296 



BOTANY 



be best observed in Mimosa, for, although so sensitive to the action 

 of external influences, it does not exhibit its irritable movements at 

 all times. Whenever the temperature of the surrounding air falls 

 below a certain level, no movements take place, and the whole 

 plant passes into a condition known as COLD RIGOR, while, on the 

 other hand, at a temperature of about 40, HEAT RIGOR occurs. 

 DROUGHT RIGOR is induced, just before withering, by an insufficient 

 supply of water, and a DARK RIGOR by a prolonged retention in 

 darkness, probably owing to the derangement of the chloroplasts. In 



a vacuum, or on exposure to hydrogen 

 and other gases chloroform vapour, 

 coal gas, etc. movement also ceases, 

 partly on account of insufficient oxygen, 

 and partly from the actual poisonous 

 action of the gases themselves. If the 

 state of rigor is not continued too 

 long, the original irritability will again 

 return on the restoration of normal 

 conditions ( 10 ). 



The movements of irritability exhibited by 

 the staminal leaves of some Berberidaceae 

 (Herberts, Mahonia) and Compositae (Cynareae 

 and Liguliflorae) bear a certain relation to 

 those of foliage leaves. The bow-shaped 

 filaments of the stamens of the Compositae 

 straighten upon mechanical irritation. As 

 they frequently contract 10-20 per cent of their 

 length, the style becomes extended beyond 

 the anther-tube (Fig. 238). The reduction 



FIG. 238. -A single flower of Centaurea {n the j h Qf the filaments is accompan i e d 

 jacea with perianth removed. A, sta- . . . . . , 



mens in normal position ; B, stamens by a moderate increase in their thickness, due 

 contracted ; c, lower i>art of tubular to the elastic contraction of the cell walls, 

 perianth ; s, stamens ; a, anther-tube ; an( j the consequent expulsion of water into 

 ;/, style ; P, pollen. (After PFEFFER, th(j interce ll u l ar spac es. The stamens of 



Berberis and Mahonia are only sensitive to 



contact on the inner side near their base, and as their contraction occurs only on 

 the inner side, the anthers are thus brought into contact with the stigma ( 101 ). 

 Examples of variation movements of carpellary leaves may be seen in the flowers 

 of Mimulus, StroUlanthes (Goldfussia), 'Martynia, Torenia, and other plants. 

 The two lobes of the styles of these flowers fold together when irritated. The 

 stigmas of Mimulus and Torenia open again shortly unless active pollen has been 

 deposited when they remain in the closed condition. Similarly, in the flowers of 

 Stylidium, a sudden upward movement of the bent style occurs when it is 

 irritated by a touch. 



