SECT. I MOKPHOLOGY 117 



cell walls of which calcium carbonate or silica has been deposited 

 (Fig. 126, below, to the right). 



The STINGING HAIRS (Fig. 126), such as those of Nettles {Urtica) 

 and of the Loasaceae, are special forms of bristles, and arise as 

 prolongations of single epidermal cells. These, however, swell in 

 the course of their development, and becoming surrounded by 

 adjoining epidermal cells present the appearance of being set in 

 sockets ; while, at the same time, by the multiplication of the cells 

 in the tissue at their base, the whole hair becomes elevated on a 

 column-like protuberance. The hair tapers towards the apex and 

 terminates, somewhat obliquely, in a small head, just below which 

 the wall of the hair remains unthickened. As the wall of the hair is 

 silicified at the end and calcified for the rest of its length, the 

 whole hair is extremely stifl". Such hairs furnish a means of 

 defence against animals. The heads break off at the slightest touch, 

 and the hairs piercing the skin pour out their poisonous contents, 

 which, especially in the case of some tropical nettles, may cause 

 severe inflammation. According to G. Haberlandt (^^') this is due 

 to the presence of a toxin of albuminous nature. 



The cells surrounding the base of a hair are often arranged in 

 a ring or in radiating lines, or are otherwise different from the 

 surrounding epidermal cells. Such cells may be called subsidiary cells 

 to the hairs. 



Unicellular hairs, such as we have so far considered, may 

 terminate in well-defined heads resulting from the swelling of their 

 tips, or their side walls may develop irregular excrescences ; on the 

 other hand, they may remain short and expanded like a balloon, or 

 remain close to the surface of the e2)idermis as spindle-shaped or 

 stellate hairs. Multicellular hairs may be merely simple rows of 

 similar cells, as the hairs on the stamens of Tradescantia (Fig. 59); or 

 their terminal cells may become swollen into globular heads (Fig. 127), 

 like those on the Chinese Primrose {Primula sinensis) ; or an epidermis 

 may be covered with disc-, star-, or bowl-shaped hairs (Fig. 128). 

 Sometimes the hairs become variously branched, lose their living 

 contents, and form a silky or woolly protective covering similar to 

 that formed by unicellular hairs. In special cases, as in the scale 

 hairs of Ferns, they may even have the shape of a small leaf. 



Emergences, unlike hairs, are not formed solely by epidermal 

 cells, but a number of cells, lying more or less deeply in the sub- 

 epidermal tissues, also take part in their formation. Thus, for 

 example, while only a few rows of sub-epidermal cells enter into the 

 formation of the emergences (Fig. 129) on the margins of the stipules 

 of the Pansy {Viola tricolor), much deeper-lying tissue participates in 

 the development of the emergences which, as trickles, serve in the 

 case of Roses as a means of protection, and at the same time are of 

 assistance in climbing. The thick emergences which spring from the 



