LIVING MOTOR-TISSUES 561 



metrical growth on the part of the curving structure, but without the 

 assistance of specialised motor-organs or -tissues. The curvature 

 always takes place principally in that portion of the organ which is 

 undergoing longitudinal extension at the time. Such growth-curvatures 

 may be autonomous in character, in which case they are also known as 

 " nutations " ; " tropistic " growth curvatures, on the other hand (such 

 as geotropic, helio tropic, hydrotropic curvatures, etc.), are induced [and 

 directed] by external stimuli. Growth-movements of this type cannot 

 be further considered here, because they are generally executed as it 

 were incidentally by parenchymatous tissues which are possessed of 

 principal functions of a totally different kind. The third and last class 

 of physiological movements are characterised by the fact that, whether 

 due to asymmetrical growth, to turgor- variation or to some other cause, 

 they are invariably carried out with the aid of special motor-tissues, 

 which may be located in special motor-organs. Here also, the move- 

 ment almost always takes the form of a curvature, which may be 

 either autonomous or induced. 



We shall start by discussing a few cases in which the motor-tissues 

 are not restricted to definite motor-organs. This simple condition is 

 exemplified by " sensitive " stamens, styles and stigmatic lobes. The 

 characteristic feature of the mechanism is the active longitudinal con- 

 traction which the motor-tissues undergo, as the result of mechanical 

 stimulation. 275 



In Centaurea and other Cynareae the five staminal filaments of 

 each floret are bent outwards in the resting condition ; when touched, 

 they undergo a contraction amounting to between 10 and 25 per cent, 

 of their length. As a consequence, the filaments straighten themselves, 

 and the whole anther-tube is drawn forcibly downwards. The pollen, 

 which was previously contained in the anther-tube, being kept in place 

 by a brush of hairs on the style, thus becomes exposed, when the fila- 

 ments contract, in such a position that it is likely to adhere to the 

 insect visitors which, under normal conditions, apply the mechanical 

 stimulus to the filaments. The ecological significance of this particular 

 mechanism is therefore perfectly clear. Pfeffer has shown that the 

 contraction of the filaments is due to a sudden fall of turgor, induced 

 by mechanical stimulation affecting the parenchyma which intervenes 

 between the epidermis of the filament, and the central vascular bundle; 

 this parenchyma, therefore, represents the special motor-tissue in this 

 case. In the unstimulated filament, the longitudinal walls of the 

 motor-cells are fully extended by turgor-pressure ; the subsequent 

 contraction will accordingly be proportional to the amount by which 

 the turgor falls. As already stated, the net result is a very consider- 

 able decrease in the length of the filament. Since the total volume 



2n 



