INTRODUCTION TO PLANT HISTOLOGY 



471 



Fig. 459. — Rosa sp. Transverse section of stem passing 

 through base of prickle showing its superficial origin. 



tissues. Thus they may sometimes become 

 indirectly useful to the plant. For example, 

 mineral materials may serve to strengthen 

 tissues. Again, the aromatic terpenes, to 

 which plant odours are due, are superfluous 

 substances from the point of view of meta- 

 bolism, but they serve an essential biological 

 purpose in attracting insects to flowers, whereby 

 pollination is efi^ected. 



From the anatomical standpoint it is simpler 

 to classify glands as either superficial or internal. 

 The superficial glands are usually modified 

 hairs in which one or more cells contain 

 the secreted material. A gland of this type 

 is the stinging hair of the Nettle {Urtica dioica) 

 (Fig. 460). The secretory cell is flask-shaped, 

 the bottom of which is seated on a pedestal 

 of small cells. The neck is drawn out into 

 a long, tapering cone, the wall of which is 

 impregnated with Calcium carbonate, and the 

 tip is bent sideways and forms a small 

 silicified bulb. When touched this bulb breaks 

 ofi", leaving a sharp point, with a lateral 

 opening like a hypodermal needle, which 

 pierces the skin. Pressure forces the poison 

 upwards from the flask-like base of the cell 

 and into the subcutaneous tissues. The gland 

 contains a mixture of histamine and acetyl 



Fig. 460. — Urtica dioica. 

 Stinging hair showing 

 pedestal of small cells, 

 flask-shaped secretory cell 

 with long neck and ter- 

 minal bulb. 



