CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 743 



stigma of Mimulus, Martynia, and Bignonia ; and by the style of 

 Goldfussia anisophylla. 



The most familiar case is that of Mimosa pudica, the Sensitive Plant. The 

 leaf of this plant is bipinnate, consisting of a primary petiole bearing at its free 

 end four secondary petioles, upon which the leaflets or pinnae are inserted (see 

 Fig. 472). The primary petiole is articulated to the stem ; each secondary 

 petiole to the primary petiole ; and each pinna to the secondary petiole, by a 

 pulvinus. When stimulated, the pinnae fold together forwards and upwards : 

 the secondary petioles move sideways so as to come closer together and to lie 

 almost parallel ; and the primary petiole sinks downwards ; the pulvini act as 

 hinges upon which the various parts move. 



It is only a few growing members which react perceptibly to 

 mechanical stimulation; such are tendrils, the petioles of leaf- 

 climbers (e.g. Tropaeolum, Lophospermum scandens, Maurandia, 

 Solanum jasminoides) , the stem of at least one stem-climber, 

 namely that of Cuscuta (Dodder), and roots. In these cases the 

 contact must be of relatively long duration, becoming, in fact 

 pressure. 



It is still a question whether the internodes of all stem-climbers may not be 

 in some degree sensitive to contact ; but if so, they are sensitive only to long- 

 continued contact. 



The irritability of growing members to mechanical stimulation 

 is, however, less marked than that of the mature motile members 

 mentioned above. Even in the most sensitive growing members, 

 such as tendrils, the resulting movement is comparatively slow. 

 The movement induced in these members is that they tend to 

 curve round the object with which they have come into contact. 

 The result of this is that fresh portions of the member come into 

 contact and are stimulated to curve, so that the member forms 

 coils round the object, and thus becomes firmly attached to it. In 

 the case of roots, when the growing-point is more or less injured 

 by pressure or otherwise, a curvature is induced of such a kind 

 that the injured side becomes convex, with the result that the 

 growing-point, and consequently the direction of growth, is de- 

 flected from the obstacle or other cause of injury. 



b. Irritability to Variations of Temperature. Movement, like the 

 other functions (see p. 671), is affected by temperature, but this 

 influence is not stimulating but tonic : it does not induce move- 

 ment, but merely modifies the activity of movement (see p. 760). 

 A sudden variation of temperature may, however, act as a stimulus 

 and induce a movement. This kind of irritability has been de- 



