568 MOTOR SYSTEM 



quite unknown. It seems quite clear, however, that it forms an essen- 

 tial part of the whole mechanism ; otherwise, it is impossible to account 

 for the occurrence of this feature in two genera such as Mimosa and 

 Biophytum, which are widely separated in the natural system, but 

 which have a striking physiological peculiarity in common with one 

 another, inasmuch as the pulvini of both execute rapid movements 

 when stimulated by shock. In the case of Biophytum, it is only the 

 pulvini of the pinnae that respond in this way ; the motor-tissue of 

 the petiolar pulvinus, which only executes nyctitropic movements, is 

 thin-walled throughout. Hence thick-walled motor-cells are evidently 

 confined to pulvini which carry out rapid movements in response to 

 seismic stimulation. 



Gardiner has found that the limiting membranes of the pits in the 

 piotor-cells of Mimosa pudica are traversed by delicate protoplasmic 

 connecting threads ; it is probable that this feature is characteristic of 

 pulvini in general. According to the author's own observations, all 

 the longitudinal walls in the motor-tissues of Grass- nodes (e.g. in 

 Secale cereale, Festuca gigantca, Dactylis glomerata), are provided with 

 transversely elongated pits. From this circumstance it may be concluded, 

 that the cohesion of these walls is smallest in the longitudinal direction, 

 that is, in the direction in which growth-extension takes place [in 

 response to geotropic stimulation]. The transverse walls, on the other 

 hand, bear numerous -circular pits. 



In addition to a nucleated protoplast, the motor-cells of pulvini 

 usually contain small chloroplasts, and frequently also irregular aggre- 

 gations of tannin (Mimosa pudica and other Legtjminosae). 



Schwendener has drawn attention to an interesting point with 

 regard to the development of intercellular air-spaces in pulvini ; as a 

 rule, these spaces are feebly developed, or altogether absent, in the 

 peripheral motor-tissue of pulvini that are sensitive to plastic 

 stimulation. Such pulvini accordingly often present a peculiar trans- 

 lucent or glassy appearance. Schwendener explains this peculiarity 

 on the ground that the motor-tissue must be translucent, if it is to be 

 capable of responding to variations in the intensity of illumination, 

 and that its transparency would be seriously impaired by the presence 

 of numerous air-containing intercellular spaces. The ventilating 

 spaces are larger and more numerous in the internal portions of such 

 pulvini i.e. in the neighbourhood of the central vascular strand 

 as well as in all parts of pulvini that move principally in response 

 to geotropic stimulation (Tradescantia, Gramineae). 



Comparatively few pulvini (e.g. those of the lateral pinnae of 

 Desmodium [Redysarum] gyrans) perform autonomous movements ; 

 paratonic responses are the rule in the case of these structures. 



