50] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 221 



in those of Mutisia the inferior and lateral surfaces are irritable, 

 but not the superior. The irritability of the root to the pressure 

 of obstacles (see pp. 169, 212) is localised in the.tip. 



The foregoing examples sufficiently prove the localisation of 

 irritability to mechanical stimulation : and the question arises 

 whether or not irritability to other stimuli is also localised. It 

 has been ascertained that this is the case, in connexion with 

 heliotropism and geotropism, at least in certain plants. Thus, the 

 heliotropic irritability (i.e. sensitiveness to the directive influence 

 of light) of the cotyledons of certain Grasses, though not abso- 

 lutely confined to the tip, has been found to reside especially in 

 that part, and the same is the case with the primary shoot of 

 many dicotyledonous seedlings and with young shoots of. various 

 plants. The geotropic irritability of roots also resides in the tip, 

 and this appears to be also true of other members. 



50. Transmission of Stimuli. The most striking in- 

 stances of this are offered by motile leaves, such as those of the 

 Sensitive plant and of Drosera. If the terminal pair of leaflets of 

 a pinna of the leaf of the Sensitive Plant be irritated, not only will 

 they fold up, but each of the other pairs of leaflets of the same 

 pinna will fold up in succession ; if the stimulus is sufficiently 

 strong, its effect may extend to other pinnae causing their leaflets 

 to fold up, or to the secondary petioles causing them to converge, or 

 even to the main petiole which then sinks downward (see Fig. 126). 

 Stimulation of one leaf, if sufficiently powerful, will cause move- 

 ment in another. In the case of Drosera, stimulation of the central 

 tentacles of a leaf causes the inflexion of the marginal tentacles 

 (p. 48). 



In so far as heliotropic and geotropic irritability is localised in 

 the tips of growing members, these must also afford instances of 

 transmission of stimuli. The stimulus acts upon the irritable tip 

 and the impulse is transmitted to the region in which the curva- 

 ture takes place. 



The means by which stimuli are transmitted is a matter which 

 is still under discussion ; but it appears that the means of trans- 

 mission is not the same in all cases. Whilst in some, such as ten- 

 drils and the leaves of Drosera, the stimulus is probably transmit- 

 ted by means of the delicate protoplasmic filaments which connect 

 the protoplasm of adjacent cells (see p. 65) ; in others, for instance 

 Mimosa pudica^ the stimulus is transmitted as a disturbance of the 

 hydrostatic equilibrium of the cells : it would, in fact, appear that 



