756 PART IV. THE PHYSIOLOGY OF PLANTS. 



the irritability of the tentacles is localised in the terminal gland. 

 In Dionsea, movement only ensues when the irritable hairs on the 

 upper surface of the leaf are touched. 



Among growing organs, tendrils offer well-marked localisation 

 of irritability. In most tendrils the lower or basal part is either 

 not at all sensitive, or is sensitive only to prolonged contact. 

 Most tendrils have their tips slightly hooked, and their irritability 

 is localised in the concavity of this curvature. The tendrils of 

 Coboea scandens and of Cissus discolor are irritable on all sides ; 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. 683, 743) 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 riot 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. 



14. Transmission of Stimuli. The most striking instances 

 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. 472). 

 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. 



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. 



