CHAPTER V. 



PERIODIC MOVEMENTS OF THE MATURE PARTS 

 OF PLANTS AND MOVEMENTS DEPENDENT 

 ON IRRITATION. 



Sect. 27. — Introduction \ The greater number of the movements which are 

 brought into play during growth — as the curvatures caused by heliotropism or geo- 

 tropism or by the pressure of supports on tendrils and climbing plants — produce 

 new permanent conditions, since it is growth that is modified. It is only when the 

 action has been a very transitory one that heliotropic or geotropic curvature or that 

 of tendrils due to irritability can again be effaced by further growth. During these 

 processes the organ is advancing towards maturity; the changes which have not 

 been effaced are therefore, as it were, stereotyped. 



The case is quite different with the changes now to be described. They take 

 place in organs whose growth is completed, but whose structure allows the tissues 

 to assume different conditions which alternate under the influence of external or 

 internal causes. 



In those movements which occur during growth the tension of the tissue is 

 concerned only so far as any change in it reacts on growth and modifies it. Periodic 

 movements and those due to irritation, on the contrary, depend entirely on changes 

 in the tension of the tissues, which, in this case, are fully developed only when the 

 organ has attained maturity. These alterations of the tension of the tissues do not 

 however induce new permanent conditions, but can be effaced; every change is 

 again reversed by internal forces, and the previous condition restored so long as 

 there has been no structural injury. 



Various objections might be raised to the distinction drawn between the move- 

 ments exhibited by growing organs and those performed by organs which have 

 ceased to grow. It might be argued that motile organs begin to be irritable and to 

 perform movements whilst they are still growing. In reply to this it may be urged 

 that the motility persists after growth has ceased, and that it is only then that it is 

 fully developed, whereas the motility of the organs considered in the previous chapter 

 ceases with growth. A possible objection is that the motile organs under consider- 

 ation have not ceased to grow, as a matter of fact, at the time when they are 

 especially irritable and in periodic movement, for they are capable, when in this 



^ [Pfefifer, Physiologische Untersuchungen, 1873; id., Die periodischen Bewegungen der Blatt- 

 organe, 1875. — Darwin, Movements of Plants, 1880.] 



