212 PART III. PHYSIOLOGY. [ 48 



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. 126). The primary petiole is articulated to the 

 stem; each secondary petiole to the primary petiole ; and each pinna to 

 the secoridar3- 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. Tropseolum, Clematis, 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. 



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. 



1). Irritability to Variations of Temperature. Movement, like 

 the other functions (see p. 160), is affected by temperature, but 

 this influence is not stimulating but tonic : it does not induce 

 movement, but merely modifies the activity of movement. A 

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

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

 tected in various leaves : for instance, a rise of temperature causes 

 certain flowers (e.g. Tulip, Crocus) to open, and a fall of tempera- 

 ture causes them to close : similarly a fall of temperature causes 

 the leaves of such plants as the Sensitive Plant and the Wood- 

 Sorrel (Oxalis Acetosetta) to fold up, whereas a rise of temperature 

 causes them to expand (see Tigs. 125, 126). 



