THE MOLECULAR FORCES IN PLANTS. 189 



vascular bundles. The parenchyma of the lower side does not 

 border directly upon the epidermis, being separated from it by 

 a layer of sclerenchyma, consisting of about three layers of strongly 

 thickened elements arranged side by side without interspaces. 

 This sclerenchyma it is which causes the movements described, 

 since its constituent elements elongate very much more than, 

 those of the parenchyma when the leaves are moistened, and vice 

 versil shorten much more when the leaves are dried. The move- 

 ments exhibited by the involucral leaves of Carlina are not 

 directly dependent on the vital activity of the cells ; the leaves 

 are also capable of movement when dry and dead. 1 



If we observe plants of Carlina in their natural habitats, we 

 shall at once see that in damp, rainy weather their inflorescences 

 are shut, the involucral leaves having closed together in conse- 

 quence of being wetted, so that they protect the flowers against 

 the injurious effects of the weather. Characteristic movements 

 due to absorption of water, but just as little directly associated 

 with the life of the cells as those of the involucral leaves of 

 Carlina, are to be observed in the 

 beaks of Erodium fruits, and in the 

 awns of Stipa. In Fig. 69 is de- 

 picted a dry mericarp of Erodium 

 with its beak. The lower part, s, of 

 the beak is spirally twisted, but not 

 the upper part, -'. We lay such a 

 fruit for a short time (about half a 

 minute) in water, and then stick it 

 with its lower end in loose moist FIG. 69.-Erodium fruit, 



sand, and cover with a bell-glass. 



The movement which takes place is readily seen, and in the moist 

 air it continues till the lower part of the beak is unwound, and 

 the entire beak is stretched straight. If we stick a moistened 

 Erodium fruit into moist sand, and by means of a vertically placed 

 piece of wood prevent the movement of the beak, which when dry 

 is directed almost horizontally, the induced movement is trans- 

 ferred to the fruit, which being acted on by a vertical pressure 

 is screwed into the sand. In nature the fruits of Erodium do 

 actually bury themselves in the ground by means of the character- 

 istic power of movement of their beaks. 



Fig. 70 depicts the fruit and awn of Stipa. In the dry fruit 

 the lower part, e, of the awn is spirally wound. The part k of 



