62 



MOVEMENTS OF CURVATURE 



the plant is shaken, the main petiole sinking, the secondary petioles 

 becoming less spreading, and the leaflets folding up in pairs. If the tip 

 of a single leaflet is cut off, the stimulus first affects its pulvinus, but 

 then spreads down the leaf-segments, the leaflets folding up in pairs, and 

 then to the other segments and to the main pulvinus until the whole 

 leaf is in the stimulated condition. The leaves of Biophytum sensitivum 

 also respond readily, whereas repeated strong shaking is necessary to 

 produce a complete sinking of the leaves of Oxalis acetosella (Fig. 20). 

 The leaflets of Robinia pseudacacia are still less sensitive, and the strongest 

 shaking only produces a slight movement in the leaflets of Acacia lo- 

 phantha, although in the Tropics the sensitiveness may approach that of 

 Mimosa l . 



The power of response varies much among the stamens of different 



- ^ 



FIG. 20. Trifoliate leaf of Oxalis acetosella. A^ unstimulated ; 

 B, after repeated strong shaking. The pulvini are shown 



FIG. 21. Flower of Centanrea jacea after 

 the removal of the corolla. The stamens are 

 shown at A in the unstimulated, at B in the 

 stimulated condition (magnified). c= corolla 

 tube ; J = filaments ; a = anther tube ; ,f=stigma. 



Cynareae, those of Centanrea jacca and Cynara scolymus suddenly drawing 

 together when stimulated by contact and at the same time becoming 

 10 to 30 per cent, shorter. The similar movement of all five filaments 

 pulls down the anther tube in which the pollen lies and causes the style 

 to push out pollen and protrude at the apex. In this case stimulation 

 produces a shortening as in a muscle, but when the active tissue is 

 appropriately joined to inactive or elastic tissue a curvature may be 

 produced as in Mimosa. The active region need not always be swollen 

 like a pulvinus, and indeed the irritable stamens of Berberidaceae (Fig. 22), 

 of Cistaceae, and of Sparmannia, as well as the stigmas of Mimithis 



1 Ewart, Annals of Botany, 1897, Vol. XI, p. 455. 



