MOTILE MECHANISMS IN HIGHER PLANTS 457 



going on, it will be noted that the movement occurs chiefly 

 where the principal curvature is to be seen, near the base of the 

 petals. That there is a difference between the cells of the upper 

 and lower surface in respect of the way in which the water of 

 turgescence is held may be shown easily by cutting a thin 

 longitudinal slice through the median line of a petal and placing 

 it in water. The cells will then assume their maximum of 

 turgescence and the section will become incurved in the sense 

 required to close the flower. On transference to a 3 or 4 per 

 cent, solution of common salt they will lose water but more 

 will be withdrawn from those of the lower face than from the 

 upper (inner) and the section will straighten out, i.e. motion 

 will be in the sense of opening the flower. These observations 

 may be repeated many times with the same section, always with 

 the corresponding result. The active movement is one of closure 

 and depends on a sheet of cells situated near the outer (lower) 

 surface of the petal. This is shown by killing the tissue by 

 immersion in alcohol; whatever the position previously assumed 

 the section will straighten out if now it be transferred to water. 



There is some reason for believing that a sudden reduction 

 of internal hydrostatic pressure (maintained by the presence of 

 osmotically active substances dissolved in the vacuole fluid) is 

 the proximate cause of many other bending movements ; and 

 seeing that a common result of stimulation is to provoke an 

 increased respiratory activity, it is possible perhaps to regard the 

 reduction of the amount of soluble carbohydrates, for example, 

 as the physiological means which condition the movement. 



The result might also be produced by the collocation of 

 dissolved molecules into higher aggregates and it may be that 

 such union is really connected with the slow movements often to 

 be observed in growing and elongating organs. 



A third possibility lies in a sudden or rhythmical alteration 

 of the protoplasmic membrane itself, whereby it becomes more 

 permeable and permits of the outward passage of at any rate 

 some of the dissolved substances which are otherwise retained 

 within the cell. That such effusion may happen during the 

 rapid movements of the leaves and leaflets of the Sensitive Plant 

 (Mimosa) and of the filaments of some stamens appears to be 

 not at all improbable, especially in the light of Lepeschkin's 

 experiments, though further investigation on this point is greatly 

 to be desired. 



