562 MOTOR SYSTEM 



of the filament diminishes to a marked extent, contraction must he 

 accompanied by an escape of cell-sap from the stimulated cells into the 

 intercellular spaces which are present in large numbers. It is not 

 known whether the fall of turgor depends upon a sudden diminution 

 in the osmotic strength of the cell-sap, caused by the external stimulus, 

 or whether the latter brings about an increase in the permeability of 

 the ectoplast towards the osmotically effective constituents of the cell. 

 Pfeffer favours the former alternative. 



In Centaurea jacea, the structure of the motor-tissue in an unstimu- 

 lated staminal filament may be described as follows : The epidermal cells 

 have thick, protruding outer walls. The space between the epidermis 

 and the slender central vascular strand is occupied by a mass of fairly 

 thick-walled parenchymatous tissue, which is traversed by numerous 

 intercellular spaces of various sizes (Fig. 229 a). A longitudinal section 

 shows that these motor-cells are somewhat elongated, and that it is 

 only their lateral walls that are thickened and provided with large 

 numbers of transversely elongated pits. From the orientation of the 

 pits, it may be inferred that the micellar series also run transversely in 

 these walls, or, in other words, that the tensile resistance of the 

 membrane is smallest in the direction of the long axis of the filament, 

 that is to say, in the direction in which the extension due to turgor- 

 pressure is greatest. The dense protoplasm of the motor-cells adheres 

 very firmly to the limiting-membrane of the pits, a fact which indicates 

 the presence of protoplasmic connecting threads at these points. The 

 regions of the walls that border upon intercellular spaces, likewise 

 bear a few shallow pits with ill-defined margins ; whether these pits have 

 anything to do with the escape of cell-sap into the intercellular spaces, 

 or not, is uncertain. The protoplasm separates readily from the thin 

 transverse walls of the motor-cells, which are smooth and unpitted. 

 Both the longitudinal and the transverse walls exhibit the reactions of 

 unmodified cellulose. As already stated, the protoplasts of the motor- 

 cells are massive ; their nuclei are of moderate size, but contain 

 unusually large nucleoli. 276 



Owing to the very considerable thickness of their longitudinal 

 walls, the motor-cells require a greater expenditure of osmotic energy 

 for their distension than is needed in the case of thin-walled cells. On 

 the other hand, the contraction which follows upon the fall of turgor 

 induced by mechanical stimulation, is correspondingly great, and a con- 

 siderable amount of resistance can be overcome during the retraction 

 of the anther-tube. As a matter of fact, the friction between the inner 

 surface of the anther-tube and the mass of pollen enclosed therein which 

 is prevented from slipping downwards with the anther-tube by the circle 

 of brush-hairs surrounding the tip of the style is very appreciable. 



