40 ARRANGEMENT OF THE MESOPHYLL 



also formed which repel by disagreeable odors or tastes the 

 attacks of animals. The nettles are shunned because of the 

 burning sting produced by the plant hairs. The upper portion 

 of these hairs (Fig. 25) is practically a delicate glass tube since 

 the walls are filled with silica. The basal portion of the hair is 

 composed of soft, yielding cellulose so that the fluid collecting 

 in the hair distends this portion of it. The knob at the end is 

 fastened to the hair by so thin a ring of wall, as seen in Fig. 25, 

 B, that the least touch breaks it off. In this wav the hair is 



FIG. 25. Stinging hair of nettle: A, portion of epidermis bearing hair. 

 B, tip of hair enlarged, showing easily detachable knob. I. D. Cardiff. 



transformed into a veritable hypodermic needle which easily 

 punctures the skin while the distended basal portion contracts and 

 forces out some of the fluid. The burning sensation or sting 

 that immediately follows the puncture of the skin is caused by 

 the injection of formic acid while a variety of other poisons pro- 

 duce the subsequent irritations. These poisons are so powerful 

 in some of the East Indian nettles as to produce serious results, 

 even a tetanus. 



17. Significance of Certain Internal Leaf Structures. The 

 changes or modifications produced in the chlorenchyma by vari- 

 ous stimuli are quite as striking as in the case of the epidermis. 

 The palisade cells are an excellent illustration of this point. 

 These cells are developed as a result of the stimulating influence 

 of light (Fig. 26). The photosynthetic activity caused by the 

 light increases the pressure of the fluids in the cells. This 



