THE COVERING TISSUES 99 



markings or other peculiarities which may be of impor- 

 tance in identification work. Striations extending 

 parallel with the length of the hair are often present. 

 The papillose surface of peppermint and stramonium 

 hairs is caused by small projections upon the surface 

 of the hair. In absinthium the large terminal cell 

 of the hair stands at right angles to the basal cells, 

 a characteristic observed in but a small number of 

 plants. The hair of anthemis shows a terminal cell 

 many times longer than the basal cells. One type 

 of hair occurring in thyme leaves is two-celled with 

 the terminal cell bent in a distinctly different direction 

 from that of the basal cell. Biseriate or twin hairs 

 are found in arnica flowers. Hairs frequently show 

 projections or barbs along the edge of the cell. In 

 multicellular hairs these projections may be formed 

 by individual cells, but in unicellular hairs they are 

 emergences or projections from the cell wall. Barbed 

 hairs play an important part in the protection and 

 distribution of seeds. The hairs of stinging nettle 

 (Urtica dioica) contain secretions which, if injected 

 under the skin, cause severe irritation. In other 

 instances (particularly cowage, Mucuna pruriens), 

 plant hairs are provided with recurved barbs by means 

 of which they become firmly embedded in the skin 

 and thus act as irritants. 



Glandular Trichomes. — A glandular trichome con- 

 sists essentially of a gland or secreting organ which 

 is in direct contact with the epidermal surface, or is 

 raised above this surface by means of a stem or stalk. 

 Sessile glandular hairs are directly attached to the 

 epidermis. In stalked glandular hairs the gland is 

 raised above the epidermal surface and attached to 



