THE PLANT AND THE ANIMAL 



263 



that have not stopped growing.^. The movements of 

 another class are different. These take place quickly, 

 almost instantaneously, as a result of irritation, or even 

 without any stimulus whatever, and apparently quite 

 spontaneously. Let us 

 study a few cases of 

 this kind of pheno- 

 menon, beginning 

 with the simplest, 

 observed in the well- 

 known barberry. In 

 the centre of the 

 yellow flowers of this 

 plant, which are very 

 like small roses, there 



is a pistil surrounded by six stamens (p, fig. 76).^ 

 These grow normally in the position shown at si. 

 on the left. But as soon as we touch the base of 

 the filament with a needle (as shown in the figure) , the 

 stamen suddenly moves and assumes the position st'. 

 on the right-hand side, i.e. applies itself to the stigma. 

 It remains for a time in this position, then gradually 

 returns to the normal position to again apply itself 

 to the stigma as soon as irritated. Movements as the 

 result of irritation, though of a somewhat different kind, 

 are also characteristic of the stamens of the corn-flower, 

 the thistle, the artichoke, and other plants. 



These are all movements of very small, if not exactly 

 microscopic, organs, and hence do not produce so start- 

 ling an impression as does the movement of the irritated 



' Some cases of these phenomena, however, fall rather into another 

 class; they depend on the presence of a special tissue in which the 

 quantity of water changes, hence also the tension of the cells. It is 

 obvious that in such cases the phenomenon may also be observed in 

 fully developed plants. Such are, for instance, the sleep movements of 

 leaves. 



^ Fig. 76 is a longitudinal section of the barberry flower : pst. marks the 

 position of the petals ; st. and st'., the stamens ; p, the pistil with the stigma. 



