566 MOTOR SYSTEM 



present that would be likely to possess the requisite contractile power. 

 The epidermis is not strong enough to produce the observed contrac- 

 tion ; the vascular bundles are placed too near the centre of the 

 cross-section to say nothing of other difficulties ; the elongated sub- 

 epidermal excretory sacs, finally, which occur at intervals right across 

 the width of the motor-tissue, may also be safely left out of account. 

 It is probable that a motor-tissue of the same type is present in all 

 Sapindaceous tendrils ; whether it occurs in other families as well, is a 

 matter for further enquiry. 277 



Attention may next be directed to the numerous cases in which 

 the motor-tissue is restricted to special motor-organs in the shape 

 of pulvini. The term pulvinus is applied to all pad- or cushion- 

 like swellings of steins and petioles which are directly concerned in 

 the execution of curvatures. Most often it is the basal ends of 

 internodes (Commelynaceae, Polygonaceae, Sileneae, etc \ or of the 

 primary petioles (Leguminosae), that are differentiated as pulvini ; less 

 frequently the distal extremity of the petiole, close beneath the 

 insertion of the lamina (Makantaceae, many Aroideae), exhibits this 

 specialisation. Where the leaves are pinnate or bipinnate, pulvini 

 may be developed at the bases of the secondary petioles (sub-petioles) 

 and pinnules, as well as in connection with the main petiole (Legu- 

 minosae). Among Grasses the swollen leaf-bases act as pulvinoid 

 motor-organs. 278 



Most pulvini are cylindrical and capable of curving in all direc- 

 tions. A few, such as those on the pinnules of Mimosa pudica and other 

 Leguminosae, are more or less flattened, and accordingly only bend 

 in a single plane. As regards internal structure, cylindrical pulvini 

 may be either radially symmetrical (Grasses) or dorsi-ventral (e.g. 

 Mimosa pudica and Biopliytum sensitivum, etc.); pulvini of the latter type 

 are, of course, also physiologically dorsi-ventral. Moreover, a pulvinus 

 may be anatomically radial and yet exhibit physiological dorsi-ventrality, 

 its two halves responding in different ways to the same external 

 stimulus, in spite of their apparent identity of structure. This type of 

 curvature, which depends entirely upon physiological differentiation, is 

 responsible for most " nyctitropic " movements. 



In a strictly topographical sense, the motor-cells of a pulvinus 

 correspond to cortical tissue. They form a bulky zone, which is inter- 

 polated between the epidermis and a central vascular strand, or a much 

 contracted central cylinder. The centralisation of the vascular and 

 mechanical tissues the latter always consist of the relatively flexible 

 collenchyma is connected with the fact, that an organ cannot bend 

 freely unless the mechanically resistant tissues are situated near the 

 centre. A peripheral disposition of the stereome, in particular, would 



