 

 "stimulation is decomposition"; that is to say that when 

 a stimulus, either internal or external, acts upon irritable 

 protoplasm, it determines the sudden, we may almost 

 say explosive, decomposition of some complex organic 

 substance. This view enables us to give a satisfactory 



