570 MOTOR SYSTEM 



paratonic movements of the main pulvinus are due to a decrease of 

 turgor (amounting to at least 2 to 5 atmospheres) in the parenchyma of 

 the sensitive abaxial half : the fall of turgor is accompanied by an 

 escape of water from the motor-cells into intercellular spaces which 

 previously contained air. The actual downward curvature of the 

 pulvinus is partly due to a contraction of the walls of the motor-cells 

 consequent upon the decrease of turgor, but is accentuated by expansion 

 of the insensitive adaxial half of the pulvinus, which was strongly 

 compressed in the unstimulated condition of the organ and also by 

 the weight of the leaf. After stimulation, it is the collapsed abaxial 

 half of the pulvinus that suffers compression. 



In conclusion, attention may be directed to a morphological 

 peculiarity, characteristic of many pulvini that execute large move- 

 ments (Oxalideae, many Leguminosae). In such cases, the concave 

 side of the pulvinus is often provided with transverse furrows (Fig. 

 231 a), which, as Schwendener acutely remarks, recall the folds of skin 

 associated with the joints of the human finger. Similar folds some- 

 times occur on the convex side, where, however, they are generally less 

 conspicuous. The presence of these furrows enables the pulvinus to 

 undergo great curvature, without exposing the motor-tissue to the risk 

 of undue compression or other serious deformation. 



