ITLV1N1 



567 



be highly unsuitable in the case of a motor-organ, which may have to 

 undergo a large amount of curvature. 



The motor-tissues of pulyini are usually composed of approximately 

 isodiametric parenchymatous elements, and thus offer a marked con- 

 trast to the elongated or even prosenchyniatous motor-cells of tendrils 

 and irritable stamens. In the nodes of Grasses, the motor-cells are 

 remarkably short, and sometimes approximate to a tabular form. In 



Fig. 231. 



A. L.S. through the pulvinus of a pinna of Biophyturn semitivum (semi-diagram- 

 matic); o, upper, u, lower side of the pulvinus ; r, rhachis (seen in T.S.) ; -j, vascular 

 bundle traversing the mid-rib of the pinna. B. Part of a L.S. through the pulvinus, 

 more highly magnified (see text). 



Oxalis and Biophyturn the corresponding elements are also more or less 

 flattened at right angles to the long axis of the pulvinus : in 

 longitudinal sections their ends are usually seen to be more or less 

 pointed (Fig. 231b). 



The walls of pulvinar motor-cells are generally thin, or, at most, 

 slightly thickened. In Mimosa pudica and Biophyturn sensUivum, 

 where the pulvini are sensitive to shock, the upper (adaxial) and lower 

 (abaxial) halves of each pulvinus represent two antagonistic zones, and 

 the resulting movements depend upon their combined action. In the 

 adaxial half the motor-cells have fairly thick walls, whereas they are 

 cp:iite thin- walled in the abaxial half (Fig. 231 B). The reason for 

 the thickened condition of some of the cell-walls, in such cases, is 



