554 



MOTOR SYSTEM 



This method of producing hygroscopic antagonism with the aid of 

 crossed cell-layers (" woven structure," geivebeartige Struktur) was 

 known to Kraus and Hildebrand. Steinbrinck was, however, the first 

 to point out that the mechanism of many " explosive " dry fruits 

 depends upon the same dynamical principle. Excellent illustrations 

 are provided by Papilionaceous legumes, and by the capsules of 

 Liliaceae, Eutaceae and Ericaceae (tribe Ehodoreae). The 

 dehiscence of most legumes appears to involve spiral torsion of each 

 valve, but in reality the movement consists of a simple curvature 



around an axis which is inclined 

 to the long axis of the valve. The 

 inner side of the valve is occupied 

 by a more or less extensive zone 

 of thick-walled fibrous motor-cells 

 (Fig. 225 b), which are elongated 

 in the direction of the axis of 

 curvature, that is, at an angle of 

 30-40 with the long axis of the 

 pod. The antagonistic tissue con- 

 sists of the thick-walled outer 

 epidermis (Fig. 225 e 1 ), which is 

 sometimes supplemented by a few 

 sub-epidermal layers. These an- 

 tagonistic elements are elongated 

 approximately at right angles to 

 the long axes of the motor-cells. 



The geniculate awns of many 

 Grasses and the awn-like appen- 

 dages of the mericarps of certain 

 Geraniaceae execute twisting movements of a hygroscopic character, 

 which gradually force the fruits into the ground, owing to the 

 presence of backwardly directed stiff bristles just behind their 

 boring points. The mechanism of these torsions has been studied 

 in detail by F. Darwin and by Zimmermann ; both these authors 

 conclude, that the twisting action depends upon the structure of the 

 individual elements of the fibrous tissue, which forms the bulk of the 

 hygroscopic organ in such cases. According to Zimmermann, it is 

 the more peripheral bast-fibres that are mainly responsible for the 

 movement. In these, as in many similar cells, the micellae are 

 arranged in left-handed spiral series. The more internal stereides, 

 on the contrary, do not twist actively, but serve to increase the 

 torsion of the whole organ, owing to the marked contraction which 

 they undergo on drying. The peripheral fibres may be isolated by 



Fig. 225. 



Oblique transverse section (taken parallel to the 

 axis of curvature) through one of the valves of the 

 pod of Latltyrus latifolius ; fj, outer epidermis ; 

 P, thin-walled parenchyma ; 6, thick-walled motor- 

 tissue ; e.< , inner epidermis. (ej and b are the two 

 antagonistic layers.) 



