-)f)0 



SENSORY SYSTEM 



cells. The structure of the base of the bristles varies considerably 

 in different cases. In the simplest type of bristle the mechanical 

 tissues extend right down to the base, the lowermost stereides being 

 dove-tailed into the parenchymatous tissue of the pulvinus. Bristles 

 of this type are mere stimulators and not independent sense-organs 

 (cf. p. 586). In other instances, the bristle is seated on a small 

 pediment composed of thin-walled elements resembling the cells of the 

 sensitive parenchyma. If the pediment is regarded as an integral 

 part of the bristle, the whole structure may be regarded as a tactile 

 sense-organ. The third and most elaborate form of bristle, finally, is 

 characterised by the presence of a well-defined basal cushion situated 



Fie. 241. 

 Basal part of a tactile bristle from the main pulvinus of Mimosa pudica. 



in the axil of the obliquely inserted bundle of stereides (Fig. 241). 

 The lower end of the cushion is encircled by a shallow but sharply 

 defined transverse furrow representing a rudimentary hinge, which 

 serves to accentuate the effects of slight upward or downward deflec- 

 tions of the bristle. The resemblance of these bristles to the tactile 

 hairs of Biophytum is very striking, the plan of construction being 

 identical in the two cases. There can be no doubt that the parenchyma 

 of the basal cushion is sensory in function, since it is merely a local 

 extension of the sensitive parenchyma of the pulvinus. The fact that 

 these various forms of bristle act as stimulators or as sense-organs, is 

 very easily demonstrated. If a bristle is slightly deflected by a gentle 

 touch with a needle or bristle, the characteristic movement of the 

 petiole instantly takes place. 



Two carnivorous plants, Aldrovandia vesiculosa and Dionaea 



