362 



BOTANY 



PART I 



americana, bear a certain relation to those of foliage leaves. The bow-shaped fila- 

 ments of the stamens of the Compositae straighten upon mechanical irritation. As 



they frequently contract 10-20 per cent 

 of their length, the style becomes ex- 

 tended beyond the anther-tube (Fig. 

 292). The reduction in the length of 

 the filaments is accompanied by a moder- 

 ate increase in their thickness, due to the 

 elastic contraction of the cell walls, and 

 the consequent expulsion of water into 

 the intercellular spaces. The stamens 

 of Berleris and Mahonia are only sensi- 

 tive to contact on the inner side near the 

 base, and as their contraction occurs only 

 on the inner side, the anthers are thus 

 brought into contact with the stigma. 



The two lips of the stigmas of Mimu- 

 lus, Goldfussia, Marty nia, Torenia, and 

 other plants close together when touched. 

 In a short time they open and are again 

 seismonastically sensitive. Opening 

 also takes place when pollen has been 

 brought to the stigma and germinated 

 on it. The destructive effect of the 

 pollen leads, however, to a closing 



movem ent which is not a phenomenon 



.. ., , .,., 

 of irritability. 



FIG. 292. A single- flower of Centaurea jacea with 

 perianth removed. A, Stamens in normal 

 position; B. stamens contracted; c. lower 

 part of tubular perianth; ,, stamens; a, 

 anther - tube ; g, style ; P, pollen. (After 

 PFEFFER, enlarged.) 



While SClSttlOnasty IS a peculiar 



form of irritability, it is also the 

 extreme form of haptotropism. There are plants which exhibit a 

 perception intermediate between irritability to contact and to shock. 

 This applies to certain etiolated seedlings, the haptotropism of which 

 was mentioned above ; a jet of water or gelatine is sufficient to 

 stimulate them, though more weakly than stroking with solid 

 bodies ( 119 ). 



